Viessmann Vitodens 050-W B0KA-25 Fault codes & diagnostics

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

Combi Natural GasLPG 2021-present
GC Number 47-819-55
Download the Viessmann Vitodens 050-W B0KA-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.
13 Emergency 89 High 12 Medium 17 Low
Just want the most common Viessmann Vitodens 050-W B0KA-25 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 131 documented codes

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7

Your boiler is still heating your radiators, but it has stopped making hot water for taps and showers, which usually points to a fault in the part that switches the boiler between heating water for the house and heating water for the tap.

High Engineer only £150-350

What to check first

  1. Check the boiler pressure gauge and confirm it reads between 1 and 1.5 bar when cold; top up only if the manual says how to do so safely.
  2. Turn the boiler off at its power switch, wait 30 seconds, then turn it back on and see whether hot water returns.
  3. Check that the hot water is switched on at the boiler controls or timer and that any cylinder thermostat is set to a sensible temperature.

Parts commonly replaced

  • Diverter valve actuator (motor)
  • Diverter valve
  • DHW cylinder temperature sensor

Related codes

Full explanation for 7
8

Your boiler is running but isn't heating your hot water tap supply, which usually means a fault with a sensor or valve that controls the hot water, and it should be checked by a qualified engineer soon.

High Engineer only £150-350

What to check first

  1. Check that the boiler has water pressure on its gauge, ideally between 1 and 1.5 bar when cold, and top it up using the filling loop if it is low.
  2. Switch the boiler off at the mains and back on after waiting about 30 seconds, then check whether hot water returns.
  3. Confirm that the hot water is set to on or to a suitable temperature on the boiler's controls or thermostat, and that no timer or holiday mode is stopping it.

Parts commonly replaced

  • Diverter valve motor
  • Cylinder temperature sensor (NTC)
  • Cylinder charging pump
  • Control PCB

Related codes

Full explanation for 8
13

The boiler's outside weather sensor appears to be faulty, so it is heating your home as though it were freezing cold outside, which can make it run more than needed but it is usually not dangerous.

Low DIY-safe £90-180

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, wait 2 minutes, then switch it back on and check whether the code clears.
  2. Locate the outside temperature sensor, usually a small white or grey box on a north or north-east facing external wall, and check it visually for physical damage, a loose or disconnected cable, or water ingress.
  3. If the cable and sensor look undamaged, leave everything else alone and book a Gas Safe registered engineer to test or replace the sensor. Do not open the boiler casing.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 13
14

Your boiler has lost track of the outside temperature reading, so it can't adjust its heating output to the weather, but it will usually keep heating and supplying hot water as normal.

Low DIY-safe £120-200

What to check first

  1. Turn the boiler off at its power switch, then find the outside temperature sensor, which is usually a small white or grey box on an external north-facing wall.
  2. Check the sensor cable for visible damage, chewed wires, or water getting in, and make sure the plug or terminal connections at the sensor are firm and dry.
  3. Turn the boiler back on and press the reset button on the boiler panel. If code 14 returns, leave the boiler running and book a qualified heating engineer to test or replace the outside sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 14
49

The boiler has lost contact with a sensor that measures how hot the exhaust gases are leaving through the flue, so it has shut itself down as a precaution and will need a qualified engineer to reconnect or replace the sensor.

High Engineer only £120-220

What to check first

  1. Check that the flue terminal outside the property is clear of leaves, snow, nests or other debris
  2. Reset the boiler using the reset button or the reset option on the display, then wait a few minutes to see whether the fault returns
  3. If the code returns, stop trying to reset it and call a Gas Safe registered engineer

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor lead or wiring harness
  • Flue sensor clip or gasket

Related codes

Full explanation for 49
50

Your boiler has detected a faulty reading from a sensor in its flue, which is the pipe that carries exhaust gases outside, so it has likely shut itself down as a 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 isolation switch or the mains, wait about 10 minutes, then switch it back on to try a reset.
  2. Check that the flue terminal outside the property is not blocked by leaves, nesting material, snow or debris, and clear any obstruction you can reach safely.
  3. If the code returns, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Control unit PCB (if the fault persists after sensor replacement)

Related codes

Full explanation for 50
57

Your boiler has lost contact with its room temperature sensor, so it can't read how warm your home is, but it will usually keep heating using its backup settings until a qualified engineer checks it.

Low Engineer only £90-180

What to check first

  1. Check that the room sensor or room thermostat on the wall is firmly connected and that its cable isn't visibly damaged or pinched.
  2. If the room sensor is wireless, replace its batteries.
  3. Switch the boiler off at its power supply for about a minute, switch it back on, and check whether the fault clears.
  4. If code 57 returns, stop there and book a Gas Safe registered engineer. Do not open the boiler casing yourself.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or wiring lead
  • Wireless room sensor batteries (if applicable)

Related codes

Full explanation for 57
58

Your boiler's control system has detected a problem with the sensor that measures the room temperature, so it may not be heating your home as accurately as it should, though it is usually not dangerous.

Low Engineer only £120-250

What to check first

  1. Switch the boiler off at the on/off switch and wait 5 minutes
  2. Switch it back on and see if the code clears
  3. Check the room thermostat or sensor and its wiring for obvious damage, then call a Viessmann-registered engineer if the code returns

Parts commonly replaced

  • Room temperature sensor (wired or wireless Vitotrol unit)
  • Room sensor connection cable

Related codes

Full explanation for 58
59

Your boiler has stopped because its fan is not reporting its speed correctly, so it has shut itself down as a safety precaution, and a qualified engineer should check it.

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 10 minutes.
  2. Turn the power back on and press the reset button on the boiler or controller once.
  3. Check the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up with the filling loop only if you have been shown how and it is safe to do so.
  4. If the fault returns or the boiler will not restart, stop and call a Gas Safe registered engineer. Do not open the boiler casing or touch the fan wiring yourself.

Parts commonly replaced

  • Fan assembly
  • Fan power or connecting cable
  • Fan speed sensor (Hall sensor)
  • Control PCB (if the fault persists after cable and fan checks)

Related codes

Full explanation for 59
62

Your boiler has paused to recalibrate how it burns gas, which usually means it is not shedding heat efficiently into the water, and it may need a reset and a check of your heating system.

Medium Engineer only £100-250

What to check first

  1. Check the pressure gauge on the boiler and confirm it reads between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the manual.
  2. Bleed the radiators, starting with the lowest, to remove trapped air, then re-check the boiler pressure.
  3. Switch the boiler off at the isolator for 30 seconds, switch it back on, and reset it using the reset button or the control panel. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump
  • Flow temperature sensor
  • Heat exchanger (if scaled or blocked)
  • Gas valve (if calibration repeatedly fails)

Related codes

Full explanation for 62
Showing 11–20 of 131
63

Your boiler has shut itself down because the hot exhaust gases leaving it are running too hot, which usually means the flue is partly blocked or not venting properly, so it needs a safety check before it is used again.

Emergency Engineer only £120-400

What to check first

  1. Turn the boiler off at its on/off switch and do not keep pressing reset, as repeated resets can hide a real safety problem.
  2. Open a window and make sure your carbon monoxide alarm is working, and get out and call the emergency gas line if you smell gas, notice fumes, or the alarm sounds.
  3. From ground level only, check the flue outlet outside the property for visible debris, leaves, nests or anything blocking it, and do not climb ladders or touch the flue.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue pipe or terminal section
  • Heat exchanger (in more serious cases)

Related codes

Full explanation for 63
64

The boiler's flame has gone out unexpectedly while starting up or running, so it has shut itself down as a safety measure and may not be providing heating or hot water until it is checked.

High Engineer only £120-280

What to check first

  1. Check that the gas supply is on, including the gas meter emergency control and the boiler's own isolation valve, and that other gas appliances in the home are working.
  2. Check that the boiler's pressure gauge reads roughly 1 to 1.5 bar when cold, and top it up only if you know how to do so safely.
  3. Press and hold the reset button for a few seconds to try to restart the boiler once. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Ignition unit
  • Gas valve
  • Fan

Related codes

Full explanation for 64
67

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 £90-200

What to check first

  1. Check that the gas supply is on, including the gas meter and any isolation valve, and that other gas appliances work.
  2. Check the boiler pressure gauge and top it up only if it is below 1 bar, following the manufacturer's filling instructions.
  3. Press and hold the reset button for about 3 seconds (or use the reset option on the controls) to try one restart, and wait 10 minutes before trying again. If the fault returns, stop and call a Gas Safe engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ionisation electrode gasket
  • Burner gasket

Related codes

Full explanation for 67
68

The boiler's flame sensor is detecting a flame when it should be off, so it has locked out to stay safe until an engineer checks it.

High Engineer only £120-350

What to check first

  1. Check the pressure gauge on the boiler and top it up to between 1 and 1.5 bar if it is low, using the filling loop, following the manufacturer's instructions.
  2. Turn the boiler off at the isolator switch or fused spur, wait 60 seconds, then turn it back on and press the reset button once.
  3. If the code returns, or if you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999 from outside.

Parts commonly replaced

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

Related codes

Full explanation for 68
69

The boiler has detected that its flame sensor is not reading correctly, so it has shut itself down as a safety precaution, and it will need a gas engineer to check the burner and controls.

High Engineer only £90-250

What to check first

  1. Check the boiler display for pressure below 1 bar and top it up using the filling loop if needed (the reading should sit between 1 and 1.5 bar)
  2. Wait a few minutes, then press the reset button on the boiler or the reset option on the controls once
  3. If the fault returns straight away or the boiler will not restart, stop and call a Gas Safe registered engineer

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Ignition electrode
  • Gas valve
  • Printed circuit board

Related codes

Full explanation for 69
70

Your boiler's internal control module has detected a fault and has stopped working as a precaution, so heating and hot water may be off until an engineer checks it.

