Viessmann Vitodens 100-W B1HF-25 Fault codes & diagnostics

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

System Natural GasLPG 2021-present
GC Number 41-819-49
Download the Viessmann Vitodens 100-W B1HF-25 manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
11 Emergency 101 High 15 Medium 24 Low
Just want the most common Viessmann Vitodens 100-W B1HF-25 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 151 documented codes

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7

Your boiler has lost contact with the sensor that measures the temperature of your hot water tank, so it can't properly control your hot water, though your heating should still work.

Medium Engineer only £100-200

What to check first

  1. Switch the boiler off at its on/off switch and wait about 30 seconds, then switch it back on to see if the fault clears.
  2. Check the sensor lead is firmly plugged in at the boiler and at the cylinder sensor, without pulling or forcing any connection.
  3. If the fault returns, leave the boiler off or on heating-only mode and book a Gas Safe registered engineer to test and replace the sensor.

Parts commonly replaced

  • Cylinder temperature sensor (DHW sensor)
  • Sensor cable or lead harness

Related codes

Full explanation for 7
8

The boiler's sensor that measures the temperature of the hot water tank has stopped sending a reliable reading, so the boiler may not heat the stored water properly, though it is not a safety danger.

Low Engineer only £110-200

What to check first

  1. Turn off the boiler at its power switch and at the fused spur or isolator, then wait a few minutes.
  2. Check the cable running from the cylinder sensor to the boiler for any visible damage, pinched wires, or loose plugs, and reseat any connections that look loose.
  3. Restore power, switch the boiler back on, and see if the fault clears. If it returns or the boiler still shows code 8, call a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor connection cable or wiring harness

Related codes

Full explanation for 8
11

Your boiler has lost contact with a small sensor that reads the temperature of the solar panels, so it can't use them, but your heating and hot water should still work normally.

Low Engineer only £120-220

Parts commonly replaced

  • Collector temperature sensor (solar)
  • Sensor cable or plug connection
  • Solar controller board (rarely needed)

Related codes

Full explanation for 11
12

The boiler has detected a problem with a small sensor that measures temperature on a solar panel system, so it may not be able to read it properly, but your heating and hot water will usually keep working.

Low Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait about 5 minutes, then switch it back on to see if the code clears.
  2. Check whether your home has a solar thermal (solar hot water) system, as this code only relates to that sensor.
  3. If the code returns, leave the boiler casing closed and contact a Gas Safe registered engineer or a Viessmann-approved installer.

Parts commonly replaced

  • Solar collector temperature sensor (Pt1000)
  • Sensor cable or wiring harness
  • Control PCB (rarely required)

Related codes

Full explanation for 12
13

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

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its control and wait a minute, then turn it back on to see if the fault clears.
  2. If the code returns, do not keep resetting it. Book a Gas Safe registered engineer to check the outside sensor and its cable.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 13
14

The boiler has lost track of the outdoor temperature reading it uses to adjust heating, so it may not be heating as efficiently as it should, but your heating and hot water will usually still work.

Low Engineer only £80-180

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 14
15

The boiler has lost contact with a temperature sensor on the bottom of the solar hot water cylinder, so it can no longer read the water temperature from that part of the tank, but your heating and hot water will usually still work.

Low Engineer only £90-180

What to check first

  1. Note the exact code shown on the boiler display and check whether any heating or hot water problems are present.
  2. Switch the boiler off at its on/off switch, wait about 30 seconds, then switch it back on to see whether the code clears.
  3. If it returns, do not open the boiler casing or the cylinder. Contact a Gas Safe registered engineer and tell them the code is 15 for the solar cylinder bottom sensor.

Parts commonly replaced

  • Solar cylinder bottom temperature sensor (PT1000 type)
  • Sensor cable or connector lead

Related codes

Full explanation for 15
16

The boiler has detected a fault with a sensor that measures the temperature at the bottom of your solar hot water tank, which usually means the solar part of your hot water system isn't working properly, but your main heating and hot water should still be running.

Low Engineer only £100-200

What to check first

  1. Turn off the boiler at its on/off switch, wait 30 seconds, then turn it back on to clear the fault code and see if it returns.
  2. Check that the sensor cable and plug connections at the solar cylinder and the controller are firmly seated and show no visible damage, such as cuts, crushed wires or scorch marks.
  3. If the code returns after a restart, contact a Gas Safe registered engineer to test and replace the sensor if needed.

Parts commonly replaced

  • Solar cylinder temperature sensor (bottom)
  • Sensor cable or plug connector

Related codes

Full explanation for 16
29

The boiler's temperature sensor on the heating circuit's main pipe has lost its connection or is giving unreliable readings, so the boiler has paused to protect itself and needs a qualified engineer to check it.

Low Engineer only £120-250

What to check first

  1. Check the system pressure gauge reads between 1.0 and 1.5 bar, and top up with the filling loop only if it is low and you know how.
  2. Switch the boiler off at the isolator, wait 30 seconds, then switch it back on to see if the fault clears.
  3. If the code returns, note the exact fault code and the boiler's serial number, then book a Gas Safe registered engineer.

Parts commonly replaced

  • Low loss header temperature sensor
  • Sensor wiring harness or plug
  • Low loss header sensor pocket

Related codes

Full explanation for 29
30

The boiler has detected a faulty temperature reading from a small sensor that helps it control the heating water, so it may stop or limit heating until an engineer checks it, but this is usually a fixable issue and not an immediate danger.

Low Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then turn it back on to see if the fault clears.
  2. Check that the sensor's wiring plug is fully pushed in and that the cable is not damaged, pinched or exposed.
  3. If the code returns, stop and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Low loss header temperature sensor
  • Sensor wiring lead or plug
  • Control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for 30
Showing 11–20 of 151
49

The boiler has stopped because it can't read the temperature of the gases leaving through the flue, which it needs to check for safe operation, so a qualified engineer should look at it soon.

High Engineer only £120-250

What to check first

  1. Check the boiler pressure gauge and top it up to the level in the manual (usually between 1 and 1.5 bar) if it is low.
  2. Press the reset button once or use the reset option on the display, then wait a few minutes to see if the boiler restarts.
  3. Look outside at the flue terminal and make sure nothing is blocking it, such as leaves, debris or a bird's nest, without touching any internal parts.
  4. If the fault returns or the boiler still won't run, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Flue sensor connector

Related codes

Full explanation for 49
50

Your boiler has spotted a problem with the sensor that monitors the temperature of the hot gases leaving through the flue, so it has likely shut itself down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £120-220

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 if the fault clears.
  2. Check that the flue outlet on the outside wall is not blocked by leaves, nests, debris or snow, and clear any blockage you can safely reach.
  3. If the fault returns, stop resetting the boiler and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Main control PCB (if the sensor is fine but the board is faulty)

Related codes

Full explanation for 50
57

The boiler has lost contact with the thermometer that measures the temperature in your home, so it may not adjust heating as accurately, though it will often still heat your house and water.

Low DIY-safe £90-180

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 2 minutes.
  2. Check the cable from the room temperature sensor or room control unit for loose, damaged, or disconnected wires, and reconnect any loose ends at the sensor end only. Do not open the boiler casing.
  3. Restore power, press the reset button on the boiler or controller, and check whether code 57 clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or wiring harness

Related codes

Full explanation for 57
58

The boiler's room thermostat sensor appears to be sending a faulty signal, so the boiler may not control the room temperature as accurately as it should, but it is usually not dangerous and can often be fixed simply.

Low DIY-safe £120-220

What to check first

  1. Turn off the boiler at its on/off switch and wait 2 minutes, then turn it back on to clear the fault code.
  2. If you have a wireless or wired room thermostat (Vitotrol), replace its batteries and check that the cable connections at the thermostat are secure and not damaged.
  3. Check the cable from the room sensor for visible damage, such as cuts, pinches or exposed wires, and check that it is not running against a metal surface or in a wet area. If it is damaged, leave the repair to an engineer.
  4. If the code returns, do not open the boiler casing. Contact a Gas Safe registered engineer to test and replace the room temperature sensor.

Parts commonly replaced

  • Room temperature sensor
  • Vitotrol room thermostat or remote control unit
  • Sensor connection cable
  • Control unit (rarely needed)

Related codes

Full explanation for 58
59

Your boiler's internal fan is not starting, so the boiler cannot safely switch on to heat your home or water, and a qualified engineer should check it.

High Engineer only £180-420

What to check first

  1. Check that the boiler has power: look at the display, and confirm the boiler's on/off switch and any fused spur or isolator switch nearby are on.
  2. Check the water 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 only if you know how, otherwise leave it for the engineer.
  3. Switch the boiler off at the power supply for 30 seconds, then switch it back on and wait a few minutes to see whether the fault clears.
  4. If code 59 returns, stop and call a Gas Safe registered engineer. Do not open the casing or disconnect the fan yourself.

Parts commonly replaced

  • Fan assembly
  • Fan connecting cable or wiring harness
  • Fan power supply or control board
  • Gas valve (if fan fault has caused burner lockout damage)

Related codes

Full explanation for 59
62

Your boiler's automatic self-check on how it burns gas did not finish successfully, so it has paused as a safety precaution.

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 the heating system is circulating properly by confirming the pump is running and bleeding any cold radiators that are not heating up.
  3. Switch the boiler off at its power supply for a few minutes, switch it back on, and reset it once using the reset button or control panel.
  4. If the fault code returns, stop and call a Gas Safe registered engineer to recalibrate the combustion controller and check the fan, gas valve and flue.

Parts commonly replaced

  • Fan assembly
  • Gas valve
  • Ionisation electrode

Related codes

Full explanation for 62
63

Your boiler has shut itself down because the hot gases leaving through the flue are running hotter than it's designed to allow, which usually points to a blockage or a faulty safety sensor rather than a serious failure.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 minutes, then switch it back on and press the reset button on the front panel (or use the reset option on the display) to try to restart it.
  2. From ground level only, look at the flue terminal on the outside wall and check for anything blocking it, such as leaves, nesting material, debris or a build-up of soot. Do not climb ladders or touch the flue while it is hot.
  3. If the boiler restarts and runs normally, keep an eye on it, but book a Gas Safe engineer to check the flue and the flue gas temperature limiter, as the fault may return.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue terminal or flue pipe section
  • Heat exchanger (if overheated or blocked)

Related codes

Full explanation for 63
64

The boiler's burner started but then lost its flame and shut itself down as a safety precaution, so your heating and hot water may be off until it is checked.

High Engineer only £120-300

What to check first

  1. Wait about 10 minutes, then reset the boiler using the reset button or the reset option on the display, and check whether it fires up and stays on.
  2. Check that the gas supply is on, including the meter emergency control valve and any other gas appliances in the house, and check the boiler pressure is between 1.0 and 1.5 bar on the display.
  3. Check that the flue outlet outside the house is not blocked by debris, leaves, or nesting material, and that the condensate pipe is not frozen or blocked. If the fault returns after a reset, call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Fan
  • Control PCB

Related codes

Full explanation for 64
65

The boiler's burner tried to light but its safety sensor didn't detect a steady flame, so it has stopped to protect the home, which usually means a dirty or faulty sensor or a gas supply problem that a heating engineer can quickly check.

High Engineer only £80-280

What to check first

  1. If you smell gas at any point, open windows, avoid switches and flames, leave the property, and call the National Gas Emergency number on 0800 111 999 from outside.
  2. Check the boiler display for the code 65 and note any other codes shown, then press the reset button or use the reset option on the control panel once and wait 10 minutes.
  3. Confirm the gas supply is on by checking that the gas meter emergency control valve and the boiler's isolation valve (usually under the boiler) are open, and check whether a gas hob or other gas appliance still works.
  4. If the fault returns after one reset, stop trying to reset it and book a Gas Safe registered engineer, as repeated resets can lock the boiler out further.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ignition electrode
  • Gas valve
  • Ignition unit or control PCB
  • Burner gasket or seal

Related codes

Full explanation for 65
67

Your boiler has stopped because it cannot reliably detect its gas flame, which is a common safety check that usually needs a qualified engineer to clean or replace a small sensor.

