Viessmann Vitodens 200-W B2HE-19 Fault codes & diagnostics

170 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

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Severity at a glance

Each cell = one fault code. Hover to identify.
9 Emergency 112 High 27 Medium 22 Low
Just want the most common Viessmann Vitodens 200-W B2HE-19 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 170 documented codes

Tap any card for details

F.7

The boiler's sensor that measures the temperature of your stored hot water has lost its connection, so the boiler can't tell how hot the cylinder is and may stop heating your hot water until it's repaired.

High Engineer only £100-200

What to check first

  1. Switch off the boiler at its power isolator and wait five minutes.
  2. Check that the cylinder sensor lead is firmly plugged in at both ends and not visibly damaged or pinched.
  3. Switch the power back on, press the reset button on the boiler, and see whether the code returns. If it does, call a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor lead or wiring harness
  • Cylinder sensor pocket

Related codes

Full explanation for F.7
F.8

Your boiler has detected a problem with the sensor that measures the temperature of your hot water cylinder, so it may not heat stored hot water properly, but this is not usually dangerous and a qualified engineer can usually fix it quickly.

Medium Engineer only £120-220

What to check first

  1. Note the exact fault code and any other messages shown on the boiler display.
  2. Turn the boiler off at the control panel and at the isolator switch, wait 10 minutes, then turn it back on to see if the code clears.
  3. Check, with the power off, that the cable from the hot water cylinder sensor to the boiler is not visibly damaged or pinched, and call a Gas Safe engineer if the code returns.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or plug
  • Control board (occasionally)

Related codes

Full explanation for F.8
F.9

Your boiler has lost track of the temperature of the hot water it sends to your taps, so it may stop heating your hot water while your central heating can usually carry on working.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the wall or fused spur, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the display again after it restarts. If F.9 returns, leave the boiler alone and do not open the casing.
  3. Book a Gas Safe registered engineer, mentioning that the code is F.9 on a Viessmann Vitodens 200-W, so they can check the sensor and its wiring.

Parts commonly replaced

  • DHW outlet temperature sensor
  • Sensor wiring harness or connector
  • Sensor clip or seal

Related codes

Full explanation for F.9
F.10

Your boiler's sensor that measures the temperature of the hot water going to your taps has sent a faulty reading, so the boiler has stopped to protect itself and may not be providing hot water until it is checked.

High Engineer only £100-200

What to check first

  1. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the display for any other fault codes and note them down for the engineer.
  3. If the fault returns, do not open the boiler casing. Contact a Gas Safe registered engineer to inspect the sensor.

Parts commonly replaced

  • DHW outlet temperature sensor
  • Sensor wiring harness or connector

Related codes

Full explanation for F.10
F.11

The boiler has detected a break in the wiring to a temperature sensor used by its solar heating connection, so it has flagged a fault that usually needs a qualified engineer to check, though it may not stop your heating or hot water.

Low Engineer only £90-180

What to check first

  1. Switch the boiler off at its isolator or fused spur, wait about 30 seconds, then switch it back on to see if the fault clears.
  2. Check whether your home has solar thermal panels, as this sensor is only used with them. If you have none, mention this to the engineer, as the fault may be a configuration issue.
  3. If the fault returns, do not open the boiler casing. Contact a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Collector temperature sensor
  • Sensor cable or wiring harness

Related codes

Full explanation for F.11
F.12

Your boiler has detected a temperature sensor reading that isn't making sense, so it has likely stopped to protect itself. A qualified engineer can usually find and fix this without too much trouble.

High Engineer only £120-230

What to check first

  1. Note the exact code on the display and check whether the boiler shows any other fault codes alongside F.12.
  2. Turn the boiler off at its isolation switch or fused spur, wait about 10 minutes, then switch it back on.
  3. Press the reset button on the boiler (or use the reset option in the menu) once, then check whether the heating and hot water return.
  4. If F.12 returns straight away, stop and book a Gas Safe registered engineer rather than trying further resets.

Parts commonly replaced

  • Collector temperature sensor (NTC)
  • Sensor cable or connector
  • Boiler control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for F.12
F.13

The boiler has lost the signal from its outdoor temperature sensor, so it can't adjust heating to the weather, but it will usually keep heating and supplying hot water.

Low Engineer only £100-180

What to check first

  1. Turn the boiler off at its control panel, wait about 30 seconds, then turn it back on to see whether the code clears.
  2. Look at the outdoor sensor on the outside wall and check for visible damage, such as a cracked casing or a cable pulled loose.
  3. If the code returns or the sensor looks damaged, book a Gas Safe registered engineer rather than opening the boiler or checking the wiring inside it.

Parts commonly replaced

  • Outdoor temperature sensor
  • Sensor connection cable

Related codes

Full explanation for F.13
F.14

The boiler has lost its signal from the outdoor temperature sensor, so it can't adjust heating to the weather, but it will usually still heat your home and water.

Low Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch, wait 10 minutes, then switch it back on to clear the fault.
  2. Check the outdoor sensor cable from the outside for visible damage, such as cuts, crushed sections, or water getting in where it enters the wall.
  3. If the fault returns or the cable looks fine, book a Gas Safe registered engineer to test and replace the outdoor sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for F.14
F.15

Your boiler's hot water tank thermometer appears to have lost its connection, so the boiler can't tell how hot the stored water is and may not heat it properly until a qualified engineer repairs the fault.

Medium Engineer only £120-250

What to check first

  1. Switch the boiler off at its power supply and wait 30 seconds, then switch it back on to see whether the code clears.
  2. Check the thin sensor cable running from the hot water cylinder to the boiler for visible damage, pinching or a loose connection, without disconnecting anything inside the boiler casing.
  3. If the code returns or hot water is still affected, contact a Gas Safe registered engineer to test and replace the cylinder temperature sensor.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or plug connector

Related codes

Full explanation for F.15
F.16

The boiler has spotted a faulty hot water cylinder sensor, so it may not be heating your stored hot water properly, though your central heating can usually keep working.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait a few minutes.
  2. Check the cable and plug connected to the cylinder temperature sensor (usually on or near the hot water cylinder) for visible damage, chafing, or loose connections, without disconnecting anything inside the boiler casing.
  3. Restore power, reset the boiler, and see if the fault code returns. If it does, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor (DHW sensor)
  • Sensor wiring harness or plug

Related codes

Full explanation for F.16
Showing 11–20 of 170
F.29

The boiler has lost the signal from one of its temperature sensors, so it can't reliably control the heating, and it's best to have a Gas Safe engineer check it soon.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the isolator or fused spur, then wait five minutes.
  2. Check that the cable plugs on the sensor near the low loss header are pushed in fully and look undamaged.
  3. Restore power, press the reset button on the boiler panel, and check the system pressure gauge is between 1 and 1.5 bar.
  4. If the fault returns or the plugs look damaged, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Low loss header temperature sensor
  • Sensor cable or wiring harness
  • Control PCB (rarely required)

Related codes

Full explanation for F.29
F.30

The boiler's sensor that monitors water temperature in the heating system is sending a faulty signal, so the boiler has stopped to protect itself and may not be providing heating.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on and press the reset button on the display once.
  2. Check the display for any message that mentions pressure or water, and if the pressure gauge reads below 1 bar, top it up following the boiler manual.
  3. If the fault returns within a few minutes, stop and call a Gas Safe registered engineer, as the sensor or its wiring usually needs replacing.

Parts commonly replaced

  • Low loss header temperature sensor
  • Sensor cable or plug connector
  • Control unit circuit board (if the fault persists after sensor replacement)

Related codes

Full explanation for F.30
F.49

The boiler has stopped because it cannot read one of its safety temperature checks properly, so it has shut itself down until a qualified engineer looks at it.

High Engineer only £150-280

What to check first

  1. Check the boiler pressure gauge and top up to about 1.0 to 1.5 bar using the filling loop if it is low.
  2. Reset the boiler using the reset button or the control panel, wait a few minutes, then check whether it restarts.
  3. Make sure the flue outlet outside the house is clear of leaves, nests, snow or other obstructions.
  4. If the code returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor lead or wiring harness
  • Control PCB (only if the sensor circuit is faulty)

Related codes

Full explanation for F.49
F.50

Your boiler has shut itself down because it's detecting a faulty reading from the sensor that monitors the hot gases leaving through the flue, which is a safety check, so a qualified engineer needs to look at it.

High Engineer only £120-220

What to check first

  1. Note the exact code shown on the boiler display and check whether the boiler still has heating and hot water.
  2. Check that the flue terminal outside the property is not blocked by leaves, snow, debris or nesting material, and clear anything you can safely reach from ground level.
  3. Try one reset using the boiler's reset button or control, then wait a few minutes; if the code returns, do not keep resetting and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor wiring lead or connector

Related codes

Full explanation for F.50
F.57

The boiler has lost contact with its room temperature sensor, so it can't properly judge how warm your home is, but it will usually keep heating and hot water working in a basic way until this is checked.

Low DIY-safe £90-180

What to check first

  1. Turn off the heating controls and switch the boiler off at its on/off switch, then wait 5 minutes.
  2. Check the room sensor (the separate wall-mounted or thermostat unit in your living area) is securely attached, and check that any visible wire leading to it is not loose, cut, or damaged. Do not open the boiler casing.
  3. Switch the boiler back on and reset it using the reset button or by pressing the reset option on the display, then see whether the F.57 code clears.
  4. If the code returns, stop and contact a Gas Safe registered engineer to test the sensor and its wiring at the boiler.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or connector

Related codes

Full explanation for F.57
F.58

Your boiler has lost contact with one of its temperature sensors, so it may not be controlling the heating as accurately as it should, but this is usually not an immediate danger.

Low Engineer only £100-200

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to see if the fault clears.
  2. Check that the room thermostat or controller is plugged in securely and that its batteries (if any) are working.
  3. If the code returns, stop and book a Gas Safe registered engineer, as the sensor or its wiring needs testing and replacing safely.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor wiring or connector
  • Remote control/room thermostat unit

Related codes

Full explanation for F.58
F.59

Your boiler has stopped heating because its internal fan is not running correctly, which is a common fault that a registered engineer can usually fix.

High Engineer only £200-450

What to check first

  1. Check the boiler pressure gauge and top up to about 1.0 to 1.5 bar if it is low, following the manufacturer's filling instructions.
  2. Turn the boiler off at the switch, wait five minutes, then turn it back on and press the reset button once.
  3. Make sure nothing is blocking the flue outlet outside and that there are no obvious gaps or damage around the flue.
  4. If the fault returns, stop trying to fix it yourself and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan connection cable or harness
  • Gas valve or gas-air mixing unit
  • Control PCB

Related codes

Full explanation for F.59
F.62

Your boiler has stopped because it could not finish its automatic self-check of how it burns gas, which usually means it is not getting enough heat away through the system, and it needs a manual reset.

High Engineer only £90-250

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it is below about 1 bar, following the manufacturer's instructions.
  2. Make sure all radiator valves are open and any TRVs are turned up, so the boiler has a clear route to release heat.
  3. Press the reset button on the front of the boiler or use the reset function on the controls, then wait a few minutes to see if the boiler restarts.
  4. If the fault returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve
  • Combustion fan
  • Ionisation electrode
  • Condensate trap
  • Flue gasket

Related codes

Full explanation for F.62
F.63

Your boiler has shut itself down because the exhaust gases leaving through the flue got too hot, which is usually caused by a blockage or restriction in the flue outlet, and it needs checking before it's restarted.

Emergency Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and isolate its power supply, then wait about 10 minutes.
  2. From ground level, check the flue outlet on the outside wall for anything blocking it, such as debris, a bird's nest, leaves or snow. Do not climb a ladder or touch the flue while it is hot.
  3. If the outlet looks clear, restore power and reset the boiler once. If the fault returns, or the boiler will not reset, stop and call a Gas Safe registered engineer.
  4. If you smell gas, or your carbon monoxide alarm sounds, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999 straight away.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue terminal or flue pipe seals
  • Condensate trap or condensate pipe

Related codes

Full explanation for F.63
F.65

The boiler tried to switch on its burner but could not detect a stable flame, so it has stopped to stay safe, and it will usually need a qualified engineer to find out why.

High Engineer only £100-300

What to check first

  1. Check the boiler pressure gauge. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the handbook.
  2. Check that the gas supply is on. Confirm the gas meter and the boiler's gas isolation valve are open, and that other gas appliances are working.
  3. Check that the flue outlet on the outside wall is not blocked by leaves, debris, nesting material or snow.
  4. Turn the boiler off at the switch, wait 5 minutes, then turn it back on. Press the reset button once only. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Control PCB
  • Burner gasket

Related codes

Full explanation for F.65
Showing 21–30 of 170
F.67

The boiler has shut itself down because it is not detecting the gas flame correctly, which usually means a dirty or worn sensor that needs a qualified engineer to clean or replace it.