High Engineer only £250-500

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait 60 seconds.
  2. Switch the power back on and wait a few minutes to see if the boiler restarts and clears the code.
  3. Check the pressure gauge on the boiler and, if it is below 1 bar, top it up using the filling loop as shown in the manual.
  4. If the code returns or the boiler will not restart, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Main control PCB
  • Boiler control board connections or wiring harness

Related codes

Full explanation for 70
71

Your boiler's fan isn't spinning fast enough to safely burn gas, so the boiler has shut itself down as a precaution.

High Engineer only £150-400

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 minutes, then switch it back on and check whether 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 obstruction you can reach safely.
  3. Check that the condensate pipe is not frozen or blocked, and that the boiler's water pressure is between 1 and 1.5 bar on the gauge.
  4. If the fault returns after a reset, leave the boiler off and call a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • Fan assembly
  • Fan speed control PCB
  • Flue and air intake seals

Related codes

Full explanation for 71
72

The boiler's fan has not slowed down to its resting speed when expected, so the boiler has paused itself as a safety precaution and may need a qualified engineer to check it.

High Engineer only £180-420

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 seconds, then switch it back on and check whether it restarts.
  2. Check that the flue outlet outside the house is not blocked by leaves, debris, nests or snow, and clear any obstruction you can safely reach.
  3. Check that the condensate pipe running from the boiler to the outside drain is not frozen or blocked, and that the condensate trap is not obviously clogged.
  4. If the fault returns after a reset, stop and contact a Gas Safe registered engineer rather than attempting further repairs.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor (Hall sensor)
  • Gas valve and fan combination unit
  • Condensate trap
  • Main PCB control board

Related codes

Full explanation for 72
73

Your boiler's internal parts are having trouble talking to each other, so it may shut down or stop working properly until a engineer checks the connections or control unit.

High Engineer only £150-400

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 10 minutes.
  2. Switch the power back on and check whether the display clears the code and the boiler restarts.
  3. If the fault returns or the boiler will not heat or supply hot water, turn it off and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit circuit board (PCB)
  • Vitotronic control module
  • BUS communication cable or connector
  • LON communication module

Related codes

Full explanation for 73
74

The boiler has shut down because there isn't enough water pressure in the heating system, which is usually caused by a small loss of water that can often be topped up.

High DIY-safe £90-200

What to check first

  1. Check the pressure gauge on the front of the boiler. With the system cold, the needle should sit between 1 and 1.5 bar. If it's below 1 bar, the system needs topping up.
  2. Locate the filling loop, which is usually a flexible silver braided hose with two small levers or valves at the bottom left of the boiler. Make sure both ends are closed before you start.
  3. Open the filling loop valves slowly, watching the pressure gauge. Close them once the needle reaches about 1.2 to 1.5 bar, then check the reading has held.
  4. Restart the boiler using the reset button or menu. If the pressure keeps dropping or the fault returns within a few days, call a Gas Safe registered engineer to find and fix the leak.

Parts commonly replaced

  • Expansion vessel
  • Pressure relief valve
  • Filling loop
  • Radiator valve or pipe fitting (if leaking)

Related codes

Full explanation for 74
Showing 21–30 of 131
75

Your boiler has stopped because it is not detecting enough water moving through it, so it has shut down to stay safe and needs an engineer to find out why.

High Engineer only £120-350

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read roughly 1 to 1.5 bar when cold, and if it is low, top it up using the filling loop as shown in the handbook.
  2. Make sure the isolation valves on the heating pipework under the boiler are fully open.
  3. Bleed any radiators that are cold at the top, then check the pressure again and top up if needed.
  4. Press the reset button once and wait a few minutes to see whether the boiler restarts. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Heating circulation pump
  • System or boiler filter/strainer
  • Flow or pressure sensor

Related codes

Full explanation for 75
77

Your boiler's main control board has detected a problem with the memory it uses to store its settings and operating records, so it has likely paused to protect itself until an engineer checks it.

High Engineer only £250-550

What to check first

  1. Note the exact code shown on the boiler display and check whether the boiler is still heating or providing hot water.
  2. Turn the boiler off at its isolator switch or fused spur, wait about 60 seconds, then switch it back on to see whether the code clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, giving them the fault code 77 and the model number Vitodens 050-W B0HA-19.

Parts commonly replaced

  • Heat management unit (HMU) circuit board
  • Coding card
  • Control unit software or data module

Related codes

Full explanation for 77
78

Your boiler's main control panel has lost its connection to the unit that runs the heating and hot water, so the boiler may have stopped working until the link is restored.

High DIY-safe £150-350

What to check first

  1. Switch off the boiler using its on/off switch and turn off the power at the fused spur or isolator for about 5 minutes.
  2. Switch the power back on and wait several minutes for the boiler to start up and reconnect its display.
  3. Check whether the fault code has cleared and the heating and hot water are working again.
  4. If code 78 returns, or the boiler will not restart, contact a Gas Safe registered engineer, as the connecting cable or control unit may need checking or replacing.

Parts commonly replaced

  • Operating unit (control panel)
  • Connecting data cable between control unit and operating unit
  • Heat management unit (control board)

Related codes

Full explanation for 78
87

The water pressure in your heating system has risen above the safe level, so the boiler has switched off to protect itself, and it needs checking before it is used again.

Emergency Engineer only £150-350

What to check first

  1. Turn the boiler off using its on/off switch and leave it to cool completely for at least 30 minutes.
  2. Check the pressure gauge on the front of the boiler. A normal cold reading is roughly 1 to 1.5 bar.
  3. If the reading is clearly above 2 bar, slowly open a radiator bleed valve over a towel or bucket to release a small amount of water, then close it. Stop when the gauge is back in the normal range.
  4. Do not restart the boiler repeatedly. If the pressure rises again or the fault returns, call a Gas Safe registered engineer, as the expansion vessel or pressure relief valve may have failed.

Parts commonly replaced

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

Related codes

Full explanation for 87
89

The boiler's internal pump that moves hot water around your heating has become stuck or obstructed, so it may not be heating your home or water properly until it is checked.

High Engineer only £200-400

What to check first

  1. Check the boiler pressure gauge and top it up to about 1.0 to 1.5 bar using the filling loop if it is low, then restart the boiler.
  2. Turn the boiler off at the switch, wait 10 minutes, then turn it back on to see if the fault clears.
  3. Bleed any radiators that are cold at the top, then check the pressure again, as trapped air can affect pump operation.
  4. If the code returns or the boiler still does not heat, stop and call a Gas Safe registered engineer rather than opening the pump.

Parts commonly replaced

  • Internal circulation pump
  • Pump seal or gasket kit
  • Pump control module

Related codes

Full explanation for 89
95

Your boiler isn't receiving signals from its room thermostat or controller, so it may not be heating your home as precisely as it should, though it's not a safety concern.

Medium DIY-safe £80-200

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur, then wait 2 minutes before switching it back on.
  2. Check that your room thermostat or controller has working batteries and is switched on, and replace the batteries if needed.
  3. Look at the thermostat's wiring at its base and the wall, and make sure no wires have come loose. Do not open the boiler casing.
  4. Switch the boiler back on and check whether the code clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • Thermostat connection cable
  • Boiler control PCB (uncommon)

Related codes

Full explanation for 95
104

Your boiler has received a signal from an external safety or control device connected to it, which has caused it to pause, so it may not be heating or providing hot water until that signal is cleared.

High Engineer only £80-200

What to check first

  1. Check whether any external device connected to the boiler, such as a room thermostat, alarm, or safety switch, is showing a fault or has been tripped, and reset it if it is safe to do so.
  2. Turn the boiler off at its isolator switch or fused spur, wait about 30 seconds, then switch it back on to see whether the fault clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the fault input circuit needs professional diagnosis.

Parts commonly replaced

  • External control or safety device
  • Boiler control PCB
  • Wiring connector or terminal block

Related codes

Full explanation for 104
142

Your boiler's internal parts are having trouble talking to each other, so it has paused to protect itself, and a qualified engineer can usually restore it.

High Engineer only £150-450

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait 30 seconds, then switch it back on to see if the fault clears.
  2. Check that the room thermostat or controller wiring, if accessible from outside the boiler, is firmly plugged in and undamaged.
  3. If the fault returns, stop and call a Gas Safe registered engineer rather than opening the boiler casing.

Parts commonly replaced

  • Vitotronic control unit (PCB)
  • CAN bus connection cable or plug
  • Room thermostat or external communication module

Related codes

Full explanation for 142
161

The boiler's main control board has lost access to some of its stored settings, so it has stopped to protect itself, and a qualified engineer will usually need to check or replace the part.

High Engineer only £250-450

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait about one minute.
  2. Turn the power back on and check whether the display clears the code and the boiler restarts.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the control board usually needs checking or replacing.

Parts commonly replaced

  • HBMU heat management unit (main control PCB)
  • Control unit memory module

Related codes

Full explanation for 161
163

The boiler's internal memory check has found a data mismatch in its heating control unit, so it has stopped as a precaution and will likely need a power reset or a qualified engineer to check the control system.

High Engineer only £250-500

What to check first

  1. Switch the boiler off at its on/off switch and wait 2 minutes.
  2. Switch the boiler back on and check whether the code clears and the boiler restarts.
  3. Check the pressure gauge reads between 1.0 and 1.5 bar, topping up only if the manufacturer's filling instructions are followed.
  4. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU) control board
  • Main control PCB
  • Control unit memory module

Related codes

Full explanation for 163
Showing 31–40 of 131
182

Your boiler has detected a fault with one of its water temperature sensors, so it has likely stopped to protect itself, and a Gas Safe engineer should check it so your heating and hot water can be restored.