High Engineer only £100-250

What to check first

  1. Wait 5 minutes, then press the reset button on the boiler or the reset option on its controls once.
  2. Check that the gas supply tap to the boiler is fully open and that other gas appliances in the home are working.
  3. Check that the boiler's flue outlet outside is not blocked by leaves, snow, or debris, then call a Gas Safe registered engineer if the fault returns.

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode gasket
  • Burner gasket

Related codes

Full explanation for 67
Showing 21–30 of 151
68

Your boiler has sensed a flame when it should not be burning, so it has shut itself down as a safety precaution and will need a gas engineer to check it before it is used again.

High Engineer only £150-400

What to check first

  1. Turn off the boiler at its on/off switch and do not keep pressing reset, as repeated attempts can make the fault harder to diagnose.
  2. Check that the boiler is not giving off any smell of gas, and if you notice one, leave the property, avoid switches, and call the National Gas Emergency line on 0800 111 999 from outside.
  3. Book a Gas Safe registered engineer, and tell them the fault code is 68 with a flame detected at burner start.

Parts commonly replaced

  • Gas valve (gas multiblock)
  • Ionisation electrode (flame sensor)
  • Ignition electrode
  • Control PCB

Related codes

Full explanation for 68
69

Your boiler has detected a flame signal that is weaker or stronger than it should be, so it has stopped as a safety precaution and will need a qualified engineer to check the burner.

High Engineer only £90-200

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.
  2. Make sure the boiler has mains power and the flue terminal outside is not blocked by leaves, snow or debris.
  3. Press the reset button once, wait 10 minutes and see whether the boiler restarts. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Electrode gasket

Related codes

Full explanation for 69
70

Your boiler's internal control unit has detected a fault and has likely stopped working to protect itself, so heating and hot water may be off until it is checked.

High Engineer only £250-500

What to check first

  1. Turn off the boiler's power supply at the fused spur or isolator switch and wait 5 minutes.
  2. Switch the power back on, then press the reset button on the boiler or in the control panel.
  3. Check the system pressure gauge is between 1 and 1.5 bar, and top up only if the manual instructions say it is safe to do so.
  4. If the code returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Main control PCB

Related codes

Full explanation for 70
71

Your boiler's fan isn't turning fast enough to safely burn gas, so it has paused itself as a precaution. This is usually fixable by a heating engineer, and a simple reset may get it running again for now.

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 isolator, then wait 10 minutes.
  2. Check that the outside flue terminal (the pipe exiting the wall) is not blocked by leaves, nests, snow or debris, and clear any blockage you can safely reach from the ground or a stable position.
  3. Turn the power back on and press the reset button on the boiler panel. If the fault code 71 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan control PCB (circuit board)
  • Fan speed sensor (Hall sensor)

Related codes

Full explanation for 71
72

Your boiler has stopped because its internal fan is not slowing down to the correct resting speed before it starts up, so it has shut itself down as a safety precaution.

High Engineer only £180-400

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 minutes, then turn it back on to try a reset.
  2. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is below 1 bar, follow the filling instructions in your manual.
  3. Make sure nothing is blocking the outside flue terminal (the pipe exiting the wall) and that nothing is piled against it.
  4. If the fault code returns after a reset, stop and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan power and signal cable
  • Main PCB (control board)
  • Gas valve assembly
  • Flue and combustion seals

Related codes

Full explanation for 72
73

Your boiler's internal parts have lost contact with each other, so it has stopped working as a precaution, but this is usually fixable and is not a sign of immediate danger.

High DIY-safe £100-400

What to check first

  1. Turn off the boiler using its on/off switch, then switch off the fused spur or isolator supplying it.
  2. Wait about 5 minutes, then switch the power back on and turn the boiler back on.
  3. If the fault clears and the boiler runs normally, keep an eye on it, but book a service so an engineer can check for a loose connection.
  4. If code 73 returns, stop resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB
  • Burner control unit
  • Bus communication cable or connector
  • Control panel or display unit

Related codes

Full explanation for 73
74

The water pressure in your heating system has dropped below the level the boiler needs to run safely, so it has stopped working until the pressure is topped up.

High DIY-safe £80-200

What to check first

  1. Check the pressure gauge on the front of the boiler with the system cold. A healthy reading is usually between 1.0 and 1.5 bar. If it is below about 1.0 bar, the boiler needs topping up.
  2. Locate the filling loop, which is usually a flexible silver braided hose with two small valves under or beside the boiler. Make sure both ends are securely connected.
  3. Open the filling loop valves slowly, one at a time, and let water flow in while watching the gauge. Close the valves once the needle reaches about 1.2 to 1.5 bar.
  4. Check around the boiler, pipework and radiator valves for drips, then close the filling loop fully and disconnect the hose if your model requires it.
  5. Restart the boiler using the reset button or menu as described in your manual. If the pressure drops again within a few days, or the code returns, call a Gas Safe registered engineer to find the leak.

Parts commonly replaced

  • Expansion vessel
  • Pressure relief valve
  • Filling loop valve
  • Pressure sensor

Related codes

Full explanation for 74
75

The boiler has detected that water isn't circulating through the heating system as it should, so it has stopped to protect itself, and it usually needs a quick check of the water pressure and pump.

High DIY-safe £90-350

What to check first

  1. Look at the pressure gauge on the front of the boiler with the system cold. It should read between 1 and 1.5 bar. If it is below 1 bar, top it up using the filling loop, following the boiler manual, then close the filling loop valve.
  2. Make sure the heating isolation valves under the boiler and the radiator valves are fully open, and check that any thermostat or programmer is calling for heat.
  3. Bleed the radiators, starting with the lowest one, to release any trapped air, then top the pressure back up if it has dropped.
  4. Press the reset button on the boiler once. If the code returns within a short time, stop and call a Gas Safe registered engineer, as the pump or a blocked filter may need attention.

Parts commonly replaced

  • Circulation pump
  • System magnetic filter or strainer
  • Flow sensor or flow switch
  • Expansion vessel

Related codes

Full explanation for 75
77

Your boiler's main control memory has lost or corrupted some of its stored settings, so it has stopped to protect itself until a qualified engineer checks it.

High Engineer only £200-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait 5 minutes.
  2. Switch the power back on and check whether the display clears the code and the boiler restarts.
  3. If the code returns or the boiler stays off, do not keep resetting it. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Coding card
  • Control unit PCB

Related codes

Full explanation for 77
78

The boiler's control panel and its main control unit have lost contact with each other, so the boiler may not start or run properly until the connection is restored.

High Engineer only £120-350

What to check first

  1. Switch off the boiler at its power isolator or fused spur and wait about 30 seconds.
  2. Check that the cable linking the control panel to the boiler is firmly plugged in at both ends and has no visible damage.
  3. Restore power, wait a couple of minutes, and check whether the fault clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Programming unit (control panel)
  • Connecting data cable between control panel and boiler
  • Heat management unit circuit board
Full explanation for 78
Showing 31–40 of 151
87

The boiler's water pressure has risen above its safe limit, so it has stopped to protect itself and needs checking by a heating engineer.

Emergency Engineer only £150-350

What to check first

  1. Switch the boiler off at its on/off switch and let it cool down completely.
  2. Do not open the pressure relief valve or try to drain water from the system yourself.
  3. Check the pressure gauge on the front of the boiler once it has cooled, and note the reading for the engineer.
  4. Call a Gas Safe registered engineer to inspect the expansion vessel, pressure relief valve and filling loop.

Parts commonly replaced

  • Expansion vessel
  • Pressure relief valve
  • Filling loop
  • Pressure gauge

Related codes

Full explanation for 87
89

Your boiler's internal pump that moves hot water around your heating system appears to be blocked, so the heating may not be working properly and a qualified engineer should look at it soon.

High Engineer only £250-400

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 as your boiler manual describes.
  2. Switch the boiler off at the isolator or fused spur, wait 30 seconds, then switch it back on to see if the fault clears. Do not repeat this more than once or twice.
  3. Bleed your radiators with a radiator key to release any trapped air, which can sometimes cause pump problems, then check the pressure again.

Parts commonly replaced

  • Internal circulation pump
  • Pump cartridge or seal kit
  • Magnetic filter or system cleaner (if sludge is present)

Related codes

Full explanation for 89
91

Your boiler's control system has lost contact with one of its internal add-on modules, so it has stopped working and needs a qualified engineer to check the connection or replace the faulty part.

High Engineer only £150-400

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait 5 minutes.
  2. Switch the power back on and let the boiler restart, then check whether the fault code clears.
  3. If the code returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing or disturb any wiring.

Parts commonly replaced

  • DIO (digital input/output) extension module
  • Vitotronic control unit
  • BUS communication cable

Related codes

Full explanation for 91
92

Your boiler's control system has lost its connection to one of its internal add-on modules, so it has paused itself as a precaution, and a qualified engineer can usually restore it.

High DIY-safe £150-350

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or circuit breaker, then wait at least 5 minutes.
  2. Switch the power back on and wait a few minutes for the boiler to start up and run its self-check.
  3. If the display still shows code 92, press and hold the reset button on the boiler control panel (or use the reset option in the menu) to clear the fault, then check whether heating and hot water return.
  4. If the fault returns, stop and book a Gas Safe registered engineer. Do not open the boiler casing or disconnect internal wiring yourself.

Parts commonly replaced

  • ADIO electronics module
  • Control unit PCB
  • Connecting cable or wiring harness

Related codes

Full explanation for 92
95

Your boiler has lost its connection to the room thermostat or controller, so it may not be heating your home according to your settings, although it is likely still safe to run.

Medium DIY-safe £100-250

What to check first

  1. Switch the boiler off at its on/off switch and at the mains supply, then check the room thermostat or controller has working batteries or power and that its display is lit.
  2. With the boiler off, check the two-wire connection between the room thermostat and the boiler's control terminals is firmly in place and not loose, damaged or disconnected. Do not touch any gas components or open the boiler casing beyond the terminal cover.
  3. Restore power, wait a few minutes, then reset the boiler using its reset button or the reset option on the controller. If the code clears and heating responds to your settings, no further action is needed. If the code returns, book a Gas Safe registered engineer.

Parts commonly replaced

  • Viessmann OpenTherm room thermostat or Vitotrol controller
  • OpenTherm connection cable
  • Boiler control unit circuit board (uncommon)

Related codes

Full explanation for 95
100

Your boiler's internal communication link is reporting a power problem, so it may not be controlling heating and hot water correctly until it's checked by a qualified engineer.

High Engineer only £90-250

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 2 minutes.
  2. Turn the power back on and press the reset button on the boiler panel (if it shows one) to see whether the code clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the fault usually needs internal electrical checks.

Parts commonly replaced

  • PlusBus connection cable or plug
  • Control unit (main PCB)
  • Power supply unit

Related codes

Full explanation for 100
102

Your boiler's control system has lost reliable contact with one of its communication parts, so it may not respond properly to your thermostat or app, but this is not a gas safety issue.

Medium DIY-safe £120-260

What to check first

  1. Switch the boiler off at its on/off switch and at the household isolator or fuse for at least 60 seconds, then switch it back on to reset the control system.
  2. Check that your room thermostat, Vitotrol or internet gateway (if fitted) has fresh batteries or power, and that any wireless link shows as connected.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer to check the communication module and its wiring.