High Engineer only £120-280

What to check first

  1. Check the boiler pressure gauge and top it up to about 1.0-1.5 bar if it is low, following the manufacturer's instructions.
  2. Make sure the boiler is switched on at the mains and that no other electrical fault is affecting it.
  3. Press the reset button once on the boiler. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation/ignition electrode
  • Burner gasket
  • Electrode lead or connector
  • Burner assembly

Related codes

Full explanation for F.67
F.68

The boiler has detected a flame when it should not be lit, so it has shut itself down as a safety precaution, and it will need a qualified engineer to find out why before it can heat your home again.

High Engineer only £120-350

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 2 minutes.
  2. Switch the power back on and press the reset button on the boiler (or hold reset as shown in the manual) to clear the fault.
  3. Check whether the boiler restarts normally. If the F.68 code returns, or you smell gas at any point, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flame sensor (ionisation electrode)
  • Gas valve
  • Control PCB
  • Burner gasket

Related codes

Full explanation for F.68
F.69

The boiler has stopped because its flame sensor is not reading the flame correctly, which is a common fault that a Gas Safe engineer can usually fix quickly.

High Engineer only £90-220

What to check first

  1. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, topping up with the filling loop if it is low.
  2. Switch the boiler off at the unit and at the fused spur, wait 10 minutes, then switch it back on and press the reset button once.
  3. If the fault returns straight away, stop and call a Gas Safe registered engineer, as the flame sensor needs cleaning or replacing and this should not be attempted without gas qualifications.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Electrode lead or connector

Related codes

Full explanation for F.69
F.70

The boiler's internal control electronics have detected a fault and have stopped the burner as a precaution, so the heating and hot water may not be working until it is looked at by an engineer.

High Engineer only £200-450

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait about 10 minutes.
  2. Switch the power back on and press the reset button on the boiler control panel once, then check whether the fault clears and the boiler runs normally.
  3. If the code F.70 returns, or the boiler will not restart, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (PCU)
  • Ignition electrode
  • Ionisation (flame sensor) electrode
  • Main PCB

Related codes

Full explanation for F.70
F.71

Your boiler's fan is turning more slowly than it should, so it has stopped as a safety precaution and will need a qualified engineer to check it.

High Engineer only £200-450

What to check first

  1. Check the outside flue terminal and air intake for leaves, nesting material, snow, or other debris and clear any you can safely reach.
  2. Make sure the boiler is switched on at the mains and that the pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's instructions allow it.
  3. Wait 10 minutes, then press the reset button once (if fitted) and see whether the boiler restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan speed sensor (usually integral to the fan)
  • Main control PCB

Related codes

Full explanation for F.71
F.72

The boiler's fan is not slowing down to the speed it needs before it starts up, so the boiler has stopped as a safety precaution and will need a qualified engineer to check it.

High Engineer only £200-450

What to check first

  1. Check that the boiler pressure gauge reads between 1 and 1.5 bar, topping up with the filling loop only if it is low and you know how to do this safely.
  2. Make sure the flue terminal outside the property and the condensate pipe are not blocked by leaves, snow, ice, nests or debris, and clear any blockage you can safely reach.
  3. Press and hold the reset button (or use the reset option on the display) once, wait 10 minutes, then try again. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor (Hall effect sensor)
  • Main PCB (control board)
  • Fan power or signal cable
  • Gas valve (in some cases)

Related codes

Full explanation for F.72
F.73

The boiler's internal parts have lost reliable contact with each other, so it has stopped working as normal and may need a reset or a check by a qualified engineer.

High Engineer only £150-400

What to check first

  1. Check the pressure gauge on the front of the boiler. If it is below 1 bar, top it up using the filling loop, following the boiler manual.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on.
  3. Press and hold the reset button or use the reset option on the controller, then wait up to 10 minutes to see if the boiler restarts.
  4. If the code returns or the boiler will not restart, stop and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Main PCB (printed circuit board)
  • Control unit (Vitotronic)
  • Bus connection cable or plug

Related codes

Full explanation for F.73
F.74

Your boiler has stopped because the water pressure in the central heating system has dropped below the safe level, which is usually fixable by topping it up, but if it keeps falling there may be a leak that needs checking.

High DIY-safe £80-200

What to check first

  1. Make sure the boiler is switched off and fully cooled down, then check the pressure gauge under the boiler. A reading below 1.0 bar means it needs topping up.
  2. Open the filling loop valves (or use the filling loop handle) slowly while watching the gauge, until the needle reads about 1.2 to 1.5 bar, then close the valves and disconnect the filling hose if fitted.
  3. Restart the boiler using the reset button or the control panel and check the display clears the F.74 code and the gauge holds its reading.
  4. If the pressure drops again within a few days or weeks, stop topping up and call a Gas Safe registered engineer to find and fix the leak.

Parts commonly replaced

  • Filling loop or filling valve
  • Pressure relief valve
  • Expansion vessel
  • Water pressure sensor
  • Pipe fittings and seals

Related codes

Full explanation for F.74
F.77

The boiler's control unit has lost some of its stored settings, so it has stopped working properly and may need a reset or a replacement part.

High Engineer only £350-650

What to check first

  1. Turn off the boiler using its on/off switch and switch off its power at the fused spur or isolator.
  2. Wait at least 2 minutes, then switch the power back on and turn the boiler on again.
  3. Check whether the fault code clears and the boiler restarts normally.
  4. If F.77 returns, stop and call a Gas Safe registered engineer, as the control unit usually needs replacing.

Parts commonly replaced

  • Burner control unit (PCB)
  • Vitotronic control module
  • Memory module (EEPROM)

Related codes

Full explanation for F.77
F.89

Your boiler's internal water pump appears to be stuck or blocked, so it may not be moving hot water around your heating system properly, and a qualified engineer should take a look.

High Engineer only £180-350

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to see whether the fault clears.
  2. Check the pressure gauge on the front of the boiler. It should read roughly 1 to 1.5 bar when cold. If it is very low, top it up using the filling loop only if you know how, and stop if you are unsure.
  3. If the fault returns or the pump is still blocked, stop and call a Gas Safe registered engineer. Do not attempt to open the pump or other gas components yourself.

Parts commonly replaced

  • Internal circulation pump
  • Pump head or rotor
  • Pump control board

Related codes

Full explanation for F.89
Showing 31–40 of 170
F.91

The boiler's main control circuit board has lost contact with one of its internal electronic parts, so it has shut itself down as a precaution and will likely need a qualified engineer to check it.

High Engineer only £200-450

What to check first

  1. Turn off the boiler using its on/off switch and the fused spur or isolator switch that supplies it, then wait 5 minutes.
  2. Turn the power back on, wait for the display to start, then press the reset button (or hold the reset option shown on the display) once.
  3. If the fault code returns, or the boiler will not restart, stop and book a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • Electronics module (DIO)
  • Main control PCB
  • Connection cable or wiring loom

Related codes

Full explanation for F.91
F.92

The boiler's control system has lost contact with one of its internal communication parts, so it may shut itself down until it is reset or checked by a Gas Safe engineer.

High Engineer only £180-420

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and whether F.92 returns on the display.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer. Do not open the casing, as it contains mains electrics and gas components.

Parts commonly replaced

  • ADIO electronics module
  • Control unit communication cable or connector
  • Main PCB (in some cases)

Related codes

Full explanation for F.92
F.93

The boiler's control panel and its add-on control module are not talking to each other properly, so the boiler may shut down or stop working until this is checked.

High Engineer only £150-350

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 10 minutes.
  2. Switch the power back on and wait a few minutes for the boiler to fully restart.
  3. Check whether the F.93 code has cleared and the boiler is working. If the code returns, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • M2IO module
  • Main control PCB
  • Communication cable or connector

Related codes

Full explanation for F.93
F.94

The boiler's internal control system has lost contact with one of its main parts, so it has shut itself down as a safety precaution until it can communicate properly again.

High Engineer only £180-400

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait about 2 minutes.
  2. Turn the power back on and press the reset button (or use the reset option on the display) once to see if the fault clears.
  3. If the code returns, stop and call a Gas Safe registered engineer, as the cause is usually inside the boiler's electronics and wiring.

Parts commonly replaced

  • SM1A control module
  • SDIO electronics module
  • BUS connection cable or wiring harness
  • Power supply unit

Related codes

Full explanation for F.94
F.100

The boiler's control system has detected an electrical supply problem in the wiring that links its controls and accessories, which may stop some features from working until it is checked.

Medium Engineer only £100-250

What to check first

  1. Turn the boiler off at its isolator switch and at the fused spur or consumer unit, wait two minutes, then switch it back on.
  2. Check whether the boiler restarts and whether the fault code clears or returns.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the cause is likely inside the boiler or its wiring.

Parts commonly replaced

  • PlusBus connection cable
  • Vitotronic control unit
  • Power supply board
  • Connected accessory module

Related codes

Full explanation for F.100
F.104

Your boiler has been stopped by a safety signal from a device or wiring connected to it outside the boiler itself, so it may not be providing heating or hot water until an engineer checks it.

High Engineer only £90-200

What to check first

  1. Check the boiler display for any other fault codes and note them down before doing anything else.
  2. Check the system pressure gauge on the boiler. If it is below about 1 bar, top it up using the filling loop as shown in the manual, then stop once it reaches 1.5 bar.
  3. Press the reset button once on the boiler and wait 10 minutes. If F.104 returns, leave the boiler off and book a Gas Safe registered engineer.

Parts commonly replaced

  • External safety switch or fault contact device
  • External fault input wiring or connector
  • Control PCB (rare)

Related codes

Full explanation for F.104
F.142

Your boiler's control system has lost contact with its fan, so it has shut itself down as a precaution, and a Gas Safe engineer will need to check it.

High Engineer only £150-400

What to check first

  1. Check the boiler display for any pressure warning, and if the pressure is below 1 bar, top it up using the filling loop as shown in the manual.
  2. Turn the boiler off at its isolator switch or fused spur, wait about 5 minutes, then turn it back on and see if the fault clears.
  3. Check the external flue terminal outside your home for visible blockages such as bird nests or debris, and clear any you can safely reach.
  4. If the fault returns, do not keep resetting the boiler. Call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan unit
  • Control board (PCB)
  • Fan connection cable or plug
  • Fan speed sensor

Related codes

Full explanation for F.142
F.160

Your boiler's internal parts are not talking to each other properly, so it may have shut itself down as a precaution, and a heating engineer should check it.

High Engineer only £150-400

What to check first

  1. Turn off the boiler using its on/off switch, then switch off the electricity supply to it at the fused spur or isolator for about 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and shows a code again.
  3. Check that the boiler's gas supply valve is open and that the system pressure gauge reads between 1 and 2 bar, topping up only if the manufacturer's instructions allow it.
  4. If the fault returns, stop and call a Gas Safe registered engineer, because the wiring and control modules inside the boiler should not be opened or repaired by a homeowner.

Parts commonly replaced

  • Main control PCB (printed circuit board)
  • CAN bus communication cable or harness
  • Control module connector plug
  • Expansion or extension module

Related codes

Full explanation for F.160
F.161

Your boiler's control unit has lost access to some of the information it stores about how to run, so it has stopped working properly and needs a qualified engineer to check it.

High Engineer only £350-600

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait 10 minutes.
  2. Switch the power back on and check whether the boiler restarts and the F.161 code clears.
  3. If the code returns or the boiler still won't heat or supply hot water, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (main PCB)
  • Control unit memory module

Related codes

Full explanation for F.161
F.162

Your boiler has detected that the electrical power reaching its control board is lower than it should be, so it has shut itself down as a safety precaution.

High Engineer only £150-450

What to check first

  1. Check whether other appliances in the house, such as lights and sockets, are working normally, as a wider power dip could be the cause.
  2. Switch the boiler off at its dedicated fused spur or isolator switch, wait 60 seconds, then switch it back on.
  3. If the boiler does not restart, press and hold the reset button for about 2 seconds, or follow the reset instructions in your manual, and then contact a Gas Safe engineer if the fault code returns.

Parts commonly replaced

  • Power supply unit
  • Main PCB (control board)
  • Mains supply connection terminals

Related codes

Full explanation for F.162
Showing 41–50 of 170
F.163

Your boiler's control unit has detected a problem with the memory that stores its settings, so it may shut down or stop working properly until an engineer checks it.

High Engineer only £300-550

What to check first

  1. Check the boiler is getting mains power and that its on/off switch and the fused spur are on.
  2. Switch the boiler off at the isolator or fused spur, wait about 60 seconds, then switch it back on.
  3. If the code clears and the boiler runs normally, note the code and book a Gas Safe engineer anyway, as the fault may return.
  4. If the code returns, stop trying to reset it and contact a Gas Safe registered engineer or Viessmann-approved installer.

Parts commonly replaced

  • Main control PCB (printed circuit board)
  • Memory or EEPROM chip
  • Control unit display module

Related codes

Full explanation for F.163
F.182

Your boiler has shut itself down because it has detected a fault with the sensor that measures the temperature of the water leaving the boiler, so it may not be providing heating or hot water until a qualified engineer checks it.

High Engineer only £150-300

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 2 bar when the boiler is cold. If it is low, top it up using the filling loop as shown in the manual, then stop.
  2. Wait about 10 minutes, then press and hold the reset button for a few seconds (or use the reset option on the controls) to try restarting the boiler once.
  3. If the code F.182 returns straight away, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness
  • Main PCB (in some cases)

Related codes

Full explanation for F.182
F.183

Your boiler has lost its reading of how hot the water leaving it is, so it has likely shut itself down as a precaution until a qualified engineer checks the sensor.