High Engineer only £120-220

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 handbook.
  2. Turn the boiler off at its power switch or fused spur, wait about 30 seconds, then switch it back on and press the reset button if one is displayed.
  3. If the fault returns, stop and call a Gas Safe registered engineer. Do not open the casing or disconnect the sensor yourself.

Parts commonly replaced

  • Flow temperature sensor (NTC outlet sensor)
  • Sensor wiring harness
  • Control PCB (if the sensor and wiring are fine)

Related codes

Full explanation for 182
183

Your boiler has lost the signal from a sensor that measures how hot the water leaving it is, so it has stopped as a safety precaution and needs a qualified engineer to check the sensor or its wiring.

High Engineer only £120-220

What to check first

  1. Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar if it's low, using the filling loop as shown in your manual.
  2. Switch the boiler off at the isolator, wait 5 minutes, then switch it back on and see if the boiler restarts.
  3. If the fault code returns, stop trying to fix it yourself and call a Gas Safe registered engineer, giving them the code 183.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness or connector

Related codes

Full explanation for 183
184

Your boiler has shut itself down as a precaution because it has detected a fault with one of its temperature sensors, so it isn't currently providing heating or hot water, but this is usually fixable by a qualified engineer.

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 your manual, then stop.
  2. Press the reset button once (or switch the boiler off and on at the mains) and wait 10 minutes to see if it restarts.
  3. If the fault code returns, do not keep resetting the boiler. Contact a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 184
299

Your boiler's internal clock has lost track of the time or date, so timed heating and hot water schedules may not run correctly, but the boiler itself is usually still safe to use.

Low DIY-safe £90-250

What to check first

  1. Check the date and time shown on the boiler's control panel and see whether they are wrong.
  2. Use the control dial and buttons to set the correct date, time and day of the week, following the menu in your user manual.
  3. Confirm your heating and hot water timer programmes are still correct, then switch the boiler off at the control panel for 30 seconds and back on to clear the message.

Parts commonly replaced

  • Control unit battery or clock backup component
  • Control unit (Vitotronic board) if the clock hardware has failed

Related codes

Full explanation for 299
346

Your boiler has stopped itself because it could not finish checking how it burns gas safely, which usually means a small adjustment or cleaning is needed rather than a major failure.

High Engineer only £120-280

What to check first

  1. Check the boiler pressure gauge and make sure it reads between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in your manual.
  2. Look at the outside flue terminal and make sure nothing is blocking it, such as leaves, nesting material or snow. Only check what you can safely see from ground level.
  3. Press the reset button on the boiler once, wait 10 minutes, and see if it restarts. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner gauze assembly

Related codes

Full explanation for 346
347

Your boiler has detected a problem with how it removes exhaust gases, so it has likely shut down to stay safe, and it needs a Gas Safe registered engineer to check it.

Emergency Engineer only £90-300

What to check first

  1. Turn the boiler off at its control panel and do not keep trying to restart it.
  2. Open windows and doors to ventilate the room, and keep people and pets away from the boiler while you wait.
  3. Call a Gas Safe registered engineer to inspect the flue and the boiler, and leave any further work to them.

Parts commonly replaced

  • Flue terminal
  • Flue gas recirculation pipe
  • Combustion fan
  • Flue gas temperature sensor

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 it needs a Gas Safe engineer to check it.

High Engineer only £200-400

What to check first

  1. Check that your gas supply and other appliances are working normally, and that you have not run out of gas or credit on a prepayment meter.
  2. Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar using the filling loop if it is low, then follow the manufacturer's instructions for the filling loop.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. If the fault returns or you smell gas, turn off the gas supply, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Gas modulation valve
  • Gas valve coil
  • Gas valve gasket or seal
  • Boiler control PCB

Related codes

Full explanation for 348
349

Your boiler has shut itself down because it cannot detect the right amount of air moving through it, which is a safety precaution and usually means something is blocking the airflow or a part needs checking.

High Engineer only £150-450

What to check first

  1. Switch off the boiler at its on/off switch and wait about 10 minutes, then switch it back on and press reset once. If the code returns, do not try again.
  2. Check that the flue outlet on the outside wall is not blocked by leaves, nesting material, snow or debris, and that nothing is stacked against it. Only do this from ground level or a secure position and never touch the boiler's internal components.
  3. Check that the boiler's room has adequate ventilation and that any air vents are not blocked.

Parts commonly replaced

  • Combustion fan assembly
  • Air/gas mixing unit (venturi)
  • Flue or air intake seals

Related codes

Full explanation for 349
350

Your boiler has stopped because it can't reliably detect its gas flame, so it has shut down as a safety precaution, and it usually needs a qualified engineer to clean or replace a small sensor.

High Engineer only £100-220

What to check first

  1. Wait 10 minutes, then press the reset button on the boiler or the reset option on the display once.
  2. Check that the gas supply is on by confirming your gas meter valve and any other gas appliances work normally.
  3. Check that the boiler's pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if your handbook says so.
  4. If the fault returns after one reset, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Burner
  • Control PCB (less common)

Related codes

Full explanation for 350
351

Your boiler has detected that its flame sensor isn't reading the gas flame correctly, so it has probably shut itself down as a safety precaution, and an engineer can usually restore it.

High Engineer only £90-180

What to check first

  1. Check the boiler's pressure gauge: if it is below 1 bar, top it up using the filling loop, following the boiler manual.
  2. Make sure the gas supply to the boiler is turned on at the isolation valve, and check that other gas appliances in the home are working.
  3. Press the reset button once on the boiler, wait 5 minutes, and see whether it restarts. If the 351 code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Electrode gasket
  • Gas valve

Related codes

Full explanation for 351
Showing 41–50 of 131
352

Your boiler is detecting more carbon monoxide in its exhaust than it should, which is a safety concern, so turn it off, open windows, and call a Gas Safe registered engineer straight away.

Emergency Engineer only £150-600

Parts commonly replaced

  • Burner
  • Heat exchanger
  • Combustion fan
  • Gas valve
  • Flue terminal or flue pipework

Related codes

Full explanation for 352
353

Your boiler isn't getting as much gas as it needs, so it's running at reduced power, which usually means it still works but may heat your home and water less effectively until the gas supply is checked.

Medium Engineer only £80-200

What to check first

  1. Check whether any other gas appliances in your home, such as a cooker or fire, are also not working properly or have a flame that looks weak or yellow.
  2. If you have a prepayment gas meter, check that it has credit and that the display shows no error.
  3. Make sure the boiler's gas isolation valve, usually a lever under or beside the boiler, is fully open in line with the pipe.
  4. If you smell gas at any time, leave the property immediately, do not use switches or flames, and call the National Gas Emergency number on 0800 111 999 from outside.
  5. If the fault remains after these checks, turn the boiler off at the control and contact a Gas Safe registered engineer rather than adjusting any gas components yourself.

Parts commonly replaced

  • Gas inlet filter
  • Gas valve
  • Gas pressure switch
  • Gas supply regulator (if the fault is on the supplier side)

Related codes

Full explanation for 353
354

Your boiler has detected that its gas control part is not adjusting the gas flow correctly, so it has stopped to stay safe, and it needs a qualified engineer to check it.

High Engineer only £180-420

What to check first

  1. Check that the gas supply tap to the boiler is fully open and that other gas appliances in the home are working.
  2. Check the boiler pressure gauge and make sure the pressure is within the range shown in the manual, usually around 1 to 1.5 bar when cold.
  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

  • Gas modulating valve
  • Gas/air ratio control unit
  • Gas filter
  • Burner gasket or seal

Related codes

Full explanation for 354
355

Your boiler has detected a flame signal when it should not be there, so it has stopped as a precaution and needs a qualified engineer to check it before it is used again.

High Engineer only £150-400

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 line on 0800 111 999.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait about 5 minutes.
  3. Switch the power back on and press the reset button or use the reset option on the boiler display once. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 355
357

Your boiler's gas valve didn't open when it should, so the boiler can't light and you may have no heating or hot water until it's checked.

High Engineer only £180-400

What to check first

  1. Check whether your other gas appliances, such as the hob, are working, to confirm the gas supply is on.
  2. Make sure the gas emergency control valve at your gas meter is fully open.
  3. Reset the boiler once using the reset button and wait 10 minutes to see if it restarts.
  4. If the fault returns or the boiler still won't light, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve assembly (gas solenoid valve)
  • Gas valve diaphragm
  • Gas filter

Related codes

Full explanation for 357
359

Your boiler is trying to light its burner but cannot get it started, so it has stopped working and is not providing heating or hot water until a qualified engineer checks it.

High Engineer only £180-350

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 gas meter emergency control valve and the boiler's gas isolation valve are fully open.
  3. Check the boiler pressure gauge and top it up only if it is below 1 bar, following the manufacturer's instructions.
  4. Press the reset button once on the boiler control panel, wait a few minutes, and see if it restarts.
  5. If you smell gas at any time, turn off the gas supply, open windows, leave the property, and call the National Gas Emergency number on 0800 111 999.

Parts commonly replaced

  • Ignition module (HBMU control unit)
  • Ignition electrode
  • Ignition lead
  • Gas valve

Related codes

Full explanation for 359
361

Your boiler started its burner but could not detect a steady flame, so it has stopped as a safety precaution and is not providing heating or hot water until it is checked.

High Engineer only £90-180

What to check first

  1. Check the boiler pressure gauge and top it up to the recommended level (usually around 1.0 to 1.5 bar) if it is low, following the boiler manual.
  2. Check that your gas supply is on by confirming other gas appliances, such as the hob, are working. If they are not, contact your gas supplier, not the boiler engineer.
  3. Press the reset button once on the boiler, wait a few minutes, and see whether it restarts. If it locks out again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 361
364

Your boiler's internal control system has detected a fault and may have shut down, so heating or hot water may be unavailable until a qualified engineer checks it.