Parts commonly replaced

  • Communication module
  • Control unit connection cable
  • Vitotrol or LON communication board

Related codes

Full explanation for 102
103

Your boiler's main control panel has lost reliable contact with its internal programming system, so it may switch off or refuse to heat until a engineer checks it.

High Engineer only £250-450

What to check first

  1. Turn off the boiler at its isolator switch or fused spur, and wait about 10 minutes.
  2. Switch the boiler back on and check whether the code clears or the boiler restarts normally.
  3. If the code returns, stop and call a Gas Safe registered engineer, as the fault is inside the control unit and needs professional diagnosis.

Parts commonly replaced

  • Programming unit (control unit)
  • Control PCB
  • Display panel

Related codes

Full explanation for 103
104

Your boiler has received a stop signal from an outside device or control connected to it, so it may have shut itself down until that signal is cleared.

High Engineer only £80-200

What to check first

  1. Check whether any external device is wired to the boiler, such as a fire alarm, emergency switch, or building control system, and whether one has been triggered or switched off.
  2. If nothing external is active and you have no such device, switch the boiler off at its isolator switch and on again, then wait a few minutes to see if it restarts.
  3. If the code returns, stop and call a Gas Safe registered engineer, as the cause may be in the boiler's wiring or control board.

Parts commonly replaced

  • External fault input wiring or terminal connection
  • Control PCB (main circuit board)
  • External switch or connected control device

Related codes

Full explanation for 104
142

The boiler's internal parts are having trouble talking to each other, which can interrupt heating or hot water, but this is usually fixable by a heating engineer.

High Engineer only £120-350

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait 5 minutes.
  2. Switch the power back on and let the boiler restart fully. Check whether the fault returns.
  3. If it returns, check that any external room thermostat or controller cable is pushed firmly in and not visibly damaged, then book a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic PCB)
  • CAN bus connection cable
  • Extension or communication module
  • Remote control unit

Related codes

Full explanation for 142
Showing 41–50 of 151
161

Your boiler's main control memory has hit a glitch, so it may have stopped heating or hot water until a qualified engineer checks it, though a simple reset is worth trying first.

High Engineer only £200-400

What to check first

  1. Check the boiler pressure gauge and top it up to between 1 and 1.5 bar if it has dropped, following the manual.
  2. Turn the boiler off at the isolator or power switch, wait 30 seconds, then switch it back on.
  3. Press the reset button on the front of the boiler once, if the display shows a reset prompt, and check whether the fault clears.

Parts commonly replaced

  • HBMU heat management unit control board
  • Main PCB (printed circuit board)
  • Boiler control module

Related codes

Full explanation for 161
162

Your boiler has detected that its electrical power supply is running too low, so it may shut itself down to protect its internal parts until the power problem is checked.

High Engineer only £150-400

What to check first

  1. Check that the boiler's power switch and any fused spur are switched on and that the boiler display is lit.
  2. Switch the boiler off at its power supply, wait 60 seconds, then switch it back on to see if the code clears.
  3. Check whether the fault returns when high-power appliances such as kettles, ovens or washing machines are running, which can point to a supply problem.
  4. If the code returns or the boiler stays off, call a Gas Safe registered engineer and mention fault code 162 and the undervoltage warning.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Power supply unit
  • Main control PCB
  • Wiring harness or connectors

Related codes

Full explanation for 162
163

Your boiler's internal memory check has failed, which usually means its control system has stopped working correctly and it may have shut down, so a restart is worth trying before calling an engineer.

High Engineer only £250-450

What to check first

  1. Switch 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 wait a few minutes for the boiler to start up and run its self-check.
  3. If the code 163 returns or the boiler stays locked out, stop and contact a Gas Safe registered engineer, as the control unit may need replacing.

Parts commonly replaced

  • HBMU heat management unit (control board)
  • Memory module on the control board
  • Boiler control PCB

Related codes

Full explanation for 163
176

The boiler has stopped because the water it produces when it burns gas is not draining away properly, so it has backed up and shut itself down to stay safe.

High DIY-safe £80-180

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait about 10 minutes for it to cool.
  2. Find the white plastic condensate pipe running from the boiler to an outside drain, and check it for ice, leaves, debris or a blockage. Gently clear any blockage you can reach and pour a kettle of lukewarm water through the pipe to test the flow.
  3. If the boiler has a condensate trap, place a bucket underneath, then carefully unscrew the trap cover and remove any sludge or debris. Refill the trap with water until it overflows slightly, then refit the cover.
  4. Restore power, switch the boiler back on and reset it using the reset button or the method in the user manual. If the code returns, or you see water leaking from the boiler, switch it off and call a Gas Safe registered engineer.

Parts commonly replaced

  • Condensate trap
  • Condensate drain pipe section
  • Condensate pump

Related codes

Full explanation for 176
182

Your boiler has detected an electrical fault with one of its temperature sensors, so it has likely stopped heating and may need a qualified engineer to replace the faulty part.

High Engineer only £120-250

What to check first

  1. Check the boiler display for any other fault codes and note them down for the engineer.
  2. Confirm the boiler has power and the pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's filling instructions are followed.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts; if the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet temperature sensor
  • Flow temperature sensor
  • Sensor wiring harness
  • Control PCB (if the fault persists after sensor replacement)

Related codes

Full explanation for 182
183

The boiler has lost the signal from a small sensor that tells it how hot the water leaving the boiler is, so it has stopped heating your home and water until that sensor is checked and reconnected or replaced.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait about 10 minutes before switching it back on.
  2. Check the boiler display after restarting. If code 183 returns, note it down and do not open the casing.
  3. Call a Gas Safe registered engineer and ask them to check the outlet temperature sensor and its wiring connection.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness or plug connector
  • Control board connection (if the wiring is damaged)

Related codes

Full explanation for 183
184

The boiler has shut itself down because it has detected a fault with the sensor that measures how hot the water leaving it is, so it has stopped heating your home and water until a qualified engineer checks it.

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 about 10 minutes.
  2. Check the pressure gauge with the boiler cold. It should read between 1 and 1.5 bar; if it is low, top it up using the filling loop as shown in your manual, then close the loop.
  3. Switch the power back on and press the reset button once. If the fault returns or the boiler locks out again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor
  • High limit safety cut-out (STB)
  • Sensor wiring harness or connector
  • Control PCB (occasionally)

Related codes

Full explanation for 184
185

Your boiler has shut itself down as a safety precaution because it has detected a problem with how it reads the water temperature leaving the boiler, so your heating and hot water are likely off until it is checked.

High Engineer only £120-260

What to check first

  1. Check the boiler pressure gauge. 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 user manual, then close the loop valves.
  2. Check that the boiler and heating system are not leaking, and that the radiators and pipework are not visibly cold on the flow side.
  3. Press the reset button on the boiler control panel once. If the fault returns within a short time, or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • High limit safety thermostat (STB)
  • Sensor wiring harness or connector

Related codes

Full explanation for 185
299

The boiler's internal clock has lost or confused its time and date, which usually affects timed heating schedules rather than the heating or hot water themselves.

Low DIY-safe £90-250

What to check first

  1. Press the reset button on the boiler control panel or switch the boiler off at its power supply for a minute, then switch it back on.
  2. Use the control dial and menu on the boiler to check and set the correct time and date.
  3. Check that your heating and hot water timer settings are still correct, then watch to see if the code clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Clock backup battery

Related codes

Full explanation for 299
345

The boiler has shut itself down to protect itself because the water inside got too hot, so it is not currently producing heat or hot water until it is checked and reset.

High Engineer only £150-350

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 as shown in the manual, then close the loop.
  2. Make sure all radiator and heating valves are open, and check that the boiler's flow isn't blocked by closed thermostatic valves or a closed isolation valve.
  3. Wait at least 15 minutes for the boiler to cool, then press the reset button once. If the code returns quickly, stop and call a Gas Safe registered engineer rather than resetting repeatedly.

Parts commonly replaced

  • Safety temperature limiter (STB)
  • Heating circulation pump
  • Flow temperature sensor
  • Heat exchanger (if overheating has caused damage)

Related codes

Full explanation for 345
Showing 51–60 of 151
346

Your boiler has run into a problem while setting up how it burns gas, so it has stopped to protect itself, and a qualified engineer should check it to get it running safely again.

High Engineer only £120-250

What to check first

  1. Check that the boiler's pressure gauge reads between 1 and 1.5 bar when cold, and top it up only if the manufacturer's filling instructions are followed.
  2. Make sure nothing is blocking the outside flue terminal, such as leaves, nests, or snow, and that the air vents around it are clear.
  3. Press the reset button on the boiler once, wait 10 minutes, and see whether it restarts. Do not repeat resets, as this can mask a fault that needs repair.

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze assembly gasket
  • Combustion chamber seal
  • Flue gas recirculation pipe seal

Related codes

Full explanation for 346
347

Your boiler has detected a problem with how it moves exhaust gases out through the flue, so it has paused to stay safe until it can be checked.

High Engineer only £180-450

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. Look at the outside flue terminal and remove any visible obstructions such as leaves, debris, or nesting material, without placing yourself at risk.
  3. Reset the boiler once using the reset button. If the code returns, stop and call a Gas Safe registered engineer rather than resetting repeatedly.

Parts commonly replaced

  • Combustion fan
  • Flue gas recirculation valve or pipe
  • Flue and combustion seals
  • Gaskets

Related codes

Full explanation for 347
348

Your boiler has detected a problem with the part that controls how much gas reaches the burner, so it has likely stopped to stay safe and may not be providing heating or hot water until a qualified engineer checks it.

High Engineer only £250-450

What to check first

  1. Check the boiler pressure gauge and top it 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 on the front of the boiler once and wait 5 minutes to see if it restarts.
  3. If the fault code returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas modulation valve
  • Gas valve coil
  • Gas valve diaphragm or seal kit

Related codes

Full explanation for 348
349

Your boiler has stopped because it can't correctly measure the airflow it needs to burn gas safely, so a qualified engineer should check it soon.

High Engineer only £180-400

What to check first

  1. Switch the boiler off at its on/off switch and at the wall or fused spur, wait about 10 minutes, then switch it back on.
  2. If it shows code 349 again, check the flue terminal outside the property from ground level (do not climb ladders) for visible blockages such as nests, leaves, or ice.
  3. Do not attempt to open the boiler casing or clean the fan. If the fault returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan unit
  • Air flow or pressure sensor
  • Fan gasket or seal
  • Air supply or flue connection hose

Related codes

Full explanation for 349
350

The boiler's flame sensor is not reading the flame correctly, so it has shut itself down as a safety precaution, and a Gas Safe engineer should check it.

High Engineer only £120-250

What to check first

  1. Check that the gas supply is on and that the gas meter's emergency control valve is fully open.
  2. Check the boiler's water pressure on the display and top it up if it is below 1 bar, following the manufacturer's instructions.
  3. Reset the boiler once using the reset button or menu, then wait 10 minutes to see whether it restarts.
  4. If the code returns, or you smell gas at any point, turn the boiler off, open windows, leave the property, and contact Gas Safe or the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

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

Related codes

Full explanation for 350
351

The boiler has shut itself down because it cannot reliably detect its flame, which is usually a dirty or loose sensor rather than a serious failure, but it will need a qualified engineer to get your heating and hot water working again.

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 and press the reset button once.
  2. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top it up using the filling loop if it is low.
  3. Check that other gas appliances, such as the hob, are working, and confirm the gas supply meter or emergency control valve is fully open.

Parts commonly replaced

  • Ionisation electrode (flame sensor)
  • Electrode gasket
  • Ignition electrode
  • Gas valve (occasionally)
  • Main PCB (rarely)

Related codes

Full explanation for 351
352

The boiler has detected a higher-than-safe level of carbon monoxide in its exhaust fumes, so it has shut itself down and should not be used again until a Gas Safe registered engineer has checked it.