High Engineer only £100-200

What to check first

  1. Check the boiler pressure gauge and top it up to the recommended level (usually between 1.0 and 1.5 bar) if it is low, using the filling loop.
  2. Turn the boiler off at the switch, wait 30 seconds, then turn it back on to clear the fault once.
  3. If the code returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness or connector
  • Control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for F.183
F.184

Your boiler has shut itself down because it has detected a faulty temperature sensor that controls its water heating, so it needs a qualified engineer to check and replace the faulty part.

High Engineer only £120-250

What to check first

  1. Check the system pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop only if you know how, otherwise leave this to the engineer.
  2. Wait at least 10 minutes, then press the reset button once on the boiler and see whether it restarts.
  3. If the fault code returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor
  • High limit safety thermostat
  • Sensor wiring harness or connector
  • Main PCB (only if the fault persists after the sensor is replaced)

Related codes

Full explanation for F.184
F.185

Your boiler has shut itself down as a precaution because a temperature sensor that checks the water leaving the boiler has lost its connection, so it can't safely judge the water temperature and needs a qualified engineer to reconnect or replace it.

High Engineer only £150-260

What to check first

  1. Switch the boiler off at its isolator or fused spur and wait about 5 minutes.
  2. Switch the power back on, then press and hold the reset button on the boiler (or use the reset option on the controls) for a few seconds.
  3. Check the boiler's pressure gauge and top up with the filling loop if it is below about 1 bar, then leave the boiler to restart.
  4. If the fault code F.185 returns, stop attempting repairs and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor lead or wiring harness
  • High limit safety cut-out (STB)
  • Main PCB (only if sensor and wiring are fine)

Related codes

Full explanation for F.185
F.299

Your boiler's internal clock has lost track of the correct time and date, which is usually a minor issue that is simple to correct and normally does not stop your heating or hot water.

Low DIY-safe £0-180 (£0 if resetting the clock clears the code; £90-180 for an engineer visit if the controller backup battery needs replacing)

What to check first

  1. Check the boiler's display to see whether the time and date are wrong.
  2. Use the buttons on the boiler's control panel to open the time and date settings and enter the correct time and date, then confirm.
  3. Wait a few minutes and check whether the fault code clears. If it returns within a few days or the clock keeps resetting, contact a Gas Safe registered engineer to check the controller's backup battery.

Parts commonly replaced

  • Controller backup battery

Related codes

Full explanation for F.299
F.342

The boiler's main control panel has lost proper communication with the part that runs the burner, so it has stopped working until it's checked by an engineer.

High Engineer only £180-420

What to check first

  1. Check that the boiler has a working power supply, including that its external fused spur or isolator switch is on.
  2. Turn the boiler off at the isolator, wait about 5 minutes, then switch it back on to see whether the fault clears.
  3. If the code returns, stop and call a Gas Safe registered engineer. Do not open the casing or touch the internal wiring.

Parts commonly replaced

  • Burner control unit (BCU)
  • Control unit PCB
  • Connecting cable or wiring harness

Related codes

Full explanation for F.342
F.345

The boiler has shut itself down as a safety precaution because it got too hot, usually because water isn't flowing through it properly, and it will need checking before it can safely run again.

High Engineer only £120-280

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top it up using the filling loop, then close the loop valves.
  2. Make sure all radiator valves and any isolation valves on the heating circuit are fully open, and check the boiler's pressure relief valve is not dripping.
  3. Press and hold the reset button for a few seconds or switch the boiler off at the isolator, wait 5 minutes, then switch it back on. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Safety temperature limiter (STB)
  • Circulation pump
  • Expansion vessel

Related codes

Full explanation for F.345
F.346

Your boiler has stopped because it could not finish checking how it burns gas safely, so it has paused itself and a gas engineer should take a look.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch and wait about 5 minutes.
  2. Check that the gas supply tap to the boiler is fully open and that other gas appliances are working.
  3. Check that the flue outlet on the outside wall is not blocked by leaves, nests, debris or snow.
  4. Turn the boiler back on and press the reset button once. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze assembly gasket
  • Flue gas recirculation seal
  • Combustion controller calibration service

Related codes

Full explanation for F.346
F.348

Your boiler has detected a problem with the part that controls how much gas reaches the burner, so it has likely stopped working for safety while a qualified engineer looks at it.

High Engineer only £250-450

What to check first

  1. Check that your gas meter has credit or that your gas supply has not been interrupted, and check whether other gas appliances like the hob are working.
  2. Wait about five minutes, then press the reset button on the boiler once, or use the reset option on the display if it shows one.
  3. If the boiler starts and stays on, keep an eye on it. If the code returns or the boiler switches off again, stop trying and call a Gas Safe registered engineer.
  4. Do not open or adjust the gas valve, and do not try to repair any gas component yourself.

Parts commonly replaced

  • Gas modulation valve
  • Gas valve gasket and O-ring seals
  • Gas valve connection cable

Related codes

Full explanation for F.348
Showing 51–60 of 170
F.349

Your boiler is still running, but it is holding back its power because it may not be getting a correct airflow reading, which can happen when the flue is partly blocked or restricted.

Emergency Engineer only £100-400

What to check first

  1. Turn the boiler off at its switch and do not reset it repeatedly.
  2. From ground level only, check the outside flue terminal for visible blockages such as nests, leaves, debris or ice.
  3. Do not climb ladders or try to clear anything inside the flue or boiler casing.
  4. If you smell gas, see soot or scorch marks, or notice fumes, ventilate the room, leave the property and call the National Gas Emergency Service on 0800 111 999.
  5. Otherwise, contact a Gas Safe registered engineer to inspect the flue and fan unit before using the boiler further.

Parts commonly replaced

  • Fan assembly
  • Flue terminal
  • Flue pipe seals
  • Air pressure sensor

Related codes

Full explanation for F.349
F.350

Your boiler has shut itself down for safety because its flame sensor is giving an unexpected reading, which usually means it can't detect the flame reliably, and it should be checked by a qualified engineer.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at the isolator switch or power socket and wait 10 minutes.
  2. Turn the boiler back on and press the reset button on the front panel once.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the flame sensor and burner need cleaning or replacing.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode cable
  • Burner gasket
  • Burner door seal

Related codes

Full explanation for F.350
F.351

Your boiler has stopped because it couldn't detect a steady gas flame when it lit, which is a common safety cut-out that usually has a fixable cause.

High Engineer only £90-200

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 seconds, then turn it back on and press the reset button on the front panel.
  2. Check that the boiler's gas supply is on and that the boiler pressure gauge reads between 1.0 and 1.5 bar when cold.
  3. If the boiler fails to light again after one reset, leave it off and call a Gas Safe registered engineer, as the flame sensor (ionisation electrode) usually needs cleaning or replacing.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode cable
  • Burner gasket
  • Gas valve (if flame sensing is faulty)

Related codes

Full explanation for F.351
F.352

The boiler has shut itself down because it has detected more carbon monoxide than it should, which usually means its exhaust pipe is blocked, leaking, or not venting properly, so please switch it off and get a Gas Safe engineer to check it.

Emergency Engineer only £100-400

Parts commonly replaced

  • Flue pipe sections
  • Flue terminal
  • Flue gaskets and seals
  • Condensate trap and pipe
  • Flue or combustion sensor

Related codes

Full explanation for F.352
F.353

Your boiler is not getting enough gas to run at full strength, so it is working at reduced power, and a qualified engineer can usually restore it to normal.

Medium Engineer only £90-180

What to check first

  1. Check that the gas meter's emergency control valve and any supply isolation valve are fully open.
  2. If you have a prepayment meter, check that it has credit and is not showing an error.
  3. Turn on another gas appliance, such as a hob, to see whether the supply problem affects the whole property. If it does, contact your gas supplier or network operator (National Gas Emergency Service on 0800 111 999) and do not use the boiler.
  4. Once the supply is confirmed, switch the boiler off at the mains for 30 seconds, then switch it back on to try a single reset. If the code returns, book a Gas Safe registered engineer.

Parts commonly replaced

  • Gas inlet filter (gas train screen)
  • Gas valve assembly (if the filter is not the cause)

Related codes

Full explanation for F.353
F.354

Your boiler has shut itself down for safety because the part that controls how much gas reaches the burner is not working within its normal range, so a Gas Safe engineer needs to check it.

High Engineer only £200-450

What to check first

  1. Check that the gas supply is on: make sure the gas meter emergency control and the boiler's gas isolation valve are fully open.
  2. Check the boiler pressure gauge and top up with the filling loop if it is below 1 bar, following the manufacturer's instructions.
  3. Look outside at the flue terminal and make sure nothing is blocking it, such as leaves, debris, bird nests or snow.
  4. Press the reset button once on the boiler (or use the reset function on the controls). If the code returns, do not keep resetting it.
  5. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Gas modulation valve (gas valve)
  • Gas valve connecting cable
  • Gas train assembly

Related codes

Full explanation for F.354
F.355

Your boiler has detected a flame when it should not be there, so it has switched itself off as a safety precaution, and a qualified engineer can usually find and fix the cause.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at the isolator switch and wait 2 minutes.
  2. Turn it back on and press the reset button on the front panel once. Do not keep resetting it repeatedly.
  3. Check the condensate pipe outside the house for ice, blockages or a sagging section that could hold water, and clear any blockage you can reach safely.
  4. If the boiler stays in fault or the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Condensate trap
  • Burner gasket
  • Control PCB

Related codes

Full explanation for F.355
F.357

Your boiler has stopped because it isn't getting the gas pressure it needs to light safely, so it's shut itself down to protect you and your home.

High Engineer only £120-350

What to check first

  1. Check that the gas supply to the boiler is on, including the isolation valve on the pipe near the boiler.
  2. Check whether other gas appliances, such as the hob or fire, also aren't working, which would suggest a wider supply problem.
  3. If other appliances are affected, contact your gas network operator or energy supplier, as the problem may be outside your home.
  4. Press the reset button once on the boiler display. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve
  • Gas pressure switch
  • Gas train components
  • Ignition electrode

Related codes

Full explanation for F.357
F.359

The boiler tried to light its burner but couldn't get a flame, so it has shut itself down to stay safe and needs a qualified engineer to check the ignition and gas supply.

High Engineer only £150-350

What to check first

  1. Check that other gas appliances, such as the hob, are working to confirm the gas supply is on, and check the gas meter emergency control valve is open.
  2. Check the boiler's pressure gauge and top up the pressure if it is below 1 bar, following the manufacturer's instructions in the manual.
  3. Press the reset button once on the boiler, wait five minutes, and see if it restarts. If the fault returns or the boiler won't reset, stop and call a Gas Safe registered engineer.
  4. If you smell gas at any time, leave the property, do not use any switches, and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Ignition unit (ignition module)
  • Ignition electrode
  • Gas valve
  • Burner control unit

Related codes

Full explanation for F.359
F.361

Your boiler has started up but could not detect a flame, so it has stopped as a safety precaution and will likely need a reset and a check of the burner's sensor and the air supply.

High Engineer only £120-250

What to check first

  1. Check that the boiler has power and that the gas supply is turned on at the meter and the boiler's gas isolation valve.
  2. Check the boiler pressure gauge; if it is below about 1 bar, top it up using the filling loop as shown in the manual, then stop.
  3. Check that nothing is blocking the flue terminal outside or the air vents in the room, and that no debris or dust has gathered around the boiler.
  4. Press the reset button on the boiler or use the reset function on the control panel once, then wait a few minutes. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode cable or lead
  • Burner gasket
  • Burner door seal

Related codes

Full explanation for F.361
Showing 61–70 of 170
F.365

Your boiler has shut itself down as a safety precaution because its gas control system is not confirming that the gas valve is working correctly, so please don't keep trying to restart it and arrange for a Gas Safe engineer to check it.

Emergency Engineer only £300-550

What to check first

  1. Check that the boiler's gas supply isolation valve is fully open, and that gas works on other appliances in the home.
  2. Turn the boiler off at its on/off switch and at the fused spur, wait 60 seconds, then turn it back on and try one reset.
  3. If the fault code returns or the boiler shuts down again, switch it off, leave it off, and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (BCU)
  • Gas valve assembly
  • Gas valve wiring harness or connector

Related codes

Full explanation for F.365
F.366

The boiler has detected a possible fault in its control system that stops the gas supply from shutting off properly, so it has locked out as a safety precaution and needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £250-450

Parts commonly replaced

  • BCU (burner control unit)
  • Gas valve

Related codes

Full explanation for F.366
F.367

The boiler's gas shut-off valve is not closing when it should, which is a safety issue, so turn the boiler off and have a Gas Safe registered engineer check it as soon as possible.

Emergency Engineer only £350-600

Parts commonly replaced

  • BCU burner control unit
  • Gas valve
  • Ignition electrode
  • Flame sensor

Related codes

Full explanation for F.367
F.368

The boiler is not getting the gas pressure it needs to light, so it has stopped and is waiting for the gas supply to be checked.