High Engineer only £180-450

What to check first

  1. Check the boiler display for water pressure and top it up to between 1.0 and 1.5 bar if it is low, using the filling loop.
  2. Switch the boiler off at its power switch, wait 10 minutes, then switch it back on to see whether the fault clears.
  3. If the code returns, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Burner control unit
  • Flame detection electrode

Related codes

Full explanation for 364
365

Your boiler has shut itself down as a safety precaution because its gas control system cannot confirm that the gas supply has switched off properly, so a qualified engineer needs to check it before it is used again.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve (gas block) assembly
  • Gas valve relay or control PCB
  • Gas valve wiring harness

Related codes

Full explanation for 365
366

Your boiler's safety system has noticed that a part controlling the gas supply is not switching off when it should, so please turn the boiler off and contact a Gas Safe registered engineer promptly.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve (gas block)
  • Boiler control PCB (circuit board)
  • Gas valve wiring harness

Related codes

Full explanation for 366
Showing 51–60 of 131
367

The boiler's gas shut-off valve may not be closing properly when it should, which is a safety concern, so the boiler should be switched off and a Gas Safe engineer should check it as soon as possible.

Emergency Engineer only £150-450

Parts commonly replaced

  • Gas valve assembly
  • Main PCB (control board)
  • Gas valve wiring harness

Related codes

Full explanation for 367
369

Your boiler has stopped because it is not detecting its flame properly, so it has shut itself down as a safety precaution and needs a qualified engineer to check it.

High Engineer only £100-220

What to check first

  1. Turn off the boiler at its on/off switch and wait about 10 minutes.
  2. Check that the gas supply is on by confirming that other gas appliances, such as the hob, work.
  3. Check the boiler pressure gauge and top it up only if it is below the recommended level shown in the manual.
  4. Turn the boiler back on and try one reset. If code 369 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 369
370

The boiler has shut itself down as a safety precaution because a part that controls the gas flow may not be turning off properly, so a qualified engineer needs to check it before it is used again.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve
  • Modulation valve (gas-air ratio control)
  • Ignition electrode
  • Control PCB

Related codes

Full explanation for 370
371

Your boiler has stopped its burner because its internal fan is not spinning fast enough to run safely, so it may not be producing heat or hot water until a qualified engineer looks at it.

High Engineer only £150-350

What to check first

  1. Check that the flue outlet on the outside wall is not blocked by leaves, debris, nests or snow.
  2. Check that the condensate pipe running from the boiler is not frozen or blocked, and clear it if you can safely reach it.
  3. Make sure other gas appliances in the home are working, and check that your gas supply or meter has not been turned off.
  4. Reset the boiler once using the reset button or the reset option on the display, then wait a few minutes to see if it restarts.
  5. If the fault returns or the boiler will not restart, turn off the power to the boiler and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor
  • Gas valve
  • Printed circuit board (PCB)

Related codes

Full explanation for 371
372

Your boiler's burner keeps going out while it is setting itself up, so it has stopped to protect itself and needs a check before it will heat your home and water again.

High Engineer only £120-280

What to check first

  1. Check that the gas supply is turned on at the meter and that other gas appliances work.
  2. Check the pressure gauge on the front of the boiler. It should read between 1 and 2 bar when cold. If it is low, follow the filling instructions in the handbook.
  3. Make sure the flue outlet outside is not blocked by leaves, debris or snow, and that the condensate pipe is not frozen.
  4. Switch the boiler off at the isolator or power switch, wait 30 seconds, then switch it back on to reset it once. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Gas valve
  • Combustion fan
  • Gas/air ratio control unit

Related codes

Full explanation for 372
373

Your boiler is trying to fine-tune how it burns gas, but it has not been able to complete the check, usually because heat is not moving around the system as well as it should.

Medium DIY-safe £90-250

What to check first

  1. Look at the pressure gauge on the front of the boiler. The needle should sit between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, follow the filling instructions in your boiler manual or ask an engineer to top it up.
  2. Bleed your radiators using a radiator key, starting with the radiator closest to the boiler and working through the house. Air trapped in radiators restricts water flow, and it is a common cause of this fault.
  3. Make sure all radiator valves and thermostatic valves are fully open, and check that the boiler's system filter or any isolation valves are not closed. Then switch the boiler off at the mains for 30 seconds, switch it back on, and press the reset button once. If the fault returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Condensate trap
  • Heat exchanger (if blocked or heavily scaled)
  • Gas valve (rarely required)

Related codes

Full explanation for 373
374

Your boiler has briefly switched off and restarted because the gas flame isn't steady enough yet, which often happens when the gas supply or flame detection is slightly unstable, and a qualified engineer can usually find and fix the cause.

Medium Engineer only £90-220

What to check first

  1. Press the reset button on the boiler (or use the reset option on the display) once, then wait 10 minutes to see whether the fault clears.
  2. Check the boiler pressure gauge and make sure it reads between 1.0 and 1.5 bar when cold; if it is low, use the filling loop as your boiler manual describes.
  3. Check that other gas appliances (such as a hob or fire) aren't being turned on and off at the same time as the boiler, and that the flue outlet outside is not blocked by leaves, snow or debris.
  4. If the fault returns or the boiler keeps locking out, turn the boiler off and book a Gas Safe registered engineer rather than repeating resets.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Gas valve
  • Gas pressure switch

Related codes

Full explanation for 374
375

The boiler is struggling to confirm that its gas flame is burning properly, so it keeps switching off and trying to restart, which usually means a flame-sensing part needs cleaning or adjusting by a engineer.

Medium Engineer only £120-260

What to check first

  1. Turn the boiler off at its on/off switch and wait 10 minutes.
  2. Check the boiler pressure gauge reads between 1.0 and 1.5 bar when cold, and top up using the filling loop only if it is low.
  3. Switch the boiler 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

  • Ionisation electrode
  • Ignition electrode
  • Burner gasket
  • Flame sensing lead

Related codes

Full explanation for 375
376

The boiler's flame sensor is giving a reading that doesn't match what it expected, so the boiler may switch itself off as a safety precaution until the sensor is checked.

High Engineer only £90-180

What to check first

  1. Turn the boiler off at its power switch, wait 30 seconds, then turn it back on and press the reset button on the display if one is shown.
  2. Check that the gas supply is turned on at the meter and the boiler's gas isolation valve, and that the boiler's pressure gauge reads between 1 and 1.5 bar when cold.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the flame sensor and burner must not be opened or cleaned by a homeowner.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ignition electrode assembly
  • Burner gasket

Related codes

Full explanation for 376
377

Your boiler's burner has lost its automatic setup check, so it has shut down as a precaution and will need a qualified engineer to reset and adjust it before it can safely heat your home again.

High Engineer only £120-250

What to check first

  1. Check that the gas supply and the electricity to the boiler are switched on, and that the pressure gauge reads roughly 1 to 1.5 bar.
  2. Look outside at the flue terminal and make sure it is clear of leaves, nests, snow or other debris, then press the reset button once on the boiler.
  3. If the fault returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 377
Showing 61–70 of 131
378

Your boiler's burner has briefly gone out while it was trying to start or run, so it has shut itself down as a safety precaution and may not be heating your home or water until it's checked.

High Engineer only £100-300

What to check first

  1. Wait about 10 minutes, then press the reset button on the boiler or the reset option on its display once.
  2. Check that the gas supply is on: make sure the gas meter or supply valve is open and that other gas appliances, such as a cooker or hob, are working.
  3. Check the flue terminal outside the house for blockages such as leaves, nests, snow or debris, and check the boiler's pressure gauge reads about 1 to 1.5 bar.
  4. If the fault returns after one reset, stop resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 378
379

Your boiler's burner has failed to detect a steady flame, 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 £120-250

What to check first

  1. Check that other gas appliances, such as the cooker or fire, are working, which confirms the gas supply is on.
  2. Check the boiler display for the fault code, then press the reset button once. If the boiler does not restart or the code returns, stop and call a Gas Safe registered engineer.
  3. Check that the external flue terminal is not blocked by leaves, snow or debris, and that the boiler has not been switched off at the mains or lost power recently.

Parts commonly replaced

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

Related codes

Full explanation for 379
380

Your boiler has shut itself down for safety because it is having trouble detecting its gas flame, which means it is probably not giving you heating or hot water until a registered engineer checks it.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch, wait five minutes, then switch it back on and press reset once. If it fails again, stop and call a Gas Safe engineer.
  2. Check that your gas supply is on by seeing whether your cooker or hob works, and make sure your gas meter's emergency control valve has not been turned off.
  3. 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

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

Related codes

Full explanation for 380
381

Your boiler's burner went out unexpectedly while it was running, so it has shut itself down as a safety precaution, and it will usually need a reset and a check before it will heat your home or water again.

High Engineer only £90-250

What to check first

  1. Check that the gas supply is on, including the gas meter and the isolation valve at the boiler, and check that other gas appliances such as the hob still work.
  2. Check that the flue outlet on the outside wall is clear of leaves, nests, snow or other debris, and that the condensate pipe is not frozen or blocked.
  3. Reset the boiler using the reset button or the reset function on the display, wait a few minutes, and see whether it restarts. If the fault returns, stop and call a Gas Safe registered engineer.
  4. If you smell gas at any time, leave the property, do not use switches or flames, and call the National Gas Emergency number on 0800 111 999 straight away.