Emergency Engineer only £150-400

Parts commonly replaced

  • Burner assembly
  • Heat exchanger
  • Combustion fan
  • Flue seals and gaskets
  • Ignition electrode

Related codes

Full explanation for 352
353

Your boiler is not getting enough gas to run at full strength, so it is working at reduced power, which is usually fixable but should be checked by a Gas Safe engineer soon.

Medium Engineer only £90-200

What to check first

  1. Check whether other gas appliances, such as a cooker or gas fire, are also working normally.
  2. Look at your gas meter and make sure the emergency control valve and any meter isolation valve are fully open.
  3. Ask your gas supplier or check online whether there is a local gas supply problem in your area.
  4. Turn the boiler off at the switch, wait about 30 seconds, then turn it back on to see if the fault clears.
  5. If the fault returns, or if you smell gas at any point, turn off the gas at the meter and call the National Gas Emergency number (0800 111 999) straight away.

Parts commonly replaced

  • Gas valve
  • Gas pressure switch
  • Gas filter
  • Gas regulator

Related codes

Full explanation for 353
354

Your boiler has detected that its gas control part is not adjusting the gas flow within its normal range, so it has likely stopped to stay safe until a qualified engineer checks it.

High Engineer only £250-450

What to check first

  1. Check that the gas supply to the boiler is turned on at the meter or isolation valve, and that other gas appliances in the home are working.
  2. Check the boiler's pressure gauge and make sure it sits within the range shown in the manual, without opening any panels.
  3. Press the reset button once on the boiler control panel, wait 5 minutes, and see if it restarts. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas modulation valve unit
  • Gas valve sealing gasket or O-ring
  • Combustion fan

Related codes

Full explanation for 354
355

Your boiler has stopped because it has detected a problem with its condensate drain or its flame detection, and it has shut down as a safety precaution.

High Engineer only £90-250

What to check first

  1. Turn off the boiler at the isolator switch and wait 10 minutes.
  2. Check the condensate pipe running from the boiler to the outside drain for ice, blockages or kinks, and clear any you find.
  3. Locate the condensate trap, usually a small container beneath the boiler, and check it for sludge or debris. Only clear it if you can do so without touching gas components.
  4. Restore power, press the reset button once, and see whether the boiler restarts and stays on.
  5. If the fault returns, or the boiler does not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Condensate trap
  • Condensate pipe section
  • Flame sensor (ionisation electrode)
  • Ignition electrode
  • Ignition transformer

Related codes

Full explanation for 355
Showing 61–70 of 151
357

Your boiler has stopped because it isn't getting the gas it needs to start up, so it can't heat your home or water until a qualified engineer checks the gas supply.

High Engineer only £150-400

What to check first

  1. Check whether other gas appliances, such as your hob or gas fire, also aren't working. If they aren't, the problem may be with the gas supply rather than the boiler.
  2. Check your energy supplier's website or app, or call them, to see whether there is a local gas supply problem in your area.
  3. Make sure the emergency control valve on the gas pipe at the boiler is fully open, meaning the lever is in line with the pipe.
  4. Wait a few minutes, then press the reset button on the boiler once. Do not keep pressing it, as repeated resets will not fix a gas supply fault.
  5. If you smell gas at any point, leave the property immediately, do not use switches or flames, and call the National Gas Emergency Service on 0800 111 999 from outside.

Parts commonly replaced

  • Gas valve assembly (gas solenoid valve)
  • Gas valve coil
  • Gas pressure switch

Related codes

Full explanation for 357
359

Your boiler is trying to light but cannot get the flame started, so it has stopped and is not providing heating or hot water until a qualified engineer checks the ignition and gas supply.

High Engineer only £150-350

What to check first

  1. Check that the gas supply to the property is on, including the gas meter emergency control valve, and that any prepayment meter has credit.
  2. Check whether other gas appliances, such as a gas hob, are working. If none are, contact your gas supplier, as the problem may be outside the boiler.
  3. Wait about 10 minutes, then press the reset button once on the boiler. If the fault returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • Burner control unit (HBMU)
  • Gas valve
Full explanation for 359
361

Your boiler started its burner to heat your home but could not detect the gas flame lighting, so it has paused itself as a safety precaution and will usually need a reset and a check by a qualified engineer.

High Engineer only £120-260

What to check first

  1. Check that your gas supply is on: make sure the gas meter emergency control valve and the boiler's own gas isolation valve (usually a lever under the boiler) are open.
  2. Check the boiler pressure on the display or gauge. If it is below about 1 bar, follow the filling instructions in your user manual to top it up, then wait a few minutes.
  3. Reset the boiler once using the reset button or by pressing the reset symbol on the display, then wait 10 minutes to see if it restarts.
  4. If the code returns, or the boiler will not stay on, stop trying and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Ignition lead
  • Burner gasket
  • Gas valve (less common)

Related codes

Full explanation for 361
364

Your boiler has reported a general system problem, so it may be shut down or not working properly until a qualified engineer checks it.

High Engineer only £90-200

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 system is cold.
  2. If the pressure is low, top it up using the filling loop as shown in the manual, then close the loop valves fully.
  3. Press the reset button on the boiler panel once, wait a few minutes, and see if the fault clears.
  4. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump
  • Filling loop valve
  • Pressure sensor
  • Control unit PCB

Related codes

Full explanation for 364
365

Your boiler has shut itself down because a safety component that controls the gas supply is not responding correctly, so turn the boiler off at the switch and call a Gas Safe registered engineer before using it again.

Emergency Engineer only £250-500

Parts commonly replaced

  • Gas valve
  • Main PCB (gas valve control board)
  • Gas valve connection cable

Related codes

Full explanation for 365
366

Your boiler's safety system has detected that the gas control valve may not have shut off properly, so it has stopped working to keep your home safe, and a Gas Safe registered engineer needs to check it promptly.

Emergency Engineer only £300-550

Parts commonly replaced

  • Gas valve assembly
  • Gas valve control module or relay
  • Main PCB (circuit board)
  • Gas valve wiring harness

Related codes

Full explanation for 366
367

Your boiler's gas valve is not switching off when it should, which is a safety concern, so please turn the boiler off and call a Gas Safe registered engineer straight away.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Gas valve electrical connector
  • Main control PCB

Related codes

Full explanation for 367
368

Your boiler has shut itself down as a safety precaution because it detected a problem with its gas-related checks, so it may not provide heating or hot water until a qualified engineer looks at it.

High Engineer only £120-300

What to check first

  1. Check that the gas supply to the boiler is turned on at the meter or isolation valve, and that other gas appliances in the home are working.
  2. Make sure the flue terminal outside the property and any air vents are not blocked by leaves, snow, debris or nesting material.
  3. Press the reset button on the boiler once, wait about 15 minutes, and see whether it restarts normally. If it faults again, stop and call a Gas Safe registered engineer.
  4. If you smell gas at any time, leave the property, avoid using switches or flames, and call the National Gas Emergency number on 0800 111 999 straight away.

Parts commonly replaced

  • Gas pressure switch
  • Combustion fan assembly
  • Gas valve
  • Flue or air intake seals

Related codes

Full explanation for 368
369

The boiler's flame sensor is not picking up a steady flame properly, so it has shut itself down as a safety precaution and will need a qualified engineer to check the burner and gas supply.

High Engineer only £120-260

What to check first

  1. Wait 10 minutes, then try resetting the boiler once using the reset button or control panel as shown in the manual.
  2. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, following the manual.
  3. Check that other gas appliances in the home work and that the gas meter supply has not been turned off, then call a Gas Safe registered engineer if the fault returns.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Burner gauze assembly gasket
  • Burner gasket or seal
  • Ignition electrode

Related codes

Full explanation for 369
370

The boiler's gas control valve is not shutting off properly, which is a safety concern, so the boiler should be turned off and a qualified Gas Safe engineer called straight away.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve assembly
  • Gas valve actuator or coil
  • Gas pressure switch
  • Main PCB

Related codes

Full explanation for 370
Showing 71–80 of 151
371

Your boiler's flue fan is turning more slowly than it should, so it has stopped to protect itself, and an engineer will need to find out why.

High Engineer only £220-420

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on and press the reset button on the front panel.
  2. Check that the outside flue terminal is clear of leaves, debris, nests or snow, and that nothing is blocking the air intake.
  3. Check that the condensate pipe running outside is not frozen or blocked, and that the condensate trap is not full of ice or debris, then restart the boiler once.

Parts commonly replaced

  • Fan assembly
  • Fan speed (Hall effect) sensor
  • Condensate trap
  • Gas valve (if fan fault has caused a related gas valve fault)

Related codes

Full explanation for 371
372

The boiler's flame keeps going out while it is setting itself up, so it has shut down as a safety precaution and needs a qualified engineer to find out why.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to try a reset.
  2. Check that the gas supply is on by confirming your gas meter emergency control valve and any household gas taps are fully open.
  3. Check that the boiler's flue outlet on the outside wall is not blocked by leaves, debris, or nesting material.
  4. If the boiler still shows code 372 after a reset, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Fan assembly
  • Gas pressure switch

Related codes

Full explanation for 372
373

Your boiler's combustion system failed its self-check, which usually means it is struggling to clear its heat or exhaust safely, so it has shut itself down to protect itself.

High Engineer only £120-300

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, following the filling instructions in your manual
  2. Make sure the boiler area and any vents are clear of obstructions and that the flue outlet outside is not blocked by leaves, snow or debris
  3. Press the reset button or switch the boiler off at the isolator for 30 seconds, then switch it back on and wait 10 minutes to see if it restarts
  4. If the fault returns, stop and call a Gas Safe registered engineer, as the combustion system needs professional inspection

Parts commonly replaced

  • Combustion fan
  • Gas valve
  • Flue gasket or seals
  • Flue terminal
  • Heat exchanger (if blocked or cracked)
  • Condensate trap

Related codes

Full explanation for 373
374

The boiler's burner check was briefly interrupted because the conditions needed to set it up were not quite steady, which is usually a temporary hiccup rather than a sign of serious damage.

Low DIY-safe £80-180

What to check first

  1. Press the reset button on the boiler (or use the reset option on the control panel) and wait about 10 minutes for it to restart and run its check again.
  2. Look at 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, following the manufacturer's guide, then close the loop.
  3. Make sure no other gas appliances, such as a cooker or a second boiler, are running at full power while the boiler is starting, and check that the boiler's air vents and flue outlet outside are not blocked by leaves, snow or debris.
  4. If the code returns or the boiler stays off after these steps, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Gas valve

Related codes

Full explanation for 374
375

Your boiler's flame sensor couldn't finish its routine self-check, so it may struggle to confirm the flame is lit properly, which is usually fixable with a simple service.

Medium Engineer only £100-250

What to check first

  1. Check the boiler's water pressure on the front gauge. It should sit between 1 and 1.5 bar when cold, and top it up using the filling loop only if it has dropped below 1 bar.
  2. Make sure the gas supply tap to the boiler is fully open and that other gas appliances are working normally.
  3. Press and hold the reset button for about one second to restart the boiler, then check whether the code clears and the heating and hot water return.
  4. If the code returns or the boiler stops working, turn it off at the isolator and book a Gas Safe registered engineer to clean or replace the flame sensor.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Burner gasket
  • Ignition and ionisation electrode lead

Related codes

Full explanation for 375
376

Your boiler has shut itself down for safety because its exhaust fan is not running at the correct speed, so it cannot switch on until a engineer checks it.