High Engineer only £90-250

What to check first

  1. Check whether other gas appliances, such as the hob or fire, are working. If none are, contact your gas supplier, as the problem may be outside your home.
  2. If you have a prepayment meter, check that it has credit and that the emergency control valve is fully open.
  3. Turn the boiler off at the isolator switch, wait 30 seconds, then turn it back on to reset it once. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve
  • Gas pressure switch
  • Gas filter

Related codes

Full explanation for F.368
F.369

The boiler's flame sensor is picking up a faint electrical signal even though no flame is burning, so the boiler has switched itself off as a safety precaution.

High Engineer only £100-250

What to check first

  1. Check the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up using the filling loop only if it is low
  2. Switch the boiler off at the isolator, wait 10 minutes, then switch it back on and press the reset button once
  3. If the fault returns, turn the boiler off and call a Gas Safe registered engineer, as the flame sensor and burner need inspecting

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze assembly gasket
  • Burner door gasket

Related codes

Full explanation for F.369
F.370

The boiler has shut itself down because its burner did not light or stay lit correctly, so it needs a qualified engineer to check the gas supply and burner controls.

High Engineer only £150-400

What to check first

  1. Check that your gas meter or supplier account has credit and that the gas supply to the property is on.
  2. Check that other gas appliances, such as the hob, are working normally.
  3. Check the boiler pressure gauge and top up with the filling loop only if it is below 1 bar and you know how to do this safely.
  4. Press the reset button once, wait 5 minutes, and see if the boiler restarts.
  5. If it shows the same fault again or you smell gas, turn off the gas supply at the meter, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Gas valve
  • Ionisation electrode
  • Burner control PCB
  • Fan assembly
  • Gas/air mixing unit seal

Related codes

Full explanation for F.370
F.372

The boiler keeps turning its gas flame off during its self-check, so it has stopped and locked itself out to stay safe until someone looks at it.

High Engineer only £120-300

What to check first

  1. Check the boiler pressure gauge and top up to about 1.0 to 1.5 bar if it is low, following the manufacturer's filling instructions.
  2. Make sure the gas supply tap to the boiler is fully open and that other gas appliances in the home are working.
  3. Check that the flue outlet outside is not blocked by debris, leaves, or snow, and that the condensate pipe is not frozen or blocked.
  4. Turn the boiler off at its isolator switch, wait 30 seconds, turn it back on, and press the reset button once. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode and gasket
  • Gas valve or gas/air mixture unit
  • Fan assembly

Related codes

Full explanation for F.372
F.373

Your boiler has stopped because it could not finish checking how it burns gas safely, so it needs a quick reset and possibly an engineer's check.

High Engineer only £100-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop, then close the loop valves.
  2. Make sure all radiator valves are open and bleed any radiators that are cold at the top, so water can circulate freely and heat can move around the system.
  3. Press the reset button on the boiler control panel once, wait a few minutes, and see whether the boiler restarts. If the fault returns or the boiler will not reset, call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan
  • Gas valve
  • Flue gas temperature sensor

Related codes

Full explanation for F.373
F.375

Your boiler has stopped because it could not finish its automatic self-check of how it burns gas, so it has paused for safety and will need a quick reset and a look by an engineer.

High Engineer only £120-280

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes.
  2. Switch it back on and press the reset button on the boiler or controls, then wait 15 minutes to see if it restarts.
  3. Check that the flue pipe outside (the vent on the wall) is not blocked by leaves, nests, snow or debris, and clear any blockage you can safely reach.
  4. If the fault returns, stop and call a Gas Safe registered engineer. Do not open the burner or remove any gas components yourself.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner gauze assembly
  • Flue gas recirculation valve gasket

Related codes

Full explanation for F.375
F.377

Your boiler has stopped itself after its automatic self-check of the burner's gas-to-air mixing failed, which is a protective shutdown rather than a sign of immediate danger, but it needs a Gas Safe engineer to look at it.

High Engineer only £100-250

What to check first

  1. Switch the boiler off at its on/off switch and wait about 10 minutes.
  2. Check the boiler's pressure gauge is between 1 and 1.5 bar when cold, and top up with the filling loop only if it is below 1 bar.
  3. Check that the flue terminal outside the property is clear of leaves, nests, snow or other debris.
  4. Switch the boiler back on and press the reset button once. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze assembly
  • Burner gasket
  • Flue gas recirculation component

Related codes

Full explanation for F.377
Showing 71–80 of 170
F.378

The boiler's flame switched off unexpectedly while it was running, so it has shut down as a safety precaution and will usually restart after a reset.

High Engineer only £90-250

What to check first

  1. Turn the boiler off at its on/off switch and wait about 5 minutes.
  2. Turn it back on and press the reset button or follow the reset prompt on the display.
  3. Check that other gas appliances, such as the hob, are working and that your gas meter or supply has not been cut off.
  4. If the fault returns, stop resetting it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode (flame sensor)
  • Ignition electrode
  • Gas valve
  • Fan

Related codes

Full explanation for F.378
F.380

The boiler has switched itself off because it is not detecting its flame properly, so it cannot safely heat your home or water until a qualified engineer checks it.

High Engineer only £120-250

What to check first

  1. Check that the gas supply is on, including the meter and any gas isolation valve, and check that other gas appliances in the home are working.
  2. Check that the boiler has power and that the display is not showing any other error alongside F.380.
  3. Press the reset button once. If the fault returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner gauze assembly

Related codes

Full explanation for F.380
F.381

Your boiler has briefly lost its gas flame while it was running, so it has switched itself off as a safety precaution, and it usually needs a check by a Gas Safe engineer to find out why.

High Engineer only £90-250

What to check first

  1. Check that other gas appliances in your home are working, and check your gas meter or supplier account to confirm there is no gas supply interruption.
  2. Check the boiler pressure gauge and top up with the filling loop if it is below 1 bar, following the manufacturer's instructions.
  3. Make sure the condensate pipe is not blocked or frozen, and that the flue outlet outside is clear of obstructions.
  4. Reset the boiler once using the reset button or the control panel. If the fault returns quickly or happens again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Fan assembly
  • Burner gasket
  • Condensate trap

Related codes

Full explanation for F.381
F.382

Your boiler has shut itself down after detecting the same problem several times in a row, so it is stopping to protect itself until a qualified engineer checks it.

High Engineer only £120-350

What to check first

  1. Check the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is low, top it up using the filling loop as shown in the manual, then close the loop.
  2. Wait about 10 minutes, then press the reset button on the front of the boiler, or turn the boiler off and on at its power supply, to clear the lockout.
  3. Check that the condensate pipe is not frozen or blocked, and that the gas supply and other appliances are working normally.
  4. If the fault code returns, stop trying to reset it and call a Gas Safe registered engineer. Do not repeatedly reset the boiler, as this can keep it locked out.

Parts commonly replaced

  • Ignition electrode
  • Flame detection electrode
  • Gas valve
  • Fan assembly
  • Printed circuit board
  • Condensate trap

Related codes

Full explanation for F.382
F.383

The boiler's internal gas shut-off valve may not be closing fully when it should, which is a safety issue, so turn the gas off at the supply tap or meter and call a Gas Safe registered engineer straight away.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve (gas control unit)
  • Gas valve electronic control module
  • Gas valve connecting cable or plug
  • Gas pressure switch

Related codes

Full explanation for F.383
F.384

The boiler's gas control valve may not be shutting off the gas supply properly, so please turn off the gas at the isolation valve and contact a Gas Safe registered engineer straight away.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve
  • Gas valve actuator/solenoid coil
  • Boiler control PCB

Related codes

Full explanation for F.384
F.385

The boiler's flame sensor circuit has detected an electrical fault, so it has stopped to stay safe and will need a qualified engineer to check and repair it.

High Engineer only £120-250

What to check first

  1. Check the pressure gauge on the front of the boiler and make sure it reads between 1.0 and 1.5 bar. If it is low, top it up using the filling loop as shown in the manual.
  2. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. Do not press reset repeatedly.
  3. If the fault returns or the boiler will not restart, turn the boiler off at the isolator switch and call a Gas Safe registered engineer. Do not open the casing or touch the sensor or its wiring.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode lead
  • Ignition/ionisation electrode gasket
  • Main PCB (control board)

Related codes

Full explanation for F.385
F.386

The boiler has lost track of its flame sensor, so it has shut itself down as a safety precaution. It needs a qualified engineer to check the sensor and its wiring before it can safely heat your home again.

High Engineer only £100-220

What to check first

  1. Turn the boiler off at its isolator switch and wait 10 minutes
  2. Check the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up with the filling loop only if it is below 1.0 bar
  3. Turn the boiler back on and press the reset button once; if it locks out again, call a Gas Safe registered engineer and do not keep resetting it

Parts commonly replaced

  • Ionisation electrode
  • Ionisation lead / electrode cable
  • Electrode gasket
  • Electronic control unit (PCB), if the fault persists after the electrode is replaced

Related codes

Full explanation for F.386
F.387

Your boiler has detected a fault with its flame sensor, which is a small part that confirms the gas flame is lit, so it has likely shut itself down as a safety precaution.

High Engineer only £120-250

What to check first

  1. Check the pressure gauge on the boiler and make sure it reads between 1 and 2 bar. If it is low, top it up using the filling loop as shown in the manual.
  2. Press the reset button on the boiler display, or switch the boiler off at the isolator for about 10 minutes and then back on.
  3. If the fault code returns, stop and call a Gas Safe registered engineer. Do not keep resetting the boiler or try to open the burner or sensor yourself.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode lead or connector
  • Burner gasket
  • Control PCB (if the fault persists after the electrode is replaced)

Related codes

Full explanation for F.387
F.388

The boiler has lost the connection to the sensor that checks its gas flame is lit, so it has shut itself down as a safety precaution and is not providing heating or hot water.

High Engineer only £120-220

What to check first

  1. Press the reset button on the boiler once and wait 10 minutes to see if it restarts.
  2. Check that the boiler has enough pressure on its gauge (usually between 1 and 1.5 bar) and top it up only if the manual says how.
  3. If it fails again, turn the boiler off at the switch, leave it for 30 minutes, then turn it back on. If the fault returns, do not keep resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ionisation lead and connector
  • Control PCB (only if the lead and electrode are fine)

Related codes

Full explanation for F.388
Showing 81–90 of 170
F.395

Your boiler has stopped because its flame sensor is reporting an electrical fault, which is a safety shutdown that a qualified engineer needs to check.

High Engineer only £150-300

What to check first

  1. Check the pressure gauge on the front of the boiler and, if it reads below 1 bar, top it up using the filling loop as shown in the manual.
  2. Press the reset button once and wait 10 minutes for the boiler to restart.
  3. If the fault returns, turn the boiler off at its isolator switch and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flame detection electrode (ionisation probe)
  • Ignition electrode
  • Control PCB (main circuit board)

Related codes

Full explanation for F.395
F.396

Your boiler has stopped because its flame sensor is sending a signal the control board doesn't recognise, which usually points to a loose or damaged wire, and a qualified engineer can typically fix it quickly.

High Engineer only £120-250

What to check first

  1. Check the boiler display and note whether it shows any other messages, as these can help the engineer.
  2. Check that the boiler has power, the gas supply is on, and the system pressure gauge reads between 1 and 1.5 bar.
  3. Press the reset button once on the boiler control panel and wait a few minutes to see if it restarts.
  4. If the fault returns, switch the boiler off and call a Gas Safe registered engineer. Do not open the casing or work on the gas or electrical components yourself.

Parts commonly replaced

  • Flame sensor (ionisation electrode)
  • Flame sensor lead or wiring harness
  • Control board (PCB)
  • Ignition electrode

Related codes

Full explanation for F.396
F.399

The boiler has shut itself down because its flame sensor has detected an electrical fault, so it has stopped heating and hot water until a qualified engineer checks it.

High Engineer only £120-250

What to check first

  1. Check the boiler pressure gauge reads between 1.0 and 1.5 bar when cold, and top up with the filling loop if it is low.
  2. Press the reset button once and wait 10 minutes to see if the boiler restarts normally.
  3. If the fault returns or the boiler locks out again, stop and book a Gas Safe registered engineer, as the flame sensor and its wiring need professional inspection.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation lead
  • Burner gasket
  • Main PCB (control board)

Related codes

Full explanation for F.399
F.400

The boiler has switched itself off because it could not detect a flame when it lit, which is usually a dirty or faulty flame sensor rather than a serious problem, but it needs a qualified engineer to check it.

High Engineer only £100-220

What to check first

  1. Check that the gas supply is on by confirming the gas hob or another gas appliance works.
  2. Check that the boiler pressure on the display is between 1.0 and 1.5 bar, and top it up using the filling loop only if it is low.
  3. Press the reset button on the boiler once, wait 10 minutes, and see if it restarts; if the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition/ionisation electrode lead
  • Burner gasket

Related codes

Full explanation for F.400
F.401

Your boiler has shut itself down because its flame sensor has detected an electrical fault, so it is not safely producing heat or hot water until a Gas Safe engineer checks it.