Parts commonly replaced

  • Flame detection electrode (ionisation probe)
  • Ignition electrode
  • Gas valve
  • Combustion fan
  • Gas/air mixing unit
  • Boiler control PCB

Related codes

Full explanation for 381
382

Your boiler has repeatedly hit the same fault and has locked itself out to protect itself, so it will need a qualified engineer to find the cause before heating and hot water return to normal.

High Engineer only £100-300

What to check first

  1. Check the boiler pressure gauge; it should read between 1.0 and 1.5 bar when cold, and top it up using the filling loop only if it is below 1.0 bar.
  2. Turn the boiler off at its power switch, wait 60 seconds, then switch it back on and press the reset button once if the boiler shows a reset prompt.
  3. If the fault returns straight away or the boiler locks out again, stop resetting it, note the display code, and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Gas valve
  • Fan assembly
  • Printed circuit board

Related codes

Full explanation for 382
383

The boiler has detected a possible blockage or dirt in its gas supply, which can affect how safely it burns gas, so it should be switched off and checked by a Gas Safe engineer.

Emergency Engineer only £120-350

Parts commonly replaced

  • Gas filter
  • Gas valve (gas multiblock)
  • Gas pressure switch

Related codes

Full explanation for 383
384

Your boiler has detected possible contamination in its gas supply, which needs a qualified Gas Safe engineer to check before the boiler is used again.

Emergency Engineer only £150-400

Parts commonly replaced

  • Gas filter
  • Gas valve
  • Gas supply pipework seals

Related codes

Full explanation for 384
385

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

High Engineer only £250-500

Parts commonly replaced

  • HBMU heat management unit
  • Ionisation electrode
  • Ignition electrode and lead
  • Burner gasket

Related codes

Full explanation for 385
386

Your boiler's central control unit has detected a problem with a part that manages how it distributes heat, so it may stop heating or hot water until an 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 and 1.5 bar when cold. If it is low, follow your boiler manual to top it up, or call an engineer if you are unsure.
  2. Turn the boiler off at its on/off switch and at the fused spur or isolator for about 60 seconds, then switch it back on.
  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
  • Boiler control PCB
  • Control unit wiring harness

Related codes

Full explanation for 386
387

Your boiler has shut itself down because its flame sensing circuit has detected an electrical fault, and it needs a qualified engineer to find and fix the cause before it can safely run again.

High Engineer only £250-550

What to check first

  1. Check that the boiler still has power and that its main on/off switch and fused spur are on.
  2. Wait 5 minutes, then press the reset button on the boiler or follow the reset instructions in the user manual once.
  3. If the fault code returns, stop and call a Gas Safe registered engineer rather than repeating the reset.

Parts commonly replaced

  • Heat management unit (HBMU) circuit board
  • Ionisation (flame sensing) electrode
  • Ignition electrode and lead
  • Earth wiring or connector repair

Related codes

Full explanation for 387
Showing 71–80 of 131
388

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

High Engineer only £350-650

What to check first

  1. Check the boiler pressure gauge. If it reads below 1 bar, top it up using the filling loop, following the boiler manual, then stop once it reaches about 1.5 bar.
  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 and see whether the fault clears.
  3. If code 388 returns, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU) control board
  • Boiler main PCB
  • Associated wiring harness or connectors

Related codes

Full explanation for 388
393

A sensor that checks the temperature of the boiler's exhaust gases has sent a faulty signal, so the boiler has most likely stopped as a precaution until a qualified engineer checks it.

High Engineer only £120-220

What to check first

  1. Note the exact code shown on the display and check whether the boiler has locked out or is still working.
  2. Check the water pressure gauge on the boiler. It should read between 1 and 1.5 bar. If it is low, top it up using the filling loop as shown in the manual, then reset the boiler.
  3. Try one reset: press the reset button or switch the boiler off at its power supply for 30 seconds, then switch it back on. If the code returns, stop and call a Gas Safe registered engineer.
  4. Check that the flue terminal outside the property is not blocked by debris, nesting material or snow, and clear it only if it is safe and easy to reach.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor wiring harness or connector
  • Main control PCB (if the sensor and wiring are fine)
  • Flue seals or gaskets (if the flue has been disturbed or is damaged)

Related codes

Full explanation for 393
394

The boiler has lost the signal from a sensor that monitors the temperature of the hot exhaust gases leaving through the flue, so it has likely shut itself down as a safety precaution.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at the isolator switch or fused spur, wait 10 minutes, then turn it back on to clear the fault once.
  2. Check that the flue terminal outside the property is not blocked by leaves, nests, snow or debris, and that the flue pipe is not damaged or disconnected.
  3. If the fault returns straight away, stop resetting the boiler and book a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 394
399

Your boiler has detected an electrical fault in its flame-sensing and control circuitry, so it has shut itself down as a precaution and will need a qualified engineer to inspect and repair it.

High Engineer only £150-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Switch the power back on and press the reset button once, if your boiler has one. Do not repeatedly reset it if the fault returns.
  3. If the fault code 399 reappears, leave the boiler off and book a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management module
  • Ionisation electrode
  • Ionisation electrode lead and earth connection

Related codes

Full explanation for 399
400

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

High Engineer only £350-650

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and clears the code.
  3. If the code returns or the boiler still does not work, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Main control PCB

Related codes

Full explanation for 400
401

Your boiler has shut itself down because it detected an electrical fault with the sensor that checks the gas flame, so it can't safely heat your home or water until a qualified engineer has looked at it.

High Engineer only £150-500

What to check first

  1. Check that the boiler has power and that the external isolator switch or fused spur is switched on.
  2. Check the central heating pressure gauge on the boiler and top up with the filling loop if it reads below about 1 bar.
  3. Press the reset button or use the reset option on the boiler display once. If the fault returns within a short time, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • HBMU heat management unit (control board)
  • Ionisation electrode lead
  • Earth connection or cable

Related codes

Full explanation for 401
402

Your boiler has stopped working because a control component that manages its heating and hot water has developed a fault, and it needs a qualified engineer to check and repair 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 your manual.
  2. Switch the boiler off at its isolator switch, wait 30 seconds, switch it back on, and press the reset button once.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 402
403

Your boiler has detected an electrical fault in the part that checks the gas flame is lit, so it has stopped working to stay safe until a engineer inspects it.

High Engineer only £150-450

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode cable
  • HBMU heat management unit
  • Earth connection or ignition lead

Related codes

Full explanation for 403
404

Your boiler's internal communication unit has reported a fault, which can stop it from running the heating or hot water until a qualified engineer checks it.

High Engineer only £250-450

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator for about 30 seconds, then switch it back on.
  2. Wait around 10 minutes and check whether code 404 returns on the display.
  3. If the code comes back, note it down and do not open the boiler casing. Book a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit module
  • Connecting cable or harness
  • Control board

Related codes

Full explanation for 404
405

Your boiler has shut itself down as a safety precaution because its flame sensor is reading a wiring or earthing fault, so it can't safely light, and heating and hot water are likely off until a qualified engineer checks it.

High Engineer only £120-650

What to check first

  1. Check the boiler pressure gauge and top it up to about 1.0 to 1.5 bar if it is low, following the filling instructions in the manual
  2. Turn the boiler off at its isolator switch for 60 seconds, then switch it back on
  3. Press the reset button once, or use the reset option on the display, and wait a few minutes to see if the boiler restarts
  4. If the fault code returns or the boiler will not stay on, stop and call a Gas Safe registered engineer

Parts commonly replaced

  • Ionisation electrode
  • Ionisation electrode lead
  • HBMU heat management unit (control board)
  • Earth connection or cable

Related codes

Full explanation for 405
Showing 81–90 of 131
406

Your boiler's central control unit has detected a fault that stops it from managing heating and hot water properly, so a qualified Gas Safe engineer should check it soon.

High Engineer only £300-550

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold; if it is low, top it up using the filling loop as shown in your manual.
  2. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on once to see whether the fault clears.
  3. If the fault returns or the boiler still does not work, stop and book a Gas Safe registered engineer. Do not open the casing or attempt to repair the control unit yourself.

Parts commonly replaced

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

Related codes

Full explanation for 406
408

Your boiler has detected a fault in one of its internal control units, which may stop it from providing heating or hot water, so it needs a qualified engineer to check and repair 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 is below 1 bar, following the boiler manual.
  2. Turn the boiler off at its power supply, wait 5 minutes, then turn it back on and see whether the fault clears.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 408
410

Your boiler's central control unit has detected a fault with one of its internal control modules, so it may have stopped 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. It should read between 1 and 1.5 bar when cold. If it is low, stop here and call an engineer rather than topping it up yourself.
  2. Switch the boiler off at its power isolator or fused spur, wait 5 minutes, then switch it back on to try a reset.
  3. If the code returns or the boiler will not restart, leave it off and book a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Control unit wiring harness
  • Boiler control board fuse

Related codes

Full explanation for 410
416

Your boiler has noticed that a safety sensor in the flue (the pipe that carries fumes outside) isn't sitting in the right place, so it is reporting a fault that a gas engineer can usually put right.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its on/off switch and wait five minutes.
  2. Turn it back on and press the reset button once, if the boiler is showing it is locked out.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer to check the flue sensor position.

Parts commonly replaced

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

Related codes

Full explanation for 416
417

Your boiler's main control unit has detected a fault that stops it from managing heating and hot water properly, so it needs a qualified engineer to check it.

High Engineer only £350-650

What to check first

  1. Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar if it's low, using the filling loop as shown in the manual.
  2. Turn the boiler off at its power switch or fused spur, wait 30 seconds, then switch it back on to see if the fault clears.
  3. If code 417 returns, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Heat management unit (HBMU)
  • Control unit wiring harness
  • Boiler control PCB

Related codes

Full explanation for 417
418

Your boiler's central control unit has detected a fault in one of its internal modules, which usually means it has stopped heating and hot water until a qualified engineer looks at it.