High Engineer only £200-400

What to check first

  1. Press the reset button on the boiler or controller once and wait 10 minutes to see if it restarts.
  2. Check that the outside flue terminal is not blocked by leaves, nests, or debris, and clear any obstruction you can reach safely from the ground.
  3. If the fault returns or the boiler stays locked out, turn the boiler off and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor
  • Flue gasket or seals
  • Control PCB

Related codes

Full explanation for 376
377

Your boiler has stopped because its automatic burner self-check did not complete correctly, which usually means the flame settings need a professional adjustment rather than anything being seriously broken.

High Engineer only £120-260

What to check first

  1. Switch the boiler off at the control panel and at the isolation switch, then wait 10 minutes.
  2. Check that the flue outlet on the outside wall is not blocked by leaves, debris, nesting material, or snow, and clear anything you can reach safely from the ground.
  3. Switch the boiler back on and press the reset button or use the reset function on the control panel once. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze gasket
  • Flue gas recirculation (FGR) gasket
  • Flue gas recirculation valve

Related codes

Full explanation for 377
378

The boiler is struggling to finish checking its flame sensor after a self-test, so it may pause or refuse to fire until the check completes successfully, and this is usually temporary.

Low DIY-safe £90-200

What to check first

  1. Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar if it is low, using the filling loop as shown in the manual.
  2. Switch the boiler off at its power supply or fused spur, wait 30 seconds, then switch it back on to restart the calibration.
  3. Make sure the gas supply tap is fully open and that no other gas appliance is causing a sudden pressure drop, then wait 10 minutes and see if the boiler runs normally.
  4. If the code returns, stop and call a Gas Safe registered engineer, as the flame sensor may need cleaning or replacing.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Burner gasket
  • Flame sensor lead

Related codes

Full explanation for 378
379

The boiler's burner switched off unexpectedly, so it has stopped working until a qualified engineer checks why the flame went out.

High Engineer only £90-250

What to check first

  1. Check that other gas appliances, such as the hob, are working, to confirm the gas supply is on.
  2. Check that the boiler's pressure gauge reads between 1 and 1.5 bar, and top it up if it is low.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 379
380

Your boiler has shut itself down because it is not detecting a steady flame when it lights, which is a safety check, and a qualified engineer can usually fix this without much fuss.

High Engineer only £120-250

What to check first

  1. Check that the gas supply to the boiler is turned on at the gas meter and the boiler isolation valve, then check the boiler pressure gauge reads roughly 1.0 to 1.5 bar.
  2. Press the reset button on the boiler once, wait about five minutes, and see if it restarts.
  3. If the fault returns, stop trying to reset it and call a Gas Safe registered engineer, as the flame sensor and burner need checking and gas work must be done by a professional.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner gauze assembly

Related codes

Full explanation for 380
Showing 81–90 of 151
381

The boiler lit its burner but the flame went out almost straight away, so it has shut itself down to stay safe and needs a reset and a check to find out why.

High Engineer only £90-220

What to check first

  1. Wait 10 minutes, then press the reset button on the boiler (or use the reset option on the display) once. If it locks out again, stop and call a Gas Safe engineer.
  2. Check the gas supply is on by confirming your gas meter valve is open and that the hob or other gas appliances work. If they don't, contact your gas supplier.
  3. Check the flue outlet outside your home is clear of leaves, nests, snow or debris, and that the condensate pipe is not frozen or blocked.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Gas valve
  • Fan

Related codes

Full explanation for 381
382

The boiler's flame switched off unexpectedly while it was running, so it has shut itself down as a safety measure, and it will usually need a qualified engineer to find out why.

High Engineer only £120-300

What to check first

  1. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top it up using the filling loop if it is low.
  2. Check that the gas supply is on by confirming the gas meter and the boiler's gas isolation valve are open and that other gas appliances are working.
  3. Press the reset button on the boiler once, wait 10 minutes, and see whether it restarts; if the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Gas filter
  • Control PCB

Related codes

Full explanation for 382
383

Your boiler has stopped several times in a row and has locked itself out to stay safe, so it needs a qualified engineer to find out why before it will run normally again.

High Engineer only £120-350

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 lower, top it up using the filling loop as shown in the manual.
  2. Make sure the gas supply is on and that other gas appliances, such as the hob, are working.
  3. Turn the boiler off at its power switch, wait 30 seconds, then turn it back on and press the reset button on the control panel once.
  4. If the boiler shows code 383 again or does not start, leave it alone and book a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 383
384

Your Viessmann boiler has stopped itself after repeated faults, so it needs a reset and a check by a qualified engineer to find the underlying cause.

High Engineer only £100-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top it up only if your manual explains how to do this safely.
  2. Reset the boiler once using the reset button or the reset option on the control panel, then wait a few minutes to see if it restarts and stays on.
  3. Check that the gas supply is on and that other gas appliances work. If the boiler locks out again, leave it off and book a Gas Safe registered engineer. If you smell gas, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Ignition electrode
  • Flame sensor
  • Gas valve
  • Fan
  • Control PCB
  • Flue gas sensor

Related codes

Full explanation for 384
385

Your boiler has detected a possible blockage or contamination in its gas supply, so it has stopped as a precaution and needs a qualified engineer to check it safely.

Emergency Engineer only £150-400

Parts commonly replaced

  • Gas inlet filter
  • Gas valve
  • Gas supply pipework (cleaning or flushing)

Related codes

Full explanation for 385
386

Your boiler has shut itself down as a safety precaution because its flame-sensing system has detected an electrical fault, and it will need a qualified engineer to check and repair it before it can run normally again.

High Engineer only £200-450

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 check the boiler display for the fault code.
  3. If the code returns or the boiler locks out again, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit (control PCB)
  • Ionisation electrode
  • Ignition and ionisation lead

Related codes

Full explanation for 386
387

Your boiler has shut itself down because it has detected an electrical fault in its control system, so it is not currently providing heating or hot water and needs a Gas Safe engineer to inspect it.

High Engineer only £200-450

What to check first

  1. Check that the boiler has power: make sure the mains supply switch and any fused spur feeding it are switched on.
  2. Press the reset button once. If code 387 returns, stop and do not keep resetting it.
  3. Look around the boiler for signs of water leaks or damp. If you find any, switch off the power at the mains and call a Gas Safe engineer straight away.

Parts commonly replaced

  • HBMU heat management unit (control PCB)
  • Ionisation electrode
  • Ignition electrode
  • Earth connection or wiring harness

Related codes

Full explanation for 387
388

Your boiler has detected a fault with one of its internal control parts, so it may stop heating or hot water until an engineer checks it, but this is not usually a sign of immediate danger.

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 circuit breaker, wait 5 minutes, then switch it back on to see whether the fault clears.
  2. If code 388 returns, note any other codes shown on the display and check that the boiler's external power supply and any thermostat or controller wiring are secure.
  3. If the fault remains, do not open the boiler casing. Contact a Gas Safe registered engineer or Viessmann-approved installer.

Parts commonly replaced

  • HBMU heat management unit
  • Control PCB
  • Wiring harness or connector

Related codes

Full explanation for 388
395

Your boiler has shut itself down for safety because it has detected an electrical fault near its ignition system, and it will need a qualified engineer to repair it.

High Engineer only £150-400

What to check first

  1. Check that the boiler has mains power and that the fuse or switch supplying it is on
  2. Check the system pressure gauge on the front of the boiler and top it up to between 1 and 1.5 bar if it is low, following the boiler manual
  3. Press the reset button once, wait 5 minutes, and see whether the fault returns. If code 395 comes back, stop and call a Gas Safe registered engineer

Parts commonly replaced

  • Heat management unit (HBMU)
  • Ignition electrode
  • Ignition lead
  • Earth lead

Related codes

Full explanation for 395
396

Your boiler's internal control module that manages how heat is sent around the system has detected a fault, so it may stop heating or hot water until an engineer checks it.

High Engineer only £150-350

What to check first

  1. Check the boiler display and note the code 396 exactly as shown.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait about 5 minutes, then switch it back on.
  3. Check that the system pressure gauge reads between 1 and 1.5 bar, and top up only if the manual says it is safe to do so.
  4. If the code returns or the boiler still does not heat or supply hot water, call a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Main control PCB
  • Connecting wiring harness

Related codes

Full explanation for 396
Showing 91–100 of 151
399

Your boiler's control unit has detected a fault with one of its internal management components, so it has likely stopped working to protect itself until an engineer checks it.

High Engineer only £250-500

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 only if you know how to do so safely, or contact a Gas Safe engineer.
  2. Switch the boiler off at its power supply, wait around 30 seconds, then switch it back on to see whether the fault clears.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer. Do not open the casing or attempt to repair internal components.

Parts commonly replaced

  • HBMU heat management unit
  • Main control PCB
  • Connecting cables and plugs

Related codes

Full explanation for 399
400

Your boiler's central control unit has detected a fault and has likely shut itself down to stay safe, so heating and hot water may not be working until a qualified engineer checks it.

High Engineer only £250-450

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, following the boiler manual.
  2. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on to try a reset.
  3. If the code returns, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Control unit PCB
  • Connecting cables and plugs

Related codes

Full explanation for 400
401

The boiler has detected an electrical fault in the part that checks the gas flame is lit, so it has shut down to stay safe and will likely need a qualified engineer to restore heating and hot water.

High Engineer only £150-450

What to check first

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

Parts commonly replaced

  • Ionisation electrode
  • Ignition/monitoring electrode
  • HBMU heat management unit
  • Electrode lead or connector

Related codes

Full explanation for 401
402

Your boiler's central control unit has detected a fault, so it has likely shut itself down to stay safe, and it will need a qualified engineer to inspect and repair it.

High Engineer only £250-500

What to check first

  1. Check the boiler pressure gauge. 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 only if you know how to do this safely, or call an engineer.
  2. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait about 30 seconds, then switch it back on and see whether the boiler restarts.
  3. If the fault code returns, stop trying to reset it and book a Gas Safe registered engineer. Do not open the boiler casing or attempt electrical or gas repairs yourself.

Parts commonly replaced

  • Heat management unit (HBMU) circuit board
  • Main control PCB
  • Wiring harness or connector

Related codes

Full explanation for 402
403

The boiler has detected an electrical fault in its flame-sensing and ignition parts, so it has shut itself down as a safety precaution, and it will need a qualified engineer to check it.

High Engineer only £180-350

What to check first

  1. Check the boiler display for any other fault codes or messages and note them down for the engineer.
  2. Check that the boiler's pressure gauge is between 1 and 1.5 bar, and top it up only if the boiler manual says it is safe to do so.
  3. Turn the boiler off at its on/off switch, wait 5 minutes, then switch it back on to try a reset.
  4. If the code returns, leave the boiler off and book a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Ionisation electrode
  • Ignition electrode
  • Ignition electrode lead
  • Ignition/flame sensing cable

Related codes

Full explanation for 403
404

Your boiler's central control module has detected a fault, which may stop your heating or hot water working until a qualified engineer checks it.

High Engineer only £250-500

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator supplying it.
  2. Wait about 5 minutes, then switch the power back on.
  3. Check whether the fault code has cleared and whether heating and hot water have returned.
  4. If the code returns, or the boiler will not restart, leave it off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU) circuit board
  • Vitotronic control unit
  • Connecting cable or wiring loom

Related codes

Full explanation for 404
405

Your boiler has shut itself down for safety because it has detected an electrical leak in the part that checks the gas flame, so it will need a qualified engineer to find and fix the cause.

High Engineer only £200-400

Parts commonly replaced

  • HBMU heat management unit
  • Ionisation electrode
  • Ignition/monitoring electrode
  • Electrode lead
  • Ignition cable

Related codes

Full explanation for 405
406

Your boiler has detected a communication fault inside its internal control system, so it may stop heating or hot water until a heating engineer checks it.