High Engineer only £120-260

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator if you have one, then wait about 5 minutes.
  2. Switch the boiler back on and press the reset button once to see if it restarts normally.
  3. If the fault returns or the boiler will not stay on, do not keep resetting it. Turn it off and book a Gas Safe registered engineer, and never open the casing yourself.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation/ignition lead
  • Ignition or ionisation electrode gasket
  • Main PCB (control board)

Related codes

Full explanation for F.401
F.402

Your boiler has shut itself down because its flame sensor isn't getting a clear signal, which is usually a loose or worn connection that a qualified engineer can normally fix quickly.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch and wait about 5 minutes
  2. Check the boiler pressure gauge reads between 1 and 1.5 bar, and top up with the filling loop if it is low
  3. Turn the boiler back on and press the reset button once. If the fault returns, stop and call a Gas Safe registered engineer, as the flame sensor lead and electrode need checking inside the casing

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ionisation electrode lead
  • Burner gasket
  • Main PCB (rarely required)

Related codes

Full explanation for F.402
F.403

The boiler's flame sensor circuit has detected a fault, so it has shut down to stay safe, and it will need a Gas Safe engineer to check the sensor and its wiring before heating and hot water can return.

High Engineer only £150-300

What to check first

  1. Check the boiler pressure gauge and top up to between 1.0 and 1.5 bar if it is low, using the filling loop as shown in the manual.
  2. Press the reset button or use the reset option on the boiler display once, then wait 10 minutes to see whether the boiler restarts.
  3. If the fault returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition and ionisation lead
  • Control board (PCB)

Related codes

Full explanation for F.403
F.404

Your boiler has stopped because it cannot reliably detect its flame during start-up, which is a common and usually repairable fault that needs a Gas Safe engineer to check.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at the isolator or power switch and wait about 10 minutes
  2. Switch it back on and press the reset button on the front panel once
  3. Check the pressure gauge is between 1.0 and 1.5 bar, and make sure nothing is blocking the flue or air vents
  4. If the fault returns, stop resetting it and call a Gas Safe registered engineer

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Ionisation/ignition lead
  • Control PCB (if the fault persists)

Related codes

Full explanation for F.404
F.405

The boiler's flame sensor has detected an electrical fault, so it has shut itself down as a safety precaution and will need a qualified engineer to inspect and repair it.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to see whether it restarts.
  2. Check the boiler pressure gauge and top it up using the filling loop only if it is below 1 bar and you know how to do so safely.
  3. If the fault returns, stop resetting it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition and ionisation lead
  • Burner gasket
  • Control PCB

Related codes

Full explanation for F.405
F.406

Your boiler has switched itself off because it cannot reliably detect its gas flame, which is a built-in safety check, and it needs a qualified engineer to inspect the connection.

High Engineer only £120-250

What to check first

  1. Switch the boiler off at the on/off switch, wait 10 minutes, then switch it back on and press reset once to see if the fault clears.
  2. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up with the filling loop if it is low.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the flame sensor and its lead need checking inside the boiler.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode lead
  • Burner gasket
  • Control PCB

Related codes

Full explanation for F.406
Showing 91–100 of 170
F.408

Your boiler has shut itself down as a safety precaution because its gas control check didn't pass, so it needs a qualified engineer to look at it.

High Engineer only £180-380

What to check first

  1. Check that your gas supply is on, including the gas meter and the boiler's isolation valve.
  2. Check the boiler pressure gauge and top up with the filling loop if it is below 1 bar.
  3. Reset the boiler once using the reset button on the front panel, then wait 10 minutes to see if it restarts.
  4. If the fault returns or the boiler won't restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve (gas multiblock)
  • Gas valve control relay/PCB
  • Gas pressure switch

Related codes

Full explanation for F.408
F.410

Your boiler's internal control system has detected a fault with its own electronics, so it has stopped working to stay safe, and a qualified engineer will likely need to check it.

High Engineer only £250-500

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or circuit breaker supplying it, then wait 5 minutes.
  2. Switch the power back on and wait a few minutes to see whether the boiler restarts and the code clears.
  3. If F.410 returns, do not repeat resets or open the boiler casing. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (BCU)
  • Main PCB / control board
  • Power supply board

Related codes

Full explanation for F.410
F.416

The boiler has noticed that a safety sensor in its flue (the exhaust pipe that carries fumes outside) may not be sitting in the right place, so it has stopped to be safe until a qualified engineer checks it.

Medium Engineer only £80-150

What to check first

  1. Note the exact code shown on the display and check whether the boiler still produces heat or hot water.
  2. Try one reset using the boiler's reset button or the display, then wait a few minutes to see if the code returns.
  3. Look outside at the flue terminal and check nothing is blocking it, such as leaves, nests or debris, and that the cover is intact.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor clip or holder
  • Flue gasket or seal

Related codes

Full explanation for F.416
F.417

Your boiler's internal control power has detected a short circuit, so it has shut itself down to stay safe, and it will likely need a qualified engineer to find and fix the cause before it can run normally again.

High Engineer only £150-400

What to check first

  1. Turn off the boiler at its on/off switch and isolate the electricity supply at the fused spur or consumer unit.
  2. Check any external wiring or accessories connected to the boiler, such as room thermostats, timers, or controls, for visibly damaged, pinched, or loose wires, and do not open the boiler casing.
  3. Restore power, wait a few minutes, and try resetting the boiler once. If F.417 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • 24 V power supply unit
  • Main control PCB
  • Wiring harness or connector
  • Fuse on the control board

Related codes

Full explanation for F.417
F.418

The boiler has detected a break in one of its internal low-voltage wires, so it has stopped working safely and needs a qualified engineer to reconnect or replace the faulty part.

High Engineer only £120-350

What to check first

  1. Check that the boiler has mains electricity by confirming the isolator switch or spur is on and the display is lit.
  2. Check that the fuse in the plug or spur is intact and the boiler's power switch is on.
  3. Switch the boiler off at the isolator for 30 seconds, switch it back on, and check whether the fault clears.
  4. If the fault returns, do not open the boiler casing. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • 24 V power supply unit
  • Internal connecting cable or wiring harness
  • Control unit PCB

Related codes

Full explanation for F.418
F.425

Your boiler's internal clock has lost its time setting, which is usually a minor issue that doesn't stop your heating or hot water from working.

Low DIY-safe £0-120

What to check first

  1. Check the display on the boiler's control panel to confirm the current time and date are wrong or flashing.
  2. Use the control dial and buttons to set the correct time and date manually, following the 'Time and date' section in your Vitodens user manual.
  3. If your boiler is connected to the internet via a Viessmann gateway or the ViCare app, check that the Wi-Fi or internet connection is working, then allow up to 24 hours for the time to sync automatically.
  4. If the fault returns or the clock keeps losing time, contact a Viessmann-registered heating engineer, as the control unit's clock backup may need replacing.

Parts commonly replaced

  • Control unit clock backup battery
  • Vitotronic control unit

Related codes

Full explanation for F.425
F.426

Your boiler has shut itself down because it's detecting a wiring fault on the connection that accepts signals from an outside control device, such as a timer or smart thermostat, so it's stopped as a precaution.

High Engineer only £90-200

What to check first

  1. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to see if the fault clears.
  2. If you have a smart thermostat, timer or other external controller linked to the boiler, check that its wires and plugs are firmly connected and not damaged, then restart the boiler.
  3. If the fault returns or the boiler stays locked out, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit PCB (main circuit board)
  • External unlock wiring loom or connector
  • External control module or thermostat (if faulty)

Related codes

Full explanation for F.426
F.427

The boiler has detected a broken or disconnected wire on an external control connection, so it may not be receiving a signal it expects from a timer, controller, or energy-tariff device, which can stop heating or hot water from running properly.

Medium Engineer only £90-200

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur, then wait a few minutes before doing anything else.
  2. Check whether any external timer, smart controller, or tariff device is connected to the boiler, and make sure its wires are not loose or damaged.
  3. Restore power, switch the boiler back on, and see if the fault clears; if it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • External unlock signal cable or wiring harness
  • Control unit connection terminal
  • External controller or tariff interface module

Related codes

Full explanation for F.427
F.428

The boiler's airflow sensor for the fan has an electrical fault, so the boiler has shut itself down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £180-550

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait 5 minutes.
  2. Turn it back on once and check whether the fault clears.
  3. If the code returns, stop and book a Gas Safe registered engineer. Do not open the boiler casing or touch the fan, sensor or wiring.

Parts commonly replaced

  • Air mass sensor
  • Fan unit
  • Fan or sensor wiring harness
  • Gas valve (if the sensor or fan is integrated with it)

Related codes

Full explanation for F.428
F.429

Your boiler has stopped because a fan-related wiring connection or sensor is not sending the right signal, so it has shut down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £150-400

What to check first

  1. Check that the boiler has power and that the external controls or thermostat are switched on.
  2. Check that the flue outlet outside the house is not blocked by leaves, snow, debris or nesting material.
  3. Press the reset button on the boiler once, wait about 10 minutes, and see whether it restarts. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan unit
  • Air mass sensor
  • Fan lead or wiring harness
  • Fan speed sensor connector

Related codes

Full explanation for F.429
Showing 101–110 of 170
F.430

Your boiler has lost its connection to its internet or remote-control gateway, so it may not respond to app or thermostat changes, though it will often keep heating your home.

Low DIY-safe £100-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait 2 minutes, then switch it back on to reset the communication.
  2. Check that the gateway unit (the small box connected to your router) has power and that its network cable is firmly plugged in at both ends.
  3. Confirm your home Wi-Fi or router is working and that the gateway is still connected to it, then restart the router if needed.
  4. If the code returns, leave the boiler running and book a heating engineer to check the gateway and communication cabling. Do not open the boiler casing.

Parts commonly replaced

  • Vitoconnect or Vitogate gateway module
  • LON communication cable
  • Boiler control unit (PCB), in rarer cases

Related codes

Full explanation for F.430
F.432

The boiler's fan has sent a faulty reading to its control board, so the boiler has switched itself off as a safety precaution. A qualified engineer will need to check it before it can run again.

High Engineer only £200-450

What to check first

  1. Switch the boiler off at the control panel and at the wall or fused spur, then wait 5 minutes.
  2. Switch it back on and check whether the fault code returns.
  3. If the code returns, leave the boiler off and call a Gas Safe registered engineer. Do not open the boiler casing or try to repair the fan or wiring yourself.

Parts commonly replaced

  • Combustion fan assembly
  • Fan wiring harness
  • Main PCB (control board)

Related codes

Full explanation for F.432
F.433

The boiler has lost contact with the part that tells it how fast its fan is spinning, so it has shut itself down as a safety precaution and will need a qualified engineer to reconnect or replace the faulty part.

High Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge and top it up to about 1.0-1.5 bar if it is low, following the boiler manual.
  2. Press the reset button once, wait 10 minutes, and see whether the boiler restarts.
  3. Check that the flue outlet outside is not blocked by leaves, debris or snow, and that there is no obvious damage to the flue pipe.
  4. If the code returns, stop trying to repair it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor (Hall sensor)
  • Fan wiring harness or connector
  • Main PCB (control board)

Related codes

Full explanation for F.433
F.436

The boiler has detected a faulty temperature sensor that measures the water leaving it, so it has shut itself down as a safety precaution and will need a qualified engineer to check and replace the part.

High Engineer only £150-300

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator for about 10 minutes.
  2. Switch the power back on and press the reset button on the boiler (or use the reset option on its display) once.
  3. If the fault code returns, stop and call a Gas Safe registered engineer, as the sensor and its wiring need professional inspection.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor wiring harness
  • Control PCB (if the sensor circuit is damaged)

Related codes

Full explanation for F.436
F.437

Your boiler has detected a problem with the wire connecting its flow temperature sensor, so it has likely shut down to protect itself and needs a qualified engineer to check it.

High Engineer only £150-300

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it is below 1 bar, following the manufacturer's instructions.
  2. Switch the boiler off at its isolator or fused spur, wait 30 seconds, then switch it back on to try a reset.
  3. If the fault returns, stop and contact a Gas Safe registered engineer, as the sensor and wiring must not be opened or tested by a non-qualified person.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor wiring harness
  • Boiler control PCB (if the wiring and sensor test fine)

Related codes

Full explanation for F.437
F.440

Your boiler has detected an electrical fault with one of its gas safety switches, so it has shut itself down to stay safe until a qualified engineer checks it.

High Engineer only £180-400

Parts commonly replaced

  • Gas pressure switch
  • EM-EA1 extension module
  • Wiring harness for gas pressure switch

Related codes

Full explanation for F.440
F.441

Your boiler has shut itself down because it has detected a fault with the wiring that connects its gas pressure monitor, so it isn't able to safely provide heating or hot water until a qualified engineer checks it.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on to try a single reset.
  2. Check that the boiler's gas supply is on, meaning the gas meter tap and the isolation valve at the boiler are both open.
  3. Check whether other gas appliances, such as a hob, are working, to see if there is a wider gas supply problem.
  4. If the fault returns or the boiler will not reset, stop trying and contact a Gas Safe registered engineer.
  5. If you ever smell gas, leave the property, avoid switches and flames, and call the National Gas Emergency number on 0800 111 999.