High Engineer only £250-450

Parts commonly replaced

  • Heat management unit (HBMU)
  • Connecting cables and plugs
  • Control PCB (if the HBMU is not the sole cause)

Related codes

Full explanation for 418
425

Your boiler's internal clock has lost track of the correct time, which is usually a minor issue that doesn't stop your heating or hot water working.

Low DIY-safe £0-90

What to check first

  1. Check that your heating and hot water are working normally, as this code usually doesn't affect them.
  2. Use the boiler's control panel to open the date and time settings (check the manual or menu for a clock or time option) and enter the correct date and time.
  3. Keep an eye on the clock over the next few days. If it resets again or the code returns, contact a Gas Safe registered engineer to check the clock's backup power and the control unit.

Parts commonly replaced

  • Control unit clock backup battery
  • Control unit (PCB)
Full explanation for 425
446

Your boiler has shut itself down as a precaution because it has detected a problem with how it measures the water temperature leaving the boiler, so it isn't providing heating or hot water until it's checked.

High Engineer only £150-300

What to check first

  1. Check the system pressure gauge on the boiler; it should read between 1.0 and 2.0 bar when the system is cold. If it is low, top it up using the filling loop as shown in your boiler manual, then close the filling loop valve.
  2. Press the reset button once (or use the reset function on the boiler display), wait a few minutes, and see whether the boiler restarts.
  3. If the fault returns or the pressure keeps dropping, stop and call a Gas Safe registered engineer. Do not open the boiler casing or attempt any gas or electrical repairs yourself.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • High limit safety cut-out (STB)
  • Circulation pump
  • Control PCB

Related codes

Full explanation for 446
447

Your boiler has shut itself down because it cannot reliably detect its gas flame, which is a safety feature and usually fixable by a qualified engineer.

High Engineer only £90-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar, so top up using the filling loop if it is low.
  2. Make sure the gas supply is on, including the meter tap and any emergency control, and that other gas appliances are working.
  3. Press the reset button once, wait about 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
  • Burner gasket
  • Gas valve
  • Control unit (PCB)

Related codes

Full explanation for 447
448

Your boiler has briefly lost track of its flame while it was running, so it has shut itself down as a safety precaution, and a qualified engineer should check it so it can be restarted safely.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its power switch and wait about 10 minutes, then turn it back on and see if it restarts.
  2. Check that your gas supply is working by confirming that your cooker or other gas appliances still light.
  3. Check that the flue outlet on the outside wall is clear of leaves, snow, or other blockages, and that any air vents in the room are not covered.
  4. If the fault returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation/ignition electrode
  • Electrode gasket
  • Burner gasket
  • Gas valve (less commonly)

Related codes

Full explanation for 448
Showing 91–100 of 131
449

Your boiler's timer or schedule has lost track of its settings, which usually means the clock or heating programme needs resetting rather than a serious fault with the heating itself.

Low DIY-safe £100-300

What to check first

  1. Check the time and date on the boiler's control display and correct them if they are wrong.
  2. Check your heating and hot water programme settings and reset them to your usual times if they look incorrect or have changed.
  3. Switch the boiler off at its electrical supply, wait 30 seconds, then switch it back on and see whether code 449 clears.
  4. If the code returns within a few days or the boiler stops heating or providing hot water, stop trying to fix it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Clock backup battery

Related codes

Full explanation for 449
450

Your boiler's internal timer has lost track of its heating schedule, so it may not be switching on and off at the times you set, though it is often still able to heat your home.

Low DIY-safe £90-220

What to check first

  1. Check the boiler display and reset the fault by pressing the reset or back button, as shown in your manual.
  2. Go to the time and date settings in the boiler controls and check they are correct. Set them if they are wrong, and check your heating and hot water schedules.
  3. Switch the boiler off at its on/off switch or isolator, wait about one minute, then switch it back on. If the code returns, or the time resets again, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic)
  • Real-time clock backup battery
  • Main control PCB

Related codes

Full explanation for 450
451

Your boiler's controller has lost track of its scheduled heating and hot water timings, so it may not follow your programme correctly, though it is not a safety problem.

Medium DIY-safe £80-180

What to check first

  1. Check that the date, time and programme settings on the boiler's control panel are correct and reset any that look wrong.
  2. Switch off the boiler at its power supply (the fused spur or isolator switch) for about one minute, then switch it back on. Do not touch any gas components.
  3. Watch the boiler for 15 to 20 minutes to see whether heating and hot water work as expected and the code clears. If the code returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Vitotronic control unit
  • Control unit backup battery (if fitted)

Related codes

Full explanation for 451
452

The boiler's internal timer has detected a problem with its heating schedule, which may mean your programmed heating times are not running correctly, though the boiler itself may still be working.

Medium DIY-safe £90-400

What to check first

  1. Check the date, time and heating schedule on the boiler's display and reset them if they look wrong.
  2. Switch the boiler off at its on/off switch, wait about 30 seconds, then switch it back on to clear the fault.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the control unit may need checking or replacing.

Parts commonly replaced

  • Control unit (main PCB)
  • Clock or timer module
  • Control panel display

Related codes

Full explanation for 452
453

The boiler's internal parts have lost step with each other during start-up or operation, so it has paused to protect itself, and this often clears after a simple restart.

Medium DIY-safe £120-300

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait at least 5 minutes.
  2. Switch the power back on and wait for the boiler to finish its start-up sequence.
  3. Check the pressure gauge on the front of the boiler is between 1 and 1.5 bar, and top up with the filling loop only if it is low and you know how to do it safely.
  4. If the code returns, stop resetting it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler control unit (PCB)
  • Ignition or sequence control module
  • Flow temperature sensor
  • Connecting wiring harness or plug

Related codes

Full explanation for 453
454

Your boiler's control settings for its heat management unit have become mismatched, so it may not run properly until a qualified engineer resets them.

High Engineer only £100-250 for a Gas Safe engineer to correct the settings; £450-750 if the heat management unit itself must be replaced

Parts commonly replaced

  • HBMU heat management unit
  • Control unit PCB

Related codes

Full explanation for 454
455

Your boiler has shut itself down because its control system detected an internal glitch in how it runs, and it is stopping as a safety precaution.

High DIY-safe £200-450

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator nearby, then wait about 5 minutes.
  2. Switch the power back on and press the reset button on the boiler control panel, or hold it as the manual says, to restart the boiler.
  3. Check whether the display shows normal operation and whether heating and hot water return. If the 455 code comes back, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (main PCB)
  • Power supply board
  • Fuse

Related codes

Full explanation for 455
456

Your boiler's internal control system has detected an error in how it runs its own operating sequence, so it has stopped to protect itself and will likely need a qualified engineer to check it.

High Engineer only £300-550

What to check first

  1. Turn 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 press the reset button on the boiler control panel (usually the reset symbol or a 'Reset' option in the menu).
  3. If the code 456 returns or the boiler will not restart, stop and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (main PCB)
  • Control unit fuse

Related codes

Full explanation for 456
457

Your boiler's fan is turning on but the control system isn't getting the speed confirmation it needs, so the boiler has stopped to stay safe, and a Gas Safe engineer should check it.

High Engineer only £250-450

What to check first

  1. Check that the boiler is switched on at its isolation switch and that the power supply and fused spur are working.
  2. Reset the boiler using the reset button or control panel, then wait a few minutes to see whether it restarts.
  3. Make sure the flue outlet outside is clear of leaves, debris, or nests, then contact a Gas Safe registered engineer if the fault code returns.

Parts commonly replaced

  • Combustion fan assembly
  • Fan connection cable or wiring harness
  • Fan gasket or air/gas mixing unit seal

Related codes

Full explanation for 457
458

Your boiler has stopped its burner as a safety precaution, so it may not be heating or providing hot water until it is reset correctly.

High DIY-safe £120-300

What to check first

  1. Check that the boiler pressure gauge reads between 1 and 1.5 bar, and that other gas appliances in the home are working, which confirms the gas supply is on.
  2. Wait about 10 minutes, then press the reset button on the boiler control panel once and allow the boiler a few minutes to restart.
  3. If the fault returns or the boiler will not reset, stop trying and call a Gas Safe registered engineer, as repeated resets can make the fault harder to diagnose.

Parts commonly replaced

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

Related codes

Full explanation for 458
Showing 101–110 of 131
461

Your boiler has shut itself down as a safety precaution because a part that controls the exhaust outlet isn't working correctly, so it isn't heating your home or water until a Gas Safe engineer checks it.

High Engineer only £120-300

What to check first

  1. Check that the outside flue terminal (the pipe opening on an outside wall) isn't blocked by debris, leaves, snow, or a bird's nest, and clear it only if you can reach it safely from the ground or a secure step.
  2. Wait about 10 minutes, then reset the boiler using the reset button on the front panel or by following the reset instructions in the user manual.
  3. If the boiler will not restart or the fault returns, stop trying and book a Gas Safe registered engineer. Do not open the boiler casing or attempt to repair the flue damper yourself.

Parts commonly replaced

  • Flue gas damper assembly
  • Flue gas damper actuator
  • Control unit PCB
  • Flue gas temperature or pressure sensor

Related codes

Full explanation for 461
462

Your boiler has stopped trying to light because a safety check on its gas supply did not give the expected response, so it has paused to stay safe until a qualified engineer looks at it.

High Engineer only £180-380

What to check first

  1. Check that the gas supply tap to the boiler and the gas meter emergency control are fully turned on.
  2. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the boiler is cold. If it is below 1 bar, follow the filling instructions in your manual or contact an engineer.
  3. Press the reset button once (or use the reset function on the boiler display) and wait 10 minutes to see if the boiler restarts. If the fault returns, stop and call a Gas Safe registered engineer.
  4. If you smell gas at any time, open windows, do not use switches or flames, leave the property, and call the National Gas Emergency number on 0800 111 999.