High Engineer only £250-450

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, following the boiler manual.
  2. Turn the boiler off at its isolator switch or fused spur, wait 30 seconds, then switch it back on to restart the system.
  3. If the fault code returns, do not keep resetting the boiler. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Control unit (PCU) or electronic board
  • Bus communication cable or connector

Related codes

Full explanation for 406
408

Your Viessmann boiler has detected a fault with one of its internal control modules, so it may stop producing heat or hot water until a qualified engineer checks and repairs it.

High Engineer only £200-450

What to check first

  1. Check the boiler pressure gauge; 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 only if you know how to do this safely, otherwise leave it to the engineer.
  2. Turn the boiler off at its isolator or power switch, wait 30 seconds, then switch it back on to see if the fault clears.
  3. If the fault returns, note the code and any other messages on the display, then contact a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • HBMU heat management unit
  • Control unit PCB
  • Wiring harness or connector

Related codes

Full explanation for 408
410

Your boiler's internal control unit has detected a fault, so it may stop providing heating and hot water until a qualified engineer checks it.

High Engineer only £250-450

What to check first

  1. Check the boiler pressure on the display or gauge. It should normally read between 1.0 and 1.5 bar when cold, and topping it up is only safe if you have been shown how or have the manufacturer's guidance.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait about 5 minutes, then switch it back on.
  3. If code 410 returns, stop and contact a Gas Safe registered engineer. Do not open the boiler casing or attempt internal repairs.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Control PCB
  • Wiring harness or connector

Related codes

Full explanation for 410
Showing 101–110 of 151
416

Your boiler has stopped or flagged a problem because a sensor that monitors the hot exhaust gases is not sitting in the right place, so it cannot read the temperature safely.

Medium Engineer only £80-150

What to check first

  1. Write down the code and note whether the boiler is still heating and providing hot water.
  2. Press the reset button on the boiler once (or use the reset control on the front panel) and wait a few minutes to see whether the code clears.
  3. Check that the flue pipe outside is not blocked by leaves, debris or nesting material, and that nothing is resting against it.
  4. If the code returns, stop trying to reset it and book a Gas Safe registered engineer to reposition or replace the flue gas temperature sensor.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor clip or retaining bracket

Related codes

Full explanation for 416
417

Your boiler has detected a fault in one of its internal control parts, which means it may have shut itself down to stay safe, so it's best to arrange a Gas Safe engineer to check it.

High Engineer only £300-600

What to check first

  1. Switch the boiler off at its on/off switch and at the household electrical supply (fused spur or isolator), wait 2 minutes, then switch it back on.
  2. 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, then close the filling loop.
  3. If the fault code returns or the boiler stays off, stop trying to repair it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit board
  • Boiler control PCB
  • Wiring harness or connectors

Related codes

Full explanation for 417
418

Your boiler's internal control module that manages heating and hot water has stopped communicating properly, so the boiler has likely shut itself down as a safety precaution and needs a qualified engineer to check it.

High Engineer only £250-450

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, following the manufacturer's instructions.
  2. Turn the boiler off at its isolator switch or fused spur, wait 30 seconds, then turn it back on to attempt a reset.
  3. If the fault code returns, stop trying to fix it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Control unit PCB
  • Wiring harness or connector plugs
  • Fuses

Related codes

Full explanation for 418
425

The boiler's internal clock couldn't update itself from its time signal, which is usually a minor issue that doesn't stop your heating or hot water working.

Low DIY-safe £0-120

What to check first

  1. Check the date and time shown on the boiler's controller and set them manually if they are wrong.
  2. Switch the boiler off at its power isolator or fused spur, wait about 30 seconds, then switch it back on.
  3. See whether the code clears. If it returns, or the clock keeps losing the correct time, contact a Gas Safe registered engineer to check the clock receiver or control unit.

Parts commonly replaced

  • Radio clock (DCF) receiver
  • Control unit clock module

Related codes

Full explanation for 425
446

Your boiler has shut itself down as a precaution because the water leaving it got hotter, or behaved differently, than it should, and it needs a qualified engineer to check the cause before it restarts safely.

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 1.5 bar when cold. If it is well below 1 bar, top it up using the filling loop as shown in the manual, then close the loop.
  2. Make sure all radiator and heating valves are open and the thermostat or controls are not set unusually high, then wait 10 minutes for the boiler to cool.
  3. Press the reset button once on the boiler control panel. If the fault returns or the boiler shuts down again, stop and call a Gas Safe registered engineer rather than resetting repeatedly.

Parts commonly replaced

  • High limit safety thermostat (STB)
  • Heating flow temperature sensor
  • Circulation pump
  • Expansion vessel

Related codes

Full explanation for 446
447

The boiler has stopped because its flame sensor is not reading the flame correctly, which usually means a dirty or faulty sensing part that can often be cleaned or replaced by a heating engineer.

High Engineer only £90-250

What to check first

  1. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on and press reset once to see if the fault clears.
  2. Check the boiler pressure gauge reads between 1.0 and 1.5 bar when cold, and top up with the filling loop only if it is below 1.0 bar.
  3. Check that the condensate pipe is not blocked or frozen and that the outside flue terminal is not obstructed by leaves, snow or debris.
  4. If the fault returns within a day or two, stop resetting it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ionisation electrode gasket
  • Burner gasket
  • Gas valve (less common)
  • Main PCB (rare)

Related codes

Full explanation for 447
448

Your boiler has stopped because it cannot detect its flame properly, so it has shut itself down as a safety measure, and a qualified engineer will usually need to check it.

High Engineer only £100-250

What to check first

  1. Check that the gas supply is turned on at the meter and that other gas appliances are working.
  2. Check the boiler pressure gauge reads between 1 and 1.5 bar, and top it up only if the manual says how to do so safely.
  3. Switch the boiler off at the isolator for 10 minutes, switch it back on, and press the reset button once. If the fault returns, call a Gas Safe registered engineer and do not keep resetting it.

Parts commonly replaced

  • Ionisation (flame detection) electrode
  • Burner door gasket
  • Burner assembly seal

Related codes

Full explanation for 448
449

Your boiler's control panel has noticed a problem with its built-in timer that decides when your heating and hot water should switch on, so the schedule may not be running as set.

Medium DIY-safe £120-350

What to check first

  1. Check the date and time on the boiler display and correct them if they are wrong or have reset.
  2. Review your heating and hot water timer settings and re-enter your schedule if it looks incorrect.
  3. Turn the boiler off at its fused spur or isolator switch, wait about 30 seconds, then switch it back on to restart the controller.
  4. If the code returns, do not keep resetting it. Book a Viessmann-registered engineer to check the control unit.

Parts commonly replaced

  • Vitotronic control unit
  • Control unit clock or memory module

Related codes

Full explanation for 449
451

The boiler's control panel has lost track of its heating schedule, so it may not switch on and off at the times you have set, though it can often keep running as normal.

Low DIY-safe £80-180

What to check first

  1. Check the date and time shown on the boiler's control panel and correct them if they are wrong.
  2. Check that your heating and hot water timer settings are still in place, and set them again if needed.
  3. Switch the boiler off at its on/off switch, wait about 30 seconds, then switch it back on.
  4. If the code is still showing, press the reset button on the control panel as described in your user manual, then check whether the code returns after a few hours.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Control unit display panel
  • Clock or timer module

Related codes

Full explanation for 451
452

Your boiler's controller has lost track of its heating timetable, so it may not be switching on and off at the times you set, but it is not a safety problem.

Low DIY-safe £80-180

What to check first

  1. Check that the date and time on the boiler controller display are correct, and correct them if not.
  2. Check that your heating and hot water timer programmes are set as you want them, and re-enter any that look wrong.
  3. Switch the boiler off at its on/off switch or the mains supply for about 30 seconds, then switch it back on.
  4. If the code clears and does not return over the next day, no further action is needed. If it comes back, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic controller)
  • Controller clock backup battery

Related codes

Full explanation for 452
Showing 111–120 of 151
453

The boiler's internal parts are struggling to stay in step with each other, so it may switch off or run unevenly, but this is usually a communication problem that a heating engineer can often fix.

Medium 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, wait 5 minutes, then switch it back on to see if the fault clears.
  2. If the fault returns, note the code and when it happens, then book a Gas Safe registered engineer. Do not open the boiler casing or work on its wiring yourself.

Parts commonly replaced

  • LON communication module
  • BUS connection cable or plug
  • Control unit PCB
  • Sequence controller

Related codes

Full explanation for 453
454

Your boiler's control settings for a part that manages heating and hot water have become incorrect, so it may not be running as it should, but this is usually fixable by a qualified engineer resetting the settings.

High Engineer only £80-180

What to check first

  1. Note the code 454 and any other codes shown on the display.
  2. Switch the boiler off at its power switch, wait 60 seconds, then switch it back on.
  3. If code 454 returns or the boiler still does not heat or supply hot water, leave it off and call a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit (only if the fault is not cleared by resetting the parameters)
  • Control unit or main PCB (rarely required)

Related codes

Full explanation for 454
455

Your boiler's control system has noticed its internal operating sequence went out of step, so it has paused to protect itself, and a reset will often get it running again.

High DIY-safe £150-400

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. Switch the power back on and wait for the display to show the normal standby screen.
  3. Press and hold the reset button (or use the reset option on the display) until the boiler restarts, then check that heating and hot water both work.
  4. If the code returns within a day or two, stop resetting it and book a Gas Safe registered engineer to check the control unit and power supply.

Parts commonly replaced

  • Main control PCB (control unit)
  • Power supply board

Related codes

Full explanation for 455
456

Your boiler's internal control system has detected a timing or operating-sequence problem and has shut itself down as a precaution, so it may not be providing heating or hot water until it is reset or repaired.

High Engineer only £200-450

What to check first

  1. Switch off the boiler at its power supply or isolator switch and wait 10 minutes.
  2. Switch the power back on, then press the reset button on the boiler (or the reset option on its controls) to restart it.
  3. Check the system pressure gauge reads between 1 and 1.5 bar, and that no other electrical equipment nearby is causing a power surge.
  4. If the code returns or the boiler will not restart after one or two resets, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB
  • Power supply unit

Related codes

Full explanation for 456
457

Your boiler's fan isn't starting up, so it can't safely begin heating your home or water, and it's best to have a Gas Safe engineer take a look soon.

High Engineer only £200-400

What to check first

  1. Check that the boiler is switched on at its isolator or power switch and that the controls are showing normally.
  2. Check the fused spur or circuit breaker supplying the boiler, and reset it if it has tripped.
  3. Reset the boiler once using the reset button or the controls, then wait a few minutes to see if it starts.
  4. If the fault returns or the boiler still won't start, stop and call a Gas Safe registered engineer rather than opening the casing.

Parts commonly replaced

  • Fan assembly
  • Fan connection cable
  • Main PCB (control board)

Related codes

Full explanation for 457
458

Your boiler has stopped and is unable to restart properly on its own, so it has likely locked out and needs a careful reset and check before it will work again.

High DIY-safe £90-300

What to check first

  1. Check that the boiler display is showing the code and that the gas supply and any isolator switch are turned on.
  2. Check the system pressure gauge under the boiler and top it up only if it is below the recommended level (usually about 1 bar) using the filling loop as shown in your manual.
  3. Turn off the boiler at its power switch or isolator, wait 1 minute, switch it back on, and 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

  • Main PCB (control board)
  • Ignition electrode
  • Gas valve

Related codes

Full explanation for 458
461

Your boiler has stopped because a safety part that controls the flue exhaust isn't opening and closing correctly, so it has shut itself down to stay safe.