Parts commonly replaced

  • Gas pressure switch
  • EM-EA1 extension module wiring harness
  • Gas valve connection lead

Related codes

Full explanation for F.441
F.442

The boiler has detected an electrical fault with a part that controls the flue, so it has switched itself off as a safety precaution and will need a qualified engineer to check it.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 30 seconds, then switch it back on to see if the fault clears.
  2. If the fault returns, do not open the boiler casing or touch the wiring. Note the code and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas damper actuator
  • Flue damper wiring harness
  • DIO electronics module (EM-EA1 extension)

Related codes

Full explanation for F.442
F.443

Your boiler has paused itself because it lost contact with a part that controls the flue exhaust, so heating and hot water are likely off until a qualified engineer checks the connection.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait 2 minutes.
  2. Turn the power back on and press the reset button on the boiler, or use the reset option on the display if one is shown.
  3. If the code returns, stop and call a Gas Safe registered engineer, as the fault is inside the boiler casing and must not be opened by a homeowner.

Parts commonly replaced

  • Flue gas damper
  • DIO electronics extension module (EM-EA1)
  • Connection wiring harness
  • Flue damper control cable

Related codes

Full explanation for F.443
F.444

The boiler has detected an electrical fault in the wiring that controls its gas safety valve, so it has shut itself down as a precaution and needs a qualified engineer to check it.

High Engineer only £150-350

What to check first

  1. Note the fault code shown on the boiler display and check whether the boiler's gas isolation valve is fully open.
  2. Switch the boiler off at its on/off switch, wait about 30 seconds, then switch it back on once to see whether it restarts.
  3. If the code returns, or the boiler smells of gas, hisses or shows any other warning, leave the property, turn off the gas at the meter and call the National Gas Emergency Service on 0800 111 999. Do not try to open the boiler casing or touch the wiring.

Parts commonly replaced

  • Gas safety valve (solenoid gas valve)
  • Wiring harness or connector to the gas valve
  • DIO electronics module extension (EM-EA1)

Related codes

Full explanation for F.444
Showing 111–120 of 170
F.445

Your boiler has shut itself down as a safety precaution because it has detected a problem with the wiring to its gas safety valve, so it won't be able to produce heating or hot water until a qualified engineer checks it.

High Engineer only £150-350

What to check first

  1. Note the fault code shown on the boiler display and the time it appeared.
  2. Check that the boiler has power and that the gas supply and LPG tank are not empty or turned off.
  3. Try a single reset using the reset button or option on the boiler, then wait a few minutes to see if it restarts.
  4. If the fault returns or the boiler locks out again, stop and call a Gas Safe registered engineer. Do not open the boiler casing or attempt to repair the wiring yourself.

Parts commonly replaced

  • Gas safety valve (LPG solenoid valve)
  • Wiring lead or harness to the gas valve
  • Electronics module (DIO / EM-EA1 extension) if damaged

Related codes

Full explanation for F.445
F.446

Your boiler has shut itself down as a precaution because it detected that the water temperature in the heating system is not behaving as expected, and it will need checking before it can run normally again.

High Engineer only £120-300

What to check first

  1. Check the pressure gauge on the boiler. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop, then close the loop valve.
  2. Turn off the boiler at its power switch, wait 10 minutes, then switch it back on and press the reset button if it is still showing the fault.
  3. Check that the radiators heat up and that the pump is running. If the radiators are cold at the top, bleed them to release trapped air, then let the system settle before resetting the boiler again.

Parts commonly replaced

  • Flow temperature sensor
  • Circulation pump
  • Expansion vessel
  • Diverter or pressure relief valve

Related codes

Full explanation for F.446
F.447

The boiler has noticed its flame is not being detected reliably, so it has shut itself down as a precaution, and a qualified engineer can usually restore it quickly.

High Engineer only £100-250

What to check first

  1. Check that the gas supply is turned on at the meter and the boiler's gas isolation valve is open.
  2. Check the pressure gauge on the front of the boiler. It should normally read between 1 and 2 bar when cold. If it is well below 1 bar, top it up using the filling loop, then stop.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. If the fault returns, call a Gas Safe registered engineer rather than repeating resets.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode gasket
  • Burner gasket

Related codes

Full explanation for F.447
F.448

Your boiler has stopped because it is not detecting its gas flame correctly, which is a safety check, and a qualified engineer will usually need to clean or check the flame sensor.

High Engineer only £100-220

What to check first

  1. Check that your gas supply is on and that other gas appliances, such as the hob, are working.
  2. Check the boiler pressure gauge and top up to the recommended level (usually 1.0 to 1.5 bar) if it is low, following the manual.
  3. Press the reset button once, wait 10 minutes, and see if the boiler restarts. If the fault returns or the boiler keeps locking out, call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner assembly
  • Ignition electrode

Related codes

Full explanation for F.448
F.449

Your boiler's control system has noticed a timing problem with its internal schedule, so it may have paused or stopped heating and hot water until it is checked.

Medium Engineer only £150-400

What to check first

  1. Check that the boiler's display shows the correct date and time, and that your heating programmer settings are sensible.
  2. Switch the boiler off at its on/off switch, wait 5 minutes, then switch it back on to clear the fault.
  3. If the fault returns or the boiler will not restart, leave it off and book a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Main circuit board
  • Power supply unit

Related codes

Full explanation for F.449
F.450

Your boiler's controller has lost track of its timing schedule, which can stop heating and hot water from switching on and off at the right times, but this is often fixed with a simple reset.

Medium DIY-safe £90-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 2 minutes.
  2. Switch the power back on and check the boiler's date and time on the controller display, then reset them if they are wrong.
  3. Check that your heating and hot water timer programmes are set correctly, then see whether the boiler starts working normally and whether F.450 returns.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Real-time clock or control PCB

Related codes

Full explanation for F.450
F.451

Your boiler's control unit has lost track of its internal heating schedule, so it may shut down or stop running on its timer until it is reset or checked by an engineer.

High Engineer only £250-550

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 2 minutes, then switch it back on.
  2. Check the boiler display for the code again. If it has cleared and the boiler is heating and producing hot water normally, check that your time and date settings and heating schedule are correct.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer. Do not attempt to open the casing or repeat resets repeatedly.

Parts commonly replaced

  • Vitotronic control unit (main PCB)
  • Control unit programming memory or EEPROM
  • Clock/timer module

Related codes

Full explanation for F.451
F.452

The boiler's internal timer has lost track of its heating schedule, so it may not switch on and off at the times you have set, but it is not usually a sign of danger.

Medium DIY-safe £80-250

What to check first

  1. Check the boiler display for any other fault codes and note them down.
  2. Reset the boiler by pressing the reset button on the control panel, or switch it off at the isolator or wall switch for one minute, then switch it back on.
  3. Check that the clock, date and heating programme on the controls are correct and set them again if needed.
  4. If the code returns or the heating and hot water still do not behave as scheduled, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit circuit board
  • Control panel or display unit

Related codes

Full explanation for F.452
F.453

The boiler has briefly lost track of the timing between its internal parts and has shut down as a safety precaution, so your heating and hot water may be off until it is reset.

High DIY-safe £90-250

What to check first

  1. Turn the boiler off at its on/off switch and wait about 60 seconds.
  2. Check that the boiler's power supply is stable: make sure the plug or fused spur is secure and no other large appliances are causing a power dip at the same time.
  3. Turn the boiler back on and press the reset button on the front panel (or use the reset option on the display) to clear the fault.
  4. If the fault returns within a few days, or the boiler will not restart, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Main PCB (control board)
  • Fan assembly
  • Ignition electrode

Related codes

Full explanation for F.453
F.454

Your boiler's control system has detected that its internal software doesn't match what it expects, so it has stopped to protect itself until a qualified engineer can check and correct it.

High Engineer only £120-300

What to check first

  1. Check the boiler display to confirm the code reads F.454 and note whether there is heating or hot water.
  2. Turn the boiler off at its on/off switch and at the fused spur or isolator for about 30 seconds, then switch it back on.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, preferably one with Viessmann experience, as the software needs to be checked or reloaded.

Parts commonly replaced

  • Control unit (PCB)
  • Control module software update
  • Electronics module (E3)

Related codes

Full explanation for F.454
Showing 121–130 of 170
F.455

Your boiler's internal control system has detected a glitch in how it runs its own software, so it has stopped to protect itself, and a reset may clear it if the fault was temporary.

High Engineer only £250-450

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait about 5 minutes.
  2. Turn the power back on and press the reset button on the boiler if it is lit, then wait a few minutes for it to restart.
  3. Check the pressure gauge is between 1 and 1.5 bar. If it is low, top it up using the filling loop, then note whether the code returns and call a Gas Safe engineer if it does.

Parts commonly replaced

  • Main control unit (PCB)
  • Boiler control board or electronic module

Related codes

Full explanation for F.455
F.456

Your boiler found a problem with its status when it tried to start up, so it may have stopped or refused to light, but this is usually fixable by a qualified engineer.

Medium Engineer only £90-220

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in your manual.
  2. Switch the boiler off at the isolator or power switch, wait 30 seconds, then switch it back on to clear the fault.
  3. Press the reset button on the boiler if the display shows a reset prompt. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Pressure sensor
  • Filling loop valve
  • Control circuit board (PCB)

Related codes

Full explanation for F.456
F.457

Your boiler has stopped because its internal ventilation fan is not starting up, which it needs to do safely before it can heat your home or water.

High Engineer only £200-450

What to check first

  1. Make sure the boiler is switched on at its power switch and at the wall socket or fused spur, and that no other electrical fault is affecting the supply.
  2. Wait about 10 minutes, then press the reset button on the front of the boiler once only.
  3. If the fault code returns, stop and call a Gas Safe registered engineer, as the fan and its wiring sit inside the sealed gas appliance and must not be opened by a homeowner.

Parts commonly replaced

  • Combustion fan assembly
  • Fan connection cable or wiring harness
  • Main control PCB (if the fan signal fault persists)

Related codes

Full explanation for F.457
F.458

Your boiler has stopped because its internal control system has detected a fault it cannot fix on its own, and a qualified engineer will need to check it.

High Engineer only £250-450

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes.
  2. Turn it back on and check whether the fault code clears and the boiler restarts.
  3. If the code returns, note it down and leave the boiler off, then book a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (circuit board)
  • Control panel or display unit

Related codes

Full explanation for F.458
F.461

Your boiler has stopped starting because its exhaust vent flap outside isn't confirming that it has opened, which is a safety check, so it won't switch on until an engineer looks at it.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to try a single reset.
  2. Check from ground level that the flue outlet on the outside wall is not blocked by leaves, debris, nesting material or snow.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer rather than opening the casing or touching the flue.

Parts commonly replaced

  • Flue gas damper actuator
  • Flue gas damper feedback cable or harness
  • EM-EA1 extension module (DIO electronics)
  • Flue damper control or position sensor

Related codes

Full explanation for F.461
F.462

The boiler has stopped because it is not receiving the gas it needs to switch on, so it has no heating or hot water until a qualified engineer checks the gas supply and the valve controlling it.

High Engineer only £150-300

What to check first

  1. Check that your LPG tank has fuel and that its supply valve is fully open.
  2. Switch the boiler off at its on/off switch and at the fused spur, wait 60 seconds, then switch it back on to try one reset.
  3. If the code returns or the boiler will not restart, stop trying and contact a Gas Safe registered engineer.

Parts commonly replaced

  • LPG safety solenoid valve
  • Gas valve assembly
  • DIO electronics module (EM-EA1 extension)

Related codes

Full explanation for F.462
F.463

Your boiler has shut itself down for safety because it isn't detecting its gas flame reliably, so it can't heat your home or water until a qualified engineer checks it.

High Engineer only £120-260

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, then check it is between 1 and 1.5 bar.
  2. Make sure the gas supply is on, including the gas meter and the isolation valve beside the boiler.
  3. Switch the boiler off at the isolator, wait 10 minutes, switch it back on and reset it using the reset button or the reset option on the display.
  4. If the fault returns, stop resetting it and call a Gas Safe registered engineer, as the flame sensing part needs cleaning or replacing.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Burner gasket
  • Gas valve

Related codes

Full explanation for F.463
F.464

Your boiler's flame sensor is giving a weaker reading than expected during its self-check, so it has stopped to stay safe, and a qualified engineer will usually need to clean or inspect that sensor.

High Engineer only £120-250

What to check first

  1. Check that the gas supply to the boiler is turned on, including the gas meter and any isolation valve under the boiler.
  2. Check the boiler pressure gauge and top it up if it is below the marked range, following the manufacturer's filling instructions.
  3. Turn the boiler off at the switch, wait about 30 seconds, then turn it back on once to reset it. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Burner gasket
  • Gas valve

Related codes

Full explanation for F.464
F.467

The boiler has stopped because it isn't getting enough gas to start up safely, so it has paused itself until the gas supply is checked.