Parts commonly replaced

  • Gas valve assembly
  • Gas valve actuator or diaphragm
  • Boiler control PCB
  • Ignition electrode

Related codes

Full explanation for 462
463

The boiler has shut itself down for safety because it is drawing in some of its own exhaust fumes instead of fresh air, and it needs a Gas Safe engineer to check the flue and air supply before it can be used again.

Emergency Engineer only £150-450

Parts commonly replaced

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

Related codes

Full explanation for 463
464

Your boiler has stopped because its flame sensor is not picking up the gas flame properly during start-up, which is a common fault that a qualified engineer can usually fix.

High Engineer only £120-250

What to check first

  1. Check that other gas appliances, such as the hob, are working, which shows the gas supply is on. If they are not, or you smell gas, leave the property, do not use switches, and call the National Gas Emergency Service on 0800 111 999.
  2. Check the boiler pressure gauge. When the system is 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. Check that the external flue terminal is free from leaves, snow, nests or other blockages, and that the boiler has not been left switched off at the isolator.
  4. Press the reset button or use the reset option on the boiler display, then wait about 10 minutes before trying once more. If the fault returns, stop and book a Gas Safe registered engineer. Do not open the boiler casing or try to clean or adjust the flame sensor yourself.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode and lead
  • Gas valve
  • Burner gasket
  • Fan

Related codes

Full explanation for 464
467

The boiler is shutting itself down because it is not getting enough gas to start and run safely, so it cannot heat your home or water until the gas supply is checked.

High Engineer only £150-450

What to check first

  1. Check whether other gas appliances in your home, such as the hob or a gas fire, also have weak flames or will not light.
  2. If other gas appliances are affected, or you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999 straight away.
  3. If only the boiler is affected, wait a few minutes, then reset it once using the reset button or the reset option on the display.
  4. If the fault returns, do not keep resetting the boiler. Book a Gas Safe registered engineer to check the gas supply pipework, meter and filters.

Parts commonly replaced

  • Gas supply filter
  • Gas valve inlet filter
  • Section of gas supply pipework
  • Gas pressure regulator

Related codes

Full explanation for 467
468

Your boiler has shut itself down because its flame sensor is reading an unexpectedly strong signal during its start-up check, so it cannot safely light the burner until a gas engineer inspects it.

High Engineer only £120-280

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar.
  2. Make sure the gas supply to the boiler is turned on and the gas meter isn't showing a problem.
  3. Switch the boiler off at the isolator or fused spur, wait 5 minutes, switch it back on, and press the reset button once.
  4. If it fails again, stop resetting it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition and ionisation electrode lead
  • Burner gasket
  • Control PCB (mainboard)

Related codes

Full explanation for 468
471

Your boiler has lost its reading of the water pressure in the heating system, so it may shut down as a precaution until a engineer checks the sensor.

High Engineer only £150-300

What to check first

  1. Look at the pressure gauge on the front of the boiler. A reading between 1.0 and 1.5 bar when the system is cold is normal.
  2. If the reading is below 1.0 bar, follow the filling instructions in your user manual to top it up slowly, then close the filling valve.
  3. Switch the boiler off at the mains for a minute, then back on. If the code returns or the boiler will not run, call a Gas Safe registered engineer.

Parts commonly replaced

  • System pressure sensor
  • Sensor wiring lead or harness
  • Control unit (rarely needed)

Related codes

Full explanation for 471
474

Your boiler's internal timer has lost track of one of its scheduled heating or hot water programmes, so it may not be switching on and off at the times you set, though it may still heat your home.

Medium DIY-safe £90-250

What to check first

  1. Switch the boiler off at its on/off switch and at the isolator or fused spur, wait 30 seconds, then switch it back on.
  2. Check the date, time and day settings on the boiler's control panel and correct them if they are wrong.
  3. Re-enter your heating and hot water programme times, then press the reset button on the control panel if the code is still showing.
  4. If the code returns within a day or two, or the boiler stops heating, call a Gas Safe registered engineer.

Parts commonly replaced

  • Vitotronic control unit (PCB)
  • Control unit backup battery

Related codes

Full explanation for 474
527

The boiler's internal control settings don't match the set it expects, so it has stopped to protect itself, and a qualified Gas Safe engineer will need to correct the settings.

High Engineer only £90-250

Parts commonly replaced

  • Coding card
  • Heat management unit (HBMU) board
  • Control unit PCB

Related codes

Full explanation for 527
528

Your boiler's control settings have become mismatched, so it has stopped working and needs a heating engineer to reload the correct settings.

High Engineer only £100-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is below 1 bar, do not attempt a top-up unless you know how, and call an engineer.
  2. Turn the boiler off at its on/off switch and at the fused spur for 30 seconds, then switch it back on once to see whether the code returns.
  3. Note the exact code and any other messages on the display, and check whether other boilers or appliances on the same supply are affected, then call a Gas Safe registered engineer.

Parts commonly replaced

  • HBMU heat management unit
  • Control PCB

Related codes

Full explanation for 528
Showing 111–120 of 131
540

The boiler has shut itself down because waste water from the heating process isn't draining away properly, so it's backing up inside the unit.

High DIY-safe £80-200

What to check first

  1. Turn off the boiler at its power switch and isolate the electricity supply before touching any part of it.
  2. Locate the condensate pipe, which is a small plastic pipe usually running from the bottom of the boiler to an outside drain or waste pipe, and check it for visible blockages, kinks, or ice in cold weather. Gently clear any debris or thaw any ice with warm (not boiling) water.
  3. Check the condensate trap or siphon, if accessible in your manual's instructions, and flush it through with warm water to clear any sludge or debris. Do not open the gas valve, burner, or heat exchanger.
  4. Restore power, reset the boiler following the manual, and check whether it runs normally. If the fault returns, or you cannot access the pipe safely, call a Gas Safe registered engineer.

Parts commonly replaced

  • Condensate trap / siphon
  • Condensate drain pipe
  • Condensate pump (if fitted)
  • Heat exchanger (in rare cases of damage)

Related codes

Full explanation for 540
574

Your boiler has lost contact with the room thermostat or room temperature sensor, so it may not be controlling the heating by room temperature as normal, though it can often still heat your home.

Low DIY-safe £90-200

What to check first

  1. Check the wireless room thermostat or sensor (often a Vitotrol unit) for low batteries and replace them with fresh ones if needed.
  2. Make sure the room sensor is within range of the boiler, is not blocked by metal objects, and is not in direct sunlight, draughts, or near a radiator.
  3. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on and see whether the code clears.
  4. If the code returns, contact a Gas Safe registered engineer to check the sensor wiring and replace the room sensor if faulty. Do not open the boiler casing yourself.

Parts commonly replaced

  • Room temperature sensor or wireless room thermostat (Vitotrol)
  • AAA batteries for wireless room unit
  • Room sensor cable or connection plug

Related codes

Full explanation for 574
575

Your boiler has lost the signal from its room temperature sensor, so it can't read how warm your home is, but it will usually still provide heating and hot water.

Low DIY-safe £80-180

What to check first

  1. Note the code 575 shown on the boiler display, then switch the boiler off at its on/off switch and wait about 10 seconds before switching it back on.
  2. Find the room temperature sensor or room control unit (usually a wall-mounted Vitotrol or thermostat in your main living area). Check that its batteries are fresh and that any wires or plugs connected to it are secure.
  3. Restart the boiler and check whether code 575 returns. If it does, contact a Gas Safe registered engineer, as the sensor or its cable will probably need replacing.

Parts commonly replaced

  • Room temperature sensor or room control unit
  • Sensor connection cable

Related codes

Full explanation for 575
576

The boiler's room temperature sensor for the main heating circuit is sending a faulty signal, so it may not control the heating accurately, though it should still be working in a reduced way.

Medium Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, then wait 5 minutes.
  2. Check the cable running to the room sensor or wireless room thermostat for visible damage, pinched wires, or water ingress, and if you have a wireless thermostat, replace its batteries.
  3. Switch the boiler back on and check whether the fault code clears. If it returns, book a Gas Safe registered engineer, as the sensor or its wiring needs testing and replacing.

Parts commonly replaced

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

Related codes

Full explanation for 576
577

The boiler has lost contact with the room controller for the second heating zone, so that zone may not heat properly, but the boiler itself is likely still working safely.

Low DIY-safe £100-200

What to check first

  1. Check the batteries in the wireless room thermostat or room unit for heating circuit 2, and replace them if they are low.
  2. Make sure the room unit is not blocked by furniture or curtains, and is not in direct sunlight or near a heat source.
  3. Switch the boiler off at its on/off switch for 2 minutes, then switch it back on and check whether the fault clears.
  4. If the code returns, book a Viessmann-approved heating engineer to check the room unit wiring and connections, and to replace the sensor if needed.

Parts commonly replaced

  • Room temperature sensor or Vitotrol room unit
  • Room unit batteries
  • Connecting cable or wiring terminals

Related codes

Full explanation for 577
578

The thermostat sensor in the room that controls your second heating zone has lost its connection, so that zone may not heat as accurately as it should, though your boiler is not unsafe.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its isolator switch, wait a few minutes, then turn it back on to clear the fault.
  2. Check that the room sensor cable is firmly connected at both ends and is not visibly damaged or trapped.
  3. If the code returns, leave the boiler on and contact a Gas Safe registered engineer to test or replace the room sensor.

Parts commonly replaced

  • Viessmann room temperature sensor
  • Sensor connection cable

Related codes

Full explanation for 578
579

The boiler has detected a fault with the temperature sensor that controls one of your heating zones, so that zone may not heat as accurately as it should, though the rest of the system should keep working.