High Engineer only £180-380

What to check first

  1. Check the boiler display for pressure and any other fault codes, then note them down for the engineer.
  2. Turn off the boiler at its power switch, wait five minutes, then turn it back on and press reset once, following the manual.
  3. Check that the outside flue terminal is clear of leaves, nests, snow or debris, without touching any gas components.
  4. If the fault returns or no heating or hot water comes back, call a Gas Safe registered engineer and do not keep resetting the boiler.

Parts commonly replaced

  • Flue gas damper assembly
  • Flue gas damper actuator
  • Control unit PCB
  • Flue seal or gasket

Related codes

Full explanation for 461
462

Your boiler has detected a problem with the gas control valve and has stopped itself from starting up, so it is not currently producing heating or hot water.

High Engineer only £200-400

What to check first

  1. Check that the gas supply is on by looking at the gas meter and confirming the isolation valve near the boiler is open.
  2. Check whether other gas appliances, such as a cooker or gas fire, are working. If none are, contact your gas supplier, as the problem may be outside your home.
  3. Check the boiler pressure gauge and make sure it reads between 1 and 1.5 bar when cold. Top up using the filling loop only if your handbook explains how.
  4. Press the reset button on the boiler once, wait 5 minutes, and see whether it starts. Do not keep resetting it repeatedly.
  5. If you smell gas at any point, leave the property, do not use switches, and call the National Gas Emergency Service on 0800 111 999 straight away.

Parts commonly replaced

  • Gas safety valve (gas valve unit)
  • Gas valve connector cable or wiring harness
  • Gas valve coil
  • Gas pressure switch

Related codes

Full explanation for 462
463

The boiler has shut itself down because it is sensing that its air supply may be contaminated, possibly with exhaust fumes flowing back into it, so a qualified gas engineer should inspect it before it is used again.

Emergency Engineer only £150-400

Parts commonly replaced

  • Flue terminal or concentric flue pipe
  • Air intake pipe
  • Flue and air intake seals
  • Combustion fan
  • Flue gas sensor

Related codes

Full explanation for 463
464

Your boiler's burner has stopped working safely because it couldn't reliably detect its flame during a self-check, so it has shut itself down to stay safe while a qualified engineer looks at it.

High Engineer only £120-250

What to check first

  1. Check that other gas appliances, such as your hob, are working, and check your gas meter is not showing a problem.
  2. Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is very low, follow your manual to top it up, or stop and call an engineer.
  3. Press the reset button once (or use the reset option on the display, as described in the manual), wait a few minutes, and see if the boiler restarts.
  4. If the fault returns or the boiler will not restart, do not keep resetting it. Turn the boiler off, and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation/ignition electrode
  • Burner gasket
  • Gas valve assembly

Related codes

Full explanation for 464
Showing 121–130 of 151
467

Your boiler has stopped its burner because it isn't getting enough gas flow while starting up, so it can't heat your home or water until the gas supply is checked.

High Engineer only £150-450

What to check first

  1. Check that other gas appliances, such as the hob or fire, still work. If none do, contact your gas supplier's emergency line.
  2. If you have a prepayment gas meter, check that it has credit.
  3. Make sure the gas isolation valve on the pipe beside the boiler is fully open (the lever should be in line with the pipe).
  4. Wait 10 minutes, then press the reset button once on the boiler. If the code returns, stop and call a Gas Safe registered engineer.
  5. If you smell gas at any point, leave the property, do not use switches, and call the National Gas Emergency Number on 0800 111 999.

Parts commonly replaced

  • Gas inlet filter (strainer)
  • Gas valve assembly
  • Gas pressure regulator
  • Gas supply pipe section (if restricted or contaminated)

Related codes

Full explanation for 467
468

Your boiler has shut itself down because its flame sensor is reading a stronger signal than expected during startup, so it has stopped to stay safe and needs a qualified check.

High Engineer only £120-250

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, then stop.
  2. Press and hold the reset button for about 3 seconds (or use the reset option on the display) to restart the boiler once.
  3. If the fault returns or the boiler will not restart, leave it switched off and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 468
471

Your boiler has stopped heating your home because it cannot read the water pressure in the heating system, so it is holding off until someone checks the sensor and its wiring.

High Engineer only £120-260

What to check first

  1. Look at the pressure gauge on the front of the boiler. With the system cold, the needle should sit between 1 and 1.5 bar.
  2. If the pressure is below 1 bar, top it up using the filling loop, following the boiler manual, then close the filling loop valve again.
  3. Switch the boiler off at the isolator for 30 seconds, switch it back on, and see whether the fault code clears.
  4. If the code returns or the boiler still will not heat, stop and call a Gas Safe registered engineer. The sensor or its wiring needs a professional check.

Parts commonly replaced

  • System pressure sensor
  • Sensor lead or wiring harness
  • Main control PCB (if the sensor input is faulty)

Related codes

Full explanation for 471
474

Your boiler has stopped its heating cycle because its burner did not run through its normal start-up sequence correctly, so it has paused itself as a safety precaution.

High Engineer only £120-300

What to check first

  1. Check the boiler pressure gauge; if it reads below 1 bar, top it up using the filling loop as shown in the manual, then check it is back between 1 and 1.5 bar.
  2. Make sure the gas supply is on by checking that other gas appliances, such as the hob, work normally.
  3. Press and hold the reset button for about 2 seconds, or switch the boiler off at the isolator, wait 10 minutes, then switch it back on.
  4. If the boiler shows code 474 again within a short time, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Flame detection electrode (ionisation probe)
  • Gas valve
  • Fan assembly
  • Control PCB

Related codes

Full explanation for 474
477

Your boiler's solar hot water panels on the roof aren't producing heat as they should, but your central heating and main hot water should still work normally.

Medium Engineer only £100-350

What to check first

  1. Check the solar system pressure gauge (usually near the boiler or solar station) when the system is cold, and note whether it is within the range shown in the manual.
  2. Reset the boiler or controller from the display, then check whether the fault clears during a sunny period, as the solar system only runs when the panels are warmer than the cylinder.
  3. Make sure nothing is shading or covering the roof panels, and that the cylinder and panel sensor cables are firmly connected, then contact a heating engineer if the fault returns.

Parts commonly replaced

  • Solar circulation pump
  • Collector temperature sensor
  • Cylinder temperature sensor
  • Solar heat transfer fluid

Related codes

Full explanation for 477
517

Your boiler is still heating your home and providing hot water, but the thermostat or remote control you use to adjust it has stopped communicating with the boiler, so you may not be able to change settings from it.

Low DIY-safe £120-300

What to check first

  1. Check the batteries in the remote control and replace them if they are low or flat.
  2. Make sure the remote control is switched on and the display is working.
  3. Switch the boiler off at its on/off switch, wait 30 seconds, then switch it back on to reset the connection.
  4. Check that the wires connecting the remote control to the boiler are not visibly damaged or loose at either end, but do not open the boiler casing.
  5. If the code returns, contact a Gas Safe registered engineer to check the PlusBus cable, the boiler's appliance address setting and the remote control itself.

Parts commonly replaced

  • Remote control unit (Vitotrol)
  • PlusBus cable
  • Batteries for the remote control
Full explanation for 517
527

Your boiler has stopped its burner because its internal control settings have been set up incorrectly, so it isn't producing heat or hot water until a qualified engineer corrects them.

High Engineer only £80-180

Related codes

Full explanation for 527
528

Your boiler has stopped its burner from working because its control settings have been set up incorrectly, so it needs a Gas Safe engineer to check and correct them.

High Engineer only £90-180

What to check first

  1. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top it up using the filling loop only if the manufacturer's instructions for your model say to.
  2. Make sure the gas supply is on by checking that your gas meter tap and the boiler's isolation valve are open, and that other gas appliances in the home are working.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU control unit (rarely needed, usually only reprogrammed)
  • Ignition electrode
  • Gas valve

Related codes

Full explanation for 528
540

Your boiler has shut itself down for safety because water from burning gas has backed up inside it and is blocking its normal drain, so it needs a qualified engineer to clear the blockage and restart it safely.

High Engineer only £120-250

What to check first

  1. Check whether the boiler display shows any other message, and note it down for the engineer.
  2. Check the boiler pressure gauge. If it is below 1 bar, top it up using the filling loop as shown in your manual, then stop.
  3. Go outside and check the external condensate pipe for ice, a blockage or a sagging section holding water. In cold weather, gently warm the outside of the pipe with warm (not boiling) water. Do not open the boiler casing.
  4. Try one reset using the reset button on the boiler. If the fault returns straight away, leave the boiler off and call a Gas Safe registered engineer.

Parts commonly replaced

  • Condensate trap (siphon)
  • Condensate pipe and fittings
  • Heat exchanger (if blocked or damaged)
  • Gaskets and seals

Related codes

Full explanation for 540
544

Your boiler has lost track of the water temperature for one of your heating zones, usually the one that uses a mixing valve, so that zone may not heat properly, though your boiler itself is likely still working.

Medium Engineer only £150-250

What to check first

  1. Check the boiler display for any other fault codes and note them down before doing anything else.
  2. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to see whether the code clears on its own.
  3. Check that your heating controls or thermostat for the affected zone are set to the correct times and temperatures.
  4. If the code returns, leave the boiler casing and wiring alone and book a Gas Safe registered engineer to inspect the flow temperature sensor and its connections.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor cable or plug connector
  • Heating circuit 2 mixer motor
  • Control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 544
Showing 131–140 of 151
545

The boiler has detected a faulty temperature reading for one of the heating zones that uses a mixing valve, so that zone may not heat properly, though your boiler is likely still working for other parts of your home.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at the on/off switch, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check that the boiler's display and controls are on the correct setting and that the heating zone's thermostat or room controls are working.
  3. If the code returns, leave the boiler as it is and contact a Gas Safe registered engineer, since the sensor and its wiring need to be checked by a qualified person.

Parts commonly replaced

  • Heating circuit 2 flow temperature sensor
  • Sensor connection cable or harness
  • Mixing valve motor (if the valve is also faulty)

Related codes

Full explanation for 545
574

Your boiler isn't receiving temperature readings from the sensor that checks the temperature in your home, so it may heat your house less accurately or less efficiently until this is looked at.

Medium Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch and at the isolator or fused spur, then wait 5 minutes.
  2. Check that any room thermostat or room sensor plugged into the boiler's control panel or wall is securely connected and that its cable isn't damaged or loose.
  3. Switch the boiler back on and see whether the code clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor (room thermostat or Vitotrol remote)
  • Sensor connection cable or plug
  • Vitotronic control unit (rare)

Related codes

Full explanation for 574
575

Your boiler has lost the signal from its room temperature sensor, so it can't read the room directly, but it should still keep your home heated using its backup settings.

Low DIY-safe £90-180

What to check first

  1. Check the room thermostat or room sensor on the wall. If it runs on batteries, replace them with fresh ones and check it is set to the correct heating circuit.
  2. Make sure the room sensor is not hidden behind furniture, curtains, or near a radiator or direct sunlight, and that it is mounted securely on the wall.
  3. Switch the boiler off at its on/off switch, wait 30 seconds, then switch it back on and check whether the code clears.
  4. If the code returns, leave the boiler casing closed and contact a Viessmann-registered heating engineer, as the wiring behind the room sensor and inside the boiler needs a qualified check.

Parts commonly replaced

  • Room temperature sensor or room control unit (e.g. Viessmann Vitotrol)
  • Sensor cable or connection terminal

Related codes

Full explanation for 575
576

The boiler's control panel has detected a fault with the sensor on your wall thermostat that measures room temperature, so it may not adjust heating to match the room accurately, though it should still heat your home.