High Engineer only £120-350

What to check first

  1. Check that other gas appliances, such as the hob or fire, are working. If they aren't, contact your gas supplier or the National Gas Emergency Service on 0800 111 999 straight away.
  2. Find your gas meter and check that its isolation valve is fully open and that there is credit on your meter if it is a prepayment meter.
  3. Wait a few minutes, then switch the boiler off and on at its power supply once. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve
  • Gas pressure switch
  • Gas filter

Related codes

Full explanation for F.467
F.468

Your boiler has detected an unusual reading from its flame sensor during its self-check, so it has stopped to stay safe, and a qualified engineer should look at it.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch and at the mains supply, wait 10 minutes, then switch it back on.
  2. Press the reset button on the boiler control panel once, if the boiler is still showing F.468.
  3. Check that the boiler's power cable is firmly plugged in and that the boiler's mains socket or switch is working.
  4. If the fault returns, stop and contact a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • Flame ionisation electrode
  • Ignition and ionisation electrode lead
  • Main control PCB
  • Gas valve (if the fault is traced to the burner circuit)

Related codes

Full explanation for F.468
Showing 131–140 of 170
F.471

Your boiler has lost the reading from its water pressure gauge, so it can't confirm the system is safe to run and has likely stopped heating, though this is usually fixable.

High Engineer only £150-300

What to check first

  1. Check the pressure gauge on the front of the boiler. The needle should sit between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top it up using the filling loop as shown in the manual, then close the loop.
  2. Switch the boiler off at the isolator or fused spur, wait 30 seconds, then switch it back on. Press the reset button if the boiler is showing a fault and it does not restart on its own.
  3. If the code returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing or disturb the wiring.

Parts commonly replaced

  • System pressure sensor
  • Sensor wiring harness or connector
  • Main PCB (control board), in some cases

Related codes

Full explanation for F.471
F.473

Your boiler's control system has lost contact with one of its linked components, so it may shut down or stop working properly until the connection is restored.

High Engineer only £150-350

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait two minutes.
  2. Switch the power back on and wait several minutes while the boiler starts up again.
  3. If the same code returns, leave the boiler off and contact a Gas Safe registered engineer, as the fault usually needs a qualified check of the wiring or the heat management unit.

Parts commonly replaced

  • Heat management unit (HMU)
  • BUS communication cable
  • Control unit PCB

Related codes

Full explanation for F.473
F.474

The boiler's control system has noticed a problem with its stored heating timetable, so it is showing a warning, but it may still be able to heat your home and water normally.

Medium DIY-safe £90-180

What to check first

  1. Check the time and date on the boiler display and correct them if they are wrong.
  2. Turn the boiler off at its power switch or fused spur, wait about 10 minutes, then switch it back on to reset the control system.
  3. Check that your heating and hot water programme times are set correctly. If the code returns, or the boiler stops heating or producing hot water, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic main circuit board)
  • Control panel display unit

Related codes

Full explanation for F.474
F.517

The boiler has lost its connection to the wired thermostat on the wall, so it may not be controlling the heating as set, though it can often still run on its default settings.

Medium DIY-safe £80-200

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait two minutes.
  2. Check the thermostat cable at the wall plate and at the boiler end for loose, damaged or disconnected wires, and reconnect any that are loose.
  3. Turn the power back on and wait around five minutes for the boiler and thermostat to re-establish communication.
  4. If the code returns or the heating still does not respond to the thermostat, call a Gas Safe registered engineer.

Parts commonly replaced

  • Vitotrol wired room thermostat / remote control
  • Connecting cable between thermostat and boiler
  • Boiler control unit (PCB), rarely required

Related codes

Full explanation for F.517
F.527

Your boiler has lost or damaged some of its stored settings, so it has stopped and needs a qualified engineer to restore them.

Medium Engineer only £90-250

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait about 60 seconds.
  2. Switch the power back on and let the boiler restart, then check whether the fault code clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer or Viessmann-approved installer.

Parts commonly replaced

  • Vitotronic control unit (PCB)
  • Coding card or parameter memory module

Related codes

Full explanation for F.527
F.528

The boiler's internal settings have not been fully loaded or have become corrupted, so it has stopped working until an engineer sets it up correctly again.

High Engineer only £90-180

What to check first

  1. Turn off the boiler using the on/off switch and isolate its electrical supply at the fused spur or circuit breaker.
  2. Wait about 5 minutes, then restore power and switch the boiler back on.
  3. If F.528 returns, stop and book a Gas Safe registered engineer, as the boiler needs reprogramming with the correct settings.

Parts commonly replaced

  • Control unit (PCB)
  • Coding card

Related codes

Full explanation for F.528
F.530

The boiler's control system has lost reliable temperature readings from one of its sensors, so it may have shut down to stay safe, and a qualified engineer should check it.

High Engineer only £150-350

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator for about 5 minutes
  2. Switch it back on and check whether the code clears and the heating and hot water return
  3. If the code returns or the boiler will not restart, leave it off and call a Gas Safe registered engineer

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Domestic hot water (cylinder) temperature sensor
  • Sensor wiring harness
  • Control unit PCB

Related codes

Full explanation for F.530
F.538

The boiler has lost track of how cool the water is returning from your radiators, so it has stopped as a precaution, and a qualified engineer can usually fix this with a straightforward sensor replacement.

High Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch and wait 5 minutes.
  2. Turn the boiler back on and check whether the fault clears or returns.
  3. Check that the boiler's pressure gauge reads between 1.0 and 1.5 bar, topping up only if the manufacturer's filling instructions say it's safe to do so.
  4. If the fault returns, call a Gas Safe registered engineer rather than opening the boiler casing.

Parts commonly replaced

  • System return temperature sensor (NTC thermistor)
  • Sensor lead or wiring harness, if the break is in the cable

Related codes

Full explanation for F.538
F.539

Your boiler's sensor that measures the temperature of water coming back from your radiators has sent a faulty signal, so the boiler has stopped itself as a precaution until a Gas Safe engineer checks it.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at the isolator switch or fused spur and wait 2 minutes.
  2. Switch it back on and press the reset button on the front panel once, following the boiler display prompts.
  3. If the fault returns, stop and book a Gas Safe registered engineer, as the sensor and its wiring need proper testing.

Parts commonly replaced

  • Return temperature sensor
  • Sensor wiring harness or connector
  • Control PCB (only if the sensor and wiring test fine)

Related codes

Full explanation for F.539
F.540

Your boiler has stopped because the water it produces when heating (condensate) can't drain away properly, so it has shut down as a safety precaution.

High Engineer only £90-180

What to check first

  1. Check the outside condensate pipe for ice, leaves, or debris, and gently thaw it with warm (not boiling) water if it is frozen.
  2. Turn the boiler off at the isolator switch, wait 10 minutes, then turn it back on and press the reset button once to see if it restarts.
  3. If the fault returns, do not keep resetting it. Call a Gas Safe registered engineer to clear the internal condensate trap and check the heat exchanger.

Parts commonly replaced

  • Condensate trap (siphon)
  • Condensate pipe section
  • Heat exchanger seals

Related codes

Full explanation for F.540
Showing 141–150 of 170
F.542

The boiler has lost the signal from one of its temperature sensors, which controls the water heading to one part of your heating system, so that part may not heat properly until it is checked.

Medium Engineer only £120-220

What to check first

  1. Check the boiler display for a pressure reading and, if it is below 1 bar, top it up using the filling loop as shown in the manual.
  2. Switch the boiler off at its power supply, wait about 5 minutes, then switch it back on and see whether the fault clears.
  3. If the fault returns, stop and book a Gas Safe registered engineer, as the sensor and its wiring need checking inside the boiler and mixer circuit.

Parts commonly replaced

  • Flow temperature sensor (heating circuit with mixer)
  • Sensor lead or wiring harness
  • Mixer circuit extension module

Related codes

Full explanation for F.542
F.543

The boiler has noticed that the temperature sensor controlling one of your heating zones, the one that uses a mixing valve, is giving a faulty reading, so it may not be heating that zone correctly.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the wall isolator or fused spur, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the display for any other fault codes and write them down before calling an engineer.
  3. Check that your heating controls and room thermostat are working, and note whether the fault comes back after a few hours or days.

Parts commonly replaced

  • Flow temperature sensor for heating circuit 1 with mixer
  • Sensor wiring lead or plug
  • Vitotronic control unit (rarely needed)

Related codes

Full explanation for F.543
F.544

Your boiler has lost the signal from the temperature sensor that controls one of your heating zones, so that zone may not heat properly, though the rest of your system should still work.

Medium Engineer only £120-220

What to check first

  1. Turn the heating controller off, wait 30 seconds, then turn it back on to clear the fault.
  2. Check the boiler display and controller for any other fault codes, and note them down for the engineer.
  3. If the fault returns, do not open the boiler casing; book a Gas Safe registered heating engineer.

Parts commonly replaced

  • Flow temperature sensor
  • Mixer valve module or motor
  • Sensor wiring harness or connector

Related codes

Full explanation for F.544
F.545

The boiler has spotted a faulty temperature sensor for one heating zone, so that zone may not heat properly, but the boiler and hot water should still be working.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its power supply and wait about 10 minutes, then switch it back on to see whether the fault clears.
  2. Check that the heating zone controller or thermostat is set to a sensible temperature and that the programmer is working.
  3. If the code returns, note the exact code and the affected zone, and book a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Flow temperature sensor for heating circuit 2 (mixer)
  • Sensor cable or plug connector
  • Mixer circuit control module (only if the sensor is fine but the module fails)

Related codes

Full explanation for F.545
F.546

The boiler has noticed that a sensor controlling one of the heating zones with a mixing valve is not sending a reliable reading, so that zone may not heat properly while the rest of the system carries on.

Medium Engineer only £120-260

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur, wait 5 minutes, then switch it back on to see if the fault clears.
  2. Check the boiler display for a pressure reading, and if it is below 1 bar, top it up using the filling loop as shown in the manual.
  3. If the fault returns, note the code and time it appeared and contact a Gas Safe registered engineer, as the sensor and its wiring need professional testing.

Parts commonly replaced

  • Heating circuit 3 flow temperature sensor
  • Sensor cable or wiring harness
  • Mixer extension module (where fitted)

Related codes

Full explanation for F.546
F.547

The boiler has spotted a faulty temperature sensor on one of its secondary heating zones, so that zone may not heat as precisely as it should, while the rest of the system should still work.

Medium Engineer only £120-260

What to check first

  1. Turn off the heating at the boiler control panel and switch off the boiler's power supply at the fused spur.
  2. Check the sensor cable and plug connections on the mixer circuit module for obvious looseness or damage, without unplugging or altering anything inside the control box.
  3. Switch the power back on, then reset the boiler; if the code returns, leave it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor for heating circuit 3 (mixer circuit)
  • Mixer circuit extension module (e.g. Vitotronic extension module)
  • Sensor cable or plug connector

Related codes

Full explanation for F.547
F.574

Your boiler isn't receiving readings from its room thermostat or remote control, so it's running on a backup setting that may not keep your home as evenly heated as usual.

Low DIY-safe £90-220

What to check first

  1. Check that the room thermostat or remote control has working batteries and its display is on.
  2. Make sure the thermostat is plugged in or wired to the boiler as intended, and that no connecting cable is loose or damaged.
  3. Switch the boiler off at its on/off switch, wait 30 seconds, switch it back on, and see whether the code clears.
  4. If the code returns, contact a Gas Safe registered engineer to check or replace the room thermostat or its wiring. Do not open the boiler casing yourself.

Parts commonly replaced

  • Viessmann room temperature sensor or remote control unit
  • Room sensor connecting cable
  • Batteries for wireless remote control

Related codes

Full explanation for F.574
F.575

Your boiler is still heating your home, but it can't sense the room temperature properly, so it is relying on the outside weather to decide how warm to make the house, which may make the temperature a little less precise.

Low DIY-safe £80-160

What to check first

  1. Check that the wireless room thermostat or room unit has working batteries, and replace them if it is low or flat.
  2. Make sure the room unit is not in direct sunlight, near a radiator, or behind a curtain, as these can give false readings.
  3. Turn the boiler off at the switch, wait 30 seconds, turn it back on, and see whether the code clears.
  4. If the code returns, leave the boiler alone and book a Gas Safe registered engineer to check the room sensor and its wiring.

Parts commonly replaced

  • Room temperature sensor or room unit (Vitotrol)
  • Room sensor cable or connector
  • Batteries for wireless room thermostat
Full explanation for F.575
F.576

Your boiler has lost the signal from the thermostat in your room, so it can no longer adjust heating based on the actual room temperature, but it should still keep your home heated using the outside temperature instead.

Medium Engineer only £120-220

What to check first

  1. Turn the boiler off at the on/off switch, wait 2 minutes, then turn it back on to see if the code clears.
  2. Check the room thermostat or room sensor on the wall for visible damage, loose wires, or a damaged cable.
  3. If the room sensor uses batteries, replace them and check the display for the code again.
  4. If the code returns or the cable looks damaged, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor (Vitotrol room unit)
  • Sensor connecting cable
  • Control unit wiring harness

Related codes

Full explanation for F.576
F.577

Your boiler has lost the signal from the room sensor for its second heating zone, so that zone is now being controlled by outside temperature alone and may not heat to the exact room temperature you set.