Low Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on to see whether the fault clears.
  2. Check the wiring to the room thermostat or room sensor for visible damage, loose connections, or pinched cables, but do not open the boiler casing.
  3. If the fault returns, contact a Gas Safe registered engineer to test or replace the sensor.

Parts commonly replaced

  • Room temperature sensor (heating circuit 2)
  • Room sensor connecting cable
  • Room thermostat or remote control unit

Related codes

Full explanation for 579
682

Your boiler has lost contact with a sensor that checks how much air is reaching the burner, so it has stopped as a safety precaution and will likely need an engineer to restore it.

High Engineer only £150-350

What to check first

  1. Switch the boiler off at the isolator or power switch, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check that the external flue terminal is not blocked by leaves, debris, snow or nesting material, and clear it only if you can do so safely from the ground or a secure position.
  3. If the fault returns, stop and call a Gas Safe registered engineer rather than opening the boiler casing.

Parts commonly replaced

  • Air mass flow sensor
  • Combustion fan assembly
  • Control PCB (if the fault persists after the sensor or fan is replaced)

Related codes

Full explanation for 682
683

Your boiler has lost track of how much air it needs to burn gas safely, so it has likely stopped to protect itself, and a qualified engineer should check it.

High Engineer only £150-300

What to check first

  1. Press the reset button or the reset option on the boiler's display once, then wait a few minutes to see whether the boiler restarts.
  2. Check that the boiler pressure on the display is between about 1.0 and 1.5 bar, topping it up only if the boiler's instructions say how to do so safely.
  3. Make sure the flue outlet outside the property is not blocked by leaves, nests, snow, or debris, and that nothing is covering the air intake.
  4. If the fault returns after a reset, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Air mass flow rate sensor
  • Fan assembly
  • Control unit PCB

Related codes

Full explanation for 683
684

Your boiler has detected a problem with the safety check that makes sure exhaust fumes leave safely through the flue, so it has shut down to protect your home.

Emergency Engineer only £150-350

Parts commonly replaced

  • Back draught safety device
  • Flue gas pressure switch
  • Flue terminal seals
  • Flue gasket

Related codes

Full explanation for 684
Showing 121–130 of 131
694

The boiler has shut itself down as a safety precaution because the exhaust gases leaving it are running hotter than expected, which can point to a blocked flue or a sensor problem.

Emergency Engineer only £90-300

What to check first

  1. Turn the boiler off at its on/off switch and let it cool for 15 minutes.
  2. Check that the flue terminal outside the property is not blocked by leaves, nests, debris, or snow, and that nothing is stored near it.
  3. Turn the boiler back on once using the reset button or by pressing the reset option on the display. If the fault returns, stop and call a Gas Safe registered engineer.
  4. If you smell gas, notice a hissing sound, see scorching around the boiler or flue, or your carbon monoxide alarm sounds, leave the property, open windows, and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue gas temperature limiter
  • Flue terminal or flue seals
  • Condensing heat exchanger (in some cases)

Related codes

Full explanation for 694
738

Your boiler's heating controller is connected but has not yet been set up to work with the boiler, so the two cannot fully communicate, and this is usually a simple setup fix.

Low Engineer only £60-120

What to check first

  1. Check the controller's manual to confirm it is an OpenTherm-compatible room thermostat or control unit made for this boiler.
  2. Switch the boiler off at its isolator or mains switch, wait 30 seconds, then switch it back on to see whether the code clears.
  3. Note the exact code shown on the display and contact a Gas Safe registered engineer, ideally one familiar with Viessmann boilers, to configure the connection.

Parts commonly replaced

  • OpenTherm room thermostat or control unit (only if incompatible)
  • Connection wiring between controller and boiler

Related codes

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 often points to a blockage or a build-up of dirt inside the boiler.

High Engineer only £120-350

What to check first

  1. Switch the boiler off at its on/off switch and avoid repeatedly resetting it, as this can mask a fault that needs professional attention.
  2. Look at the flue outlet outside your home and remove any visible debris, leaves, or nesting material, without touching the inside of the flue.
  3. Check the boiler pressure gauge when the system is cold and confirm it reads between 1.0 and 1.5 bar.
  4. Make sure your carbon monoxide alarm is working, and book a Gas Safe registered engineer to inspect the flue, heat exchanger, and combustion settings.

Parts commonly replaced

  • Flue gas temperature sensor
  • Heat exchanger (cleaning or replacement)
  • Flue terminal and flue pipe sections
  • Combustion fan
Full explanation for 766
767

Your boiler has switched itself off because the exhaust gases leaving through the flue are running hotter than they should, which usually points to a blockage or a part that needs checking by a qualified engineer.

High Engineer only £90-250

What to check first

  1. Turn the boiler off at the isolator switch, wait 10 minutes, then switch it back on and try one reset from the display.
  2. Go outside and check the flue terminal (the pipe or cowl where the boiler exhausts) for leaves, nests, debris or anything blocking the opening. Only do this from ground level or a safe position, and do not climb ladders to reach it.
  3. If the boiler restarts and runs normally, keep an eye on it. If code 767 returns, or the boiler locks out again, stop attempting resets and book a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 767
799

Your boiler's central heating pump has an electrical problem, so it can't move water around the system, which means your heating and hot water won't work until a qualified engineer fixes 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 the boiler is cold, and a low reading can stop the pump working properly.
  2. If the pressure is below 1.0 bar, you can top it up using the filling loop, following the steps in the Viessmann user manual, then close the loop again.
  3. Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to see if the fault clears. If it returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump (integrated pump assembly)
  • Pump electrical connector or wiring loom
  • Control PCB (main circuit board)

Related codes

Full explanation for 799
979

Your boiler is getting heating requests from two control connections at once, so it is confused about which one to follow, and it may not respond to the heating controls properly until a heating engineer checks the wiring and settings.

Medium Engineer only £80-180

Parts commonly replaced

  • Room thermostat or OpenTherm controller
  • Plug 96 wiring link or connector
  • Control wiring harness

Related codes

Full explanation for 979
980

The boiler is not moving enough hot water around your heating system, so it has paused to protect itself until the flow is restored.

High Engineer only £100-300

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 low, top it up using the filling loop, then close the loop valves.
  2. Make sure all radiator valves are open and that any system isolation valves are fully open.
  3. Switch the boiler off at the isolator for a minute, then switch it back on to try a reset. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • System filter or magnetic dirt separator
  • Circulation pump
  • Plate heat exchanger
  • Flow temperature sensor

Related codes

Full explanation for 980
981

Your boiler has detected that hot water is not circulating through the heating system fast enough, so it has shut itself down to protect itself until the flow is restored.

High Engineer only £100-350

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, then close the loop.
  2. Make sure all radiator valves and the boiler's isolating valves are fully open.
  3. Bleed any radiators that are cold at the top, then check the pressure again.
  4. Reset the boiler using the reset button or by switching it off and on again. If the fault returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump
  • Magnetic system filter or strainer
  • Flow sensor
  • Plate heat exchanger
  • Expansion vessel

Related codes

Full explanation for 981
982

Your boiler has stopped its heating pump from moving water around the radiators, which usually means the system has run low on water pressure, and it has switched heating off to protect the pump.

High DIY-safe £90-350

What to check first

  1. Look at the pressure gauge on the front of the boiler. A healthy reading is usually between 1 and 1.5 bar when the system is cold.
  2. If the reading is below 1 bar, slowly open the filling loop valve (usually a small lever or silver connector underneath the boiler) and watch the gauge until it reaches about 1.5 bar, then close the valve firmly.
  3. Check that the boiler is switched on and has no other fault showing, then reset it using the reset button or by turning the boiler off and on again.
  4. If the pressure drops again within a few days, or the boiler shows the same code again, stop and call a Gas Safe registered engineer, as there may be a leak, air in the system or a faulty pump.

Parts commonly replaced

  • Heating circulation pump
  • Pump motor or control module
  • Filling loop or pressure valve
  • Expansion vessel (if pressure keeps dropping)

Related codes

Full explanation for 982
E10

The boiler has lost its connection to your home internet, which only affects remote monitoring and app control, so your heating and hot water should still work normally.

Low DIY-safe £0-150

What to check first

  1. Check that your home router or Wi-Fi is switched on and working, and that other devices can get online.
  2. Confirm the Wi-Fi password entered in the boiler's settings or the Viessmann app is correct and matches your router.
  3. Switch your router off at the wall for 30 seconds, then back on, and wait a few minutes for the boiler to reconnect automatically.
  4. If the error persists, move the router closer to the boiler or check the boiler's Wi-Fi module is seated securely, then contact a Viessmann-approved engineer if it still will not connect.

Parts commonly replaced

  • Vitoconnect internet gateway module
  • Wi-Fi router or extender
Full explanation for E10
Showing 131–131 of 131
E12

Your boiler has lost its connection to Viessmann's online service, which is usually a temporary communication problem and does not normally stop your heating or hot water from working.

Low DIY-safe £0-40 if the fix is a router restart or network setting change; £150-250 if the Vitoconnect internet module needs replacing, including labour

What to check first

  1. Check that your home internet router is switched on and that other devices in the house can get online.
  2. Turn the boiler off at its on/off switch, wait about 30 seconds, then switch it back on.
  3. Wait 15 to 30 minutes and see whether the E12 code clears on its own, as the connection can be re-established later.
  4. If the code returns, check that the boiler's internet module (Vitoconnect) is firmly connected and that your Wi-Fi signal reaches the boiler's location.
  5. If the code persists after these steps, contact a Gas Safe registered engineer or Viessmann support to check the internet module and network settings.

Parts commonly replaced

  • Vitoconnect internet gateway module
Full explanation for E12
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