Low DIY-safe £60-150

What to check first

  1. Switch off the boiler at its isolator or fused spur and wait a few minutes.
  2. Check the cable running from the room thermostat or sensor on the wall for damage, pinched wires, or bare copper touching metal, and reconnect any loose terminals.
  3. Restore power, press the reset button on the boiler or control unit, and check whether code 576 returns.
  4. If the code returns, the room sensor itself is likely faulty. Contact a Gas Safe registered engineer to replace it. Do not open the boiler casing.

Parts commonly replaced

  • Room temperature sensor (Vitotrol or wall-mounted room sensor)
  • Connecting cable between sensor and boiler

Related codes

Full explanation for 576
577

The boiler can't get a reading from the room thermostat sensor for the second heating zone, so that zone may not heat properly, though the rest of the system should still work.

Medium Engineer only £90-180

What to check first

  1. Check whether your system actually has a second heating circuit, and whether a second room thermostat or sensor is meant to be connected.
  2. Look at the sensor's cable at the wall unit and at the boiler control panel to see if it looks loose or damaged, without opening any electrical covers.
  3. Switch the boiler off at its isolator or mains switch for 30 seconds, then back on, and check whether code 577 returns.

Parts commonly replaced

  • Room temperature sensor (heating circuit 2)
  • Sensor connecting cable
  • Heating circuit extension module (if fitted)

Related codes

Full explanation for 577
578

The boiler has lost the signal from the room thermostat sensor for its second heating zone, so it can't tell how warm that room is, but it should still heat your home and water safely.

Low Engineer only £100-200

What to check first

  1. Switch the boiler off at its on/off switch and wait 5 minutes, then switch it back on to see if the code clears.
  2. Check the sensor cable at the wall or room unit for visible damage, or loose connections where it plugs in, without opening the boiler casing.
  3. If the code returns, or the cable looks fine, contact a Gas Safe registered engineer to test and replace the sensor.

Parts commonly replaced

  • Room temperature sensor (heating circuit 2)
  • Sensor cable or connector

Related codes

Full explanation for 578
579

Your boiler's room thermostat sensor has a wiring fault, so it can't read the room temperature properly, but the heating will usually still run using a backup setting.

Low Engineer only £120-250

What to check first

  1. Turn the boiler off at its power switch and wait 5 minutes, then switch it back on to see if the code clears.
  2. Check the cable running from the room thermostat or sensor to the boiler for visible damage, such as cuts, crushed sections or loose wires at the connections.
  3. If the code returns, leave the boiler as it is and book a heating engineer to test and replace the room sensor.

Parts commonly replaced

  • Room temperature sensor (room thermostat or Vitotrol sensor)
  • Room sensor cable or connection lead

Related codes

Full explanation for 579
682

Your boiler has stopped because it cannot detect the airflow it needs to burn gas safely, so it may not be providing heating or hot water until a qualified engineer checks it.

High Engineer only £200-450

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on to see if the fault clears.
  2. Check that the flue terminal outside the property is not blocked by leaves, nests, snow or debris, and clear any obstruction you can reach safely.
  3. Check that the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if you know how to do so safely.
  4. If the fault returns, stop attempting repairs and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Air mass flow sensor
  • Combustion fan assembly
  • Gas valve (if the fan or sensor fault has affected gas control)

Related codes

Full explanation for 682
683

Your boiler has stopped because it cannot correctly measure the air it needs to burn gas safely, so it has shut itself down as a precaution, and a qualified engineer should check it.

High Engineer only £150-300

What to check first

  1. Check that the flue terminal outside your home is clear of leaves, snow, nests or other debris, without touching the boiler casing.
  2. Press the reset button on the boiler once and wait a few minutes to see whether it restarts.
  3. If the fault returns, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Air mass flow sensor
  • Combustion fan
  • Control PCB

Related codes

Full explanation for 683
684

Your boiler has detected a possible problem with how it vents its exhaust fumes, so it has stopped for safety and needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £120-250

Parts commonly replaced

  • Back draught safety device
  • Flue gas temperature sensor
  • Flue seals and gaskets
  • Flue terminal

Related codes

Full explanation for 684
Showing 141–150 of 151
694

Your boiler has shut itself down as a safety precaution because the hot exhaust gases leaving it are running hotter than they should, which often means the flue outlet is partly blocked or the boiler needs a check, so please have a Gas Safe engineer look at it before you use it again.

Emergency Engineer only £90-400

What to check first

  1. Turn the boiler off at the switch and wait at least 10 minutes.
  2. From outside, check the flue terminal (the pipe or grille where the boiler vents through the wall) for visible blockages such as nests, leaves, or debris, and clear any you can reach safely without using ladders.
  3. If the flue looks clear, switch the boiler back on and try one reset. If the fault returns or the boiler locks out again, stop and call a Gas Safe registered engineer, and do not keep resetting it.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue terminal or flue pipe section
  • Heat exchanger (in some cases)

Related codes

Full explanation for 694
738

Your boiler has spotted a room controller that is connected but has not been set up properly, so it may not be controlling your heating as intended.

Medium Engineer only £60-120

What to check first

  1. Switch the boiler off at its on/off switch and at the wall, wait two minutes, then switch it back on to see whether the code clears.
  2. Check the room controller's display for any setup or pairing menu, and make sure it is set to OpenTherm if your manual explains how.
  3. If the code returns, contact a Gas Safe registered engineer to configure the OpenTherm settings.

Parts commonly replaced

  • OpenTherm room thermostat or remote control (only if faulty)
  • Boiler control PCB (rarely required)
Full explanation for 738
766

Your boiler has shut itself down because the hot exhaust gases leaving through the flue are hotter than they should be, which usually means the flue is partly blocked or a sensor needs checking, so it needs a qualified engineer to look at it before it's used again.

Emergency Engineer only £120-300

What to check first

  1. Turn the boiler off at the switch, wait 10 minutes, then turn it back on once to see if the fault clears.
  2. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar (only if you know how to do this safely).
  3. From outside, check the flue terminal is clear of leaves, nests, debris or anything blocking it, and call a Gas Safe engineer if you're unsure.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue terminal or flue pipe sections
  • Flue gas outlet seal or gasket

Related codes

Full explanation for 766
767

The boiler has shut itself down because the hot gases leaving it through the flue are running hotter than they should, which is a safety precaution and usually means something in the flue or boiler needs checking by an engineer.

High Engineer only £120-350

What to check first

  1. Turn the boiler off at the isolator switch and leave it off for about 10 minutes
  2. Check that the flue terminal outside the property is not blocked by debris, nesting material, or plant growth, and clear anything you can reach safely
  3. Turn the boiler back on and try one reset using the reset button or display; if the code returns, stop and call a Gas Safe registered engineer

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue terminal or flue pipe section
  • Heat exchanger
  • Flue gas temperature limiter

Related codes

Full explanation for 767
799

Your boiler's central heating pump, which moves hot water around your radiators, has detected an electrical problem, so your heating may stop or run unreliably until a qualified engineer checks it.

High Engineer only £180-350

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 below 1 bar, top it up using the filling loop, following the boiler manual.
  2. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on to clear the fault.
  3. If the code returns, stop and call a Gas Safe registered engineer. The pump's electrical connections and wiring should not be handled without training.

Parts commonly replaced

  • Central heating circulation pump
  • Electronic control board (PCB)
  • Pump cable or connector

Related codes

Full explanation for 799
979

Your boiler is receiving heating requests from two control connections at the same time, so it can't tell which one to follow, and an engineer needs to check the wiring and settings.

Medium Engineer only £80-150

Parts commonly replaced

  • Room thermostat or OpenTherm receiver
  • Control wiring or plug connector

Related codes

Full explanation for 979
980

Your boiler isn't getting enough water flowing through it to heat your hot water tap supply, so it has paused hot water heating until the flow improves, though your central heating may still work.

High Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge: with the boiler cold, it should read between 1.0 and 1.5 bar. If it's low, top it up using the filling loop as shown in your manual, then close the loop valve.
  2. Make sure all isolation valves on the heating and hot water pipework are fully open, and that no hot water taps or outside taps are left partly open.
  3. Turn the boiler off at the switch, wait 30 seconds, then turn it back on to reset the fault and try a hot water tap again.
  4. If the fault returns, or the pressure drops again soon after topping up, stop and contact a Gas Safe registered engineer, as this can point to a leak or a faulty component.

Parts commonly replaced

  • Heating circulation pump
  • Diverter valve
  • Plate heat exchanger (domestic hot water)
  • Expansion vessel

Related codes

Full explanation for 980
981

Your boiler is running, but it isn't getting enough water flowing through its hot water side, so it can't heat your tap water properly, and a qualified engineer should look at the cause.

High Engineer only £120-350

What to check first

  1. Look at the pressure gauge on the boiler (usually on the front panel). It should read between 1 and 1.5 bar when the system is cold. If it's low, follow the boiler manual to top it up using the filling loop, then close the loop valves again.
  2. Make sure the boiler's isolation valves (usually under the boiler, one on the heating flow and one on the return) are fully open, and that no hot water taps are partly blocked or stuck closed.
  3. Turn the boiler off at the switch for about 30 seconds, then back on to clear any temporary fault. If code 981 returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Diverter valve actuator
  • Circulation pump
  • Plate heat exchanger (flushed or replaced)

Related codes

Full explanation for 981
982

Your boiler has stopped heating your home and your hot water because it has lost enough water in its heating system, so its internal pump is running without water.

High DIY-safe £0-30 if topping up the pressure fixes it; £250-450 if the circulation pump needs replacing or a leak needs repairing

What to check first

  1. Look at the pressure gauge on the front of the boiler. If the needle is below 1 bar, the system has lost water.
  2. Following the filling instructions in your Viessmann manual, slowly open the filling loop valve (usually under the boiler) until the gauge reads about 1.2 to 1.5 bar, then close the valve firmly.
  3. Switch the boiler off at the wall, wait 30 seconds, switch it back on and check it restarts and the pressure holds. If the pressure drops again within a few days, or you see water under the boiler or around the pipes, stop and call a Gas Safe registered engineer to find the leak.

Parts commonly replaced

  • Heating circulation pump
  • Filling loop valve
  • Pressure relief valve

Related codes

Full explanation for 982
E10

Your boiler is still heating and providing hot water, but it has lost its connection to your home WiFi, so it cannot be controlled or monitored remotely through the app.

Low DIY-safe £0-180

What to check first

  1. Check that your WiFi router is switched on and working, and that other devices in your home can get online.
  2. Confirm that the WiFi network name and password entered in the boiler or app settings are still correct, especially if you have recently changed your router or password.
  3. Restart your router and wait a few minutes, then restart the boiler by pressing and holding the reset button or switching the boiler off and on at the isolator.
  4. If the code remains, move your router closer to the boiler or check the boiler's WiFi signal strength in its settings, then re-enter the network details or repeat the connection process in the Viessmann app.

Parts commonly replaced

  • Viessmann Vitoconnect internet gateway module
  • WiFi router (only if the existing router is faulty)
Full explanation for E10
Showing 151–151 of 151
E12

Your boiler is most likely still heating your home and water, but it has lost its connection to Viessmann's online service, so remote monitoring and app control may not work until the link is restored.

Low DIY-safe £0-150

What to check first

  1. Check that your home Wi-Fi or broadband is working by loading a website on another device.
  2. Restart your router and wait about 5 minutes for it to reconnect fully.
  3. Switch the boiler off at its power supply or fused spur for about 1 minute, then switch it back on.
  4. Check that the boiler is within range of your Wi-Fi router, and that the Vitoconnect unit is firmly plugged in and its status light is on.
  5. Wait around 30 minutes, then check whether the E12 code has cleared. If it has not, contact a Viessmann-approved engineer or your heating company.

Parts commonly replaced

  • Vitoconnect internet communication module
Full explanation for E12
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