Medium DIY-safe £90-180

What to check first

  1. Check the batteries in the wireless room thermostat or sensor for heating circuit 2 and replace them if they are low or flat.
  2. Switch the boiler off at its power supply or fused spur for 30 seconds, switch it back on, and check whether the code clears.
  3. Make sure the room sensor is not behind furniture, in direct sunlight, or near a draught, and look for any visibly damaged cable. Do not open the boiler casing. If the code returns, book a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor (Vitotrol or equivalent)
  • Sensor connection cable
  • Control unit circuit board (rare)

Related codes

Full explanation for F.577
Showing 151–160 of 170
F.578

The boiler has lost the signal from the thermostat sensor in one room-heating zone, so that zone is running on a less precise backup setting until the sensor connection is restored or the sensor is replaced.

Medium DIY-safe £100-200

What to check first

  1. Turn the boiler off at its on/off switch and at the mains supply, wait about 5 minutes, then turn it back on to clear the fault.
  2. Locate the room thermostat or room sensor for heating circuit 2, usually on an inside wall, and check its cable is firmly connected at the sensor end and not cut, pinched, or loose.
  3. If the fault returns, contact a Gas Safe registered engineer to check the sensor cable connection at the boiler and test or replace the room sensor.

Parts commonly replaced

  • Room temperature sensor (heating circuit 2)
  • Sensor connection cable

Related codes

Full explanation for F.578
F.579

The boiler's control system has detected a fault with the room thermostat sensor for the second heating zone, so it can't respond properly to the temperature in that room, and that zone may not heat as expected.

Medium Engineer only £120-220

Parts commonly replaced

  • Room temperature sensor (heating circuit 2)
  • Sensor cable or connector
  • Control unit extension module (uncommon)

Related codes

Full explanation for F.579
F.580

The boiler has lost the signal from the room thermostat for its third heating zone, so it is heating that zone based on outside temperature instead, and it should still keep your home warm.

Low DIY-safe £90-180

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on and check whether the code clears.
  2. Check the room thermostat or room sensor for heating circuit 3 is switched on, and replace its batteries if it runs on them.
  3. Look at the wiring or plug connection between the room sensor and the wall, and make sure it is secure, without damage, and not loose. Do not open the boiler casing.

Parts commonly replaced

  • Room temperature sensor (room thermostat unit)
  • Sensor connecting cable

Related codes

Full explanation for F.580
F.581

The boiler has lost the signal from one of its room thermostats, so that heating zone is running on outdoor-temperature settings alone and may not heat your rooms as accurately as usual.

Medium DIY-safe £90-180

What to check first

  1. Switch off the boiler at its isolator switch or fused spur, and wait about 30 seconds.
  2. Locate the room thermostat or sensor for the third heating circuit and check that its cable is securely connected at both ends, without pulling or damaging it.
  3. Restore power, then reset the boiler from the control panel. If the code returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor (heating circuit 3)
  • Sensor connection cable
  • Vitotronic control unit (rarely needed)

Related codes

Full explanation for F.581
F.582

The boiler has lost the signal from the room thermostat sensor for one heating zone, so it is heating that zone based on outside temperature instead of the actual room temperature, which may make it less precise but usually still keeps the house warm.

Medium DIY-safe £120-220

What to check first

  1. Turn off the boiler at its on/off switch, wait 2 minutes, then turn it back on to clear the fault and see if it returns.
  2. Find the room sensor or room thermostat for heating circuit 3 (often in a hallway or living room) and check the cable at the wall and at the sensor for visible damage, pinched wires, or water staining.
  3. If the sensor is a wireless or battery unit, replace the batteries, then restart the boiler.
  4. If the fault returns or the cable looks damaged, leave it alone and book a Gas Safe registered engineer, as the cable or sensor will need replacing and the boiler's control box should not be opened by a homeowner.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or wiring
  • Heating circuit control module

Related codes

Full explanation for F.582
F.666

The boiler has noticed a fault with a sensor that monitors the temperature of a secondary hot water tank, so that extra tank may not be heating up properly, though your main heating and hot water will usually still work.

Low Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, wait 5 minutes, then switch it back on to see if the code clears.
  2. Check the display for any other codes, and note whether F.666 returns after a few hours or days.
  3. If the code returns, contact a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • DHW preheating temperature sensor (TS3)
  • Sensor wiring lead or plug connector
  • Solar transfer pump (only if found faulty)

Related codes

Full explanation for F.666
F.667

The boiler has detected a faulty temperature sensor in the hot water cylinder that is used for solar heating, so it can't properly top up the second cylinder's water, but your main heating and hot water should still be working.

Low Engineer only £120-220

What to check first

  1. Note the code F.667 and the date it appeared, and check whether the boiler still provides heating and hot water.
  2. Reset the boiler using the reset button or control panel, as described in your user manual, then wait a few minutes and see if the code returns.
  3. If the code clears but comes back, or if your hot water or heating stops working, book a Gas Safe registered engineer and do not open the boiler or cylinder yourself.

Parts commonly replaced

  • DHW cylinder temperature sensor (TS3)
  • Sensor cable and plug connector
  • Solar pump or solar control module (only if the fault persists after sensor replacement)

Related codes

Full explanation for F.667
F.668

Your boiler has noticed a problem with a temperature sensor that controls how the water tank is pre-heated by solar energy, so it may not be using that extra free heat, but your boiler can still heat your water and home.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on to clear the fault.
  2. Check the boiler display for whether F.668 returns. If it does, note the code and the date it appears.
  3. Check that your hot water is still working normally, and if it is, book a Gas Safe engineer at your convenience rather than urgently.

Parts commonly replaced

  • DHW reheating temperature sensor (TS4)
  • Sensor lead or cable
  • Cylinder sensor connector

Related codes

Full explanation for F.668
F.669

The boiler has detected a fault with a temperature sensor that helps pre-heat a secondary hot water tank and manage the solar panel pump, so that part of the system may not be working as well as it should, though the main heating and hot water are usually unaffected.

Low Engineer only £110-220

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 seconds, then switch it back on to see if the code clears.
  2. Check the boiler display and note whether the code returns after a few minutes or after a hot water demand.
  3. Look at the visible cable running from the cylinder sensor to the boiler for obvious damage, but do not open the boiler casing or disconnect any wiring.

Parts commonly replaced

  • Cylinder temperature sensor (TS4)
  • Sensor connecting cable or plug
  • Vitotronic control unit (only if the sensor and wiring are fine)

Related codes

Full explanation for F.669
F.670

Your boiler is reporting a problem with the sensor that checks the temperature of the stored hot water in your solar set-up, so the solar backup isn't working, though the boiler may still heat your home and water using gas.

Low Engineer only £90-180

Parts commonly replaced

  • Buffer cylinder temperature sensor (TS3)
  • Sensor cable or connector

Related codes

Full explanation for F.670
Showing 161–170 of 170
F.671

Your boiler's backup solar heating sensor is reporting a fault, which usually means the main heating and hot water can keep working while the backup system is switched off until it is checked.

Low Engineer only £90-180

What to check first

  1. Switch the boiler off at the control panel and at the isolator switch, wait 5 minutes, then switch it back on to see if the code clears.
  2. Check that the boiler still heats your home and provides hot water, which confirms the main system is working normally.
  3. If the code returns, leave the boiler as it is and book a Gas Safe registered engineer to test and replace the buffer cylinder sensor.

Parts commonly replaced

  • Buffer cylinder temperature sensor (TS3)
  • Sensor wiring lead or connector

Related codes

Full explanation for F.671
F.672

Your boiler has lost the signal from one of its solar heating temperature sensors, so it can no longer control the solar panel pump properly, but your main heating and hot water should still work.

Low Engineer only £90-180

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, wait 2 minutes, then switch it back on to see if the code clears.
  2. Check the display for any other fault codes or messages, and note them down for the engineer.
  3. If the code returns, leave the boiler on so it keeps heating and hot water, and book a Gas Safe registered engineer to check the solar sensor and its wiring.

Parts commonly replaced

  • Solar temperature sensor (TS3)
  • Sensor cable or plug connection

Related codes

Full explanation for F.672
F.673

Your boiler has noticed a fault in the sensor that monitors the solar water heating panels on your roof, so the solar part of your hot water system may not be working properly, though your main heating and hot water should still be running.

Low Engineer only £120-250

What to check first

  1. Note the exact code shown on the boiler display and check whether your heating and hot water are still working normally.
  2. Check that your boiler is still under pressure at the recommended level on its display or gauge, and note the reading.
  3. Turn the boiler off at its power switch for about a minute, then turn it back on and see whether the code clears or returns.

Parts commonly replaced

  • Cylinder temperature sensor (TS3)
  • Sensor cable or plug connector
  • Solar transfer pump
  • Solar control module

Related codes

Full explanation for F.673
F.682

The boiler has stopped itself from firing up because its burner check system isn't reading correctly, so it needs an engineer to look at it.

High Engineer only £150-400

What to check first

  1. Check the pressure gauge on the front of the boiler. If it reads below 1 bar, top it up using the filling loop as shown in the manual, then stop once it reaches 1.5 bar.
  2. Make sure the boiler has power and that the flue outlet and any air vents outside are not blocked by leaves, snow or debris.
  3. Press the reset button once, wait 10 minutes, and see if the boiler restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Air mass flow rate sensor
  • Combustion fan assembly
  • Burner control unit

Related codes

Full explanation for F.682
F.683

Your boiler has shut itself down because a sensor that measures the air flowing into the burner isn't giving a reliable reading, so it can't safely switch on.

High Engineer only £180-380

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on to try a reset.
  2. Check that the flue terminal outside the property is not blocked by leaves, nesting material, snow or debris, and clear any blockage you can safely reach.
  3. Check the boiler's pressure gauge is between 1.0 and 1.5 bar when cold, and top up using the filling loop only if your boiler manual says how to do so.
  4. If the boiler still shows F.683 after a reset, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Air mass flow rate sensor
  • Gas and air ratio control unit
  • Burner fan
  • Air intake or flue seals

Related codes

Full explanation for F.683
F.684

Your boiler has switched itself off as a safety precaution because a part that makes sure exhaust fumes leave safely through the flue is not working properly, so it needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £150-350

Parts commonly replaced

  • Back draught safety device
  • Flue pipe and seals
  • Flue terminal

Related codes

Full explanation for F.684
I.55

The boiler's hot water has reached its maximum set temperature, so it has paused heating the water until it cools a little, which is usually normal and not a sign of danger.

Low DIY-safe £80-180

What to check first

  1. Check the hot water temperature setting on the boiler's control panel and make sure it is not set higher than you need, typically around 60°C.
  2. Turn off the boiler at its on/off switch, wait about 30 seconds, then switch it back on to clear the message.
  3. Run a hot tap for a minute to see whether the message clears as the cylinder water is used. If it returns repeatedly, or hot water is not reaching the taps, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Diverter valve motor
  • Control PCB

Related codes

Full explanation for I.55
I.56

Your boiler is reporting that a separate heating control, such as a thermostat, timer or hot water cylinder sensor, is currently asking it to switch on, which is usually normal and not a sign of damage.

Low DIY-safe £0-150

What to check first

  1. Check your room thermostat and programmer or timer to confirm they are set as you expect, and that the heating is not scheduled to run when you do not want it to.
  2. Look at any hot water cylinder thermostat and check the cylinder is not calling for heat when it is already warm.
  3. Make sure any zone valves or external controls are not stuck open or on, then wait a few minutes and see whether the code clears by itself.
  4. If the code stays and the heating runs constantly, switch the boiler off at its isolator, wait 10 seconds, switch it back on, and check again. If it persists, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Room thermostat
  • Cylinder thermostat
  • Programmer or time clock
  • Zone valve actuator
  • External control wiring

Related codes

Full explanation for I.56
I.57

Your boiler has been told by an external control, such as a timer, smart controller or energy supplier switch, to hold off heating, so it is pausing rather than faulting.

High DIY-safe £80-150

What to check first

  1. Check your room thermostat, programmer and any smart heating app to see whether heating has been switched off or set to a holiday or boost mode.
  2. Look for any external controller, timer, load-control device or smart plug connected to the heating system and check whether it is currently active.
  3. Once the blocking source is turned off or its schedule has ended, wait a few minutes; if the code stays, switch the boiler off at its on/off switch, wait 30 seconds, and switch it back on. If the code returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • External blocking wiring or terminal connection
  • Time switch or external control unit
  • Boiler control PCB (rare)

Related codes

Full explanation for I.57
I.59

Your boiler has reloaded its saved settings, which is usually harmless, but you may need to check the clock, date and heating schedule to make sure everything is set the way you want.

Low DIY-safe £0-150

What to check first

  1. Check the boiler display for any other fault codes, and note them down if there are any.
  2. Check the time, date and heating and hot water schedules on the boiler or thermostat, and set them back to how you like them.
  3. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on and see whether the code clears.
  4. If the code returns or the boiler stops heating or producing hot water, contact a Gas Safe registered engineer.

Parts commonly replaced

  • BCU electronics module (only if the fault persists after an engineer's diagnosis)

Related codes

Full explanation for I.59
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