Viessmann Vitodens 200-W WB2C 26kW System Fault codes & diagnostics

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

System Natural GasLPG 2009-2013 Discontinued
GC Number 41-819-15
Download the Viessmann Vitodens 200-W WB2C 26kW System 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.
2 Emergency 47 High 22 Medium 11 Low
Just want the most common Viessmann Vitodens 200-W WB2C 26kW System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 82 documented codes

Tap any card for details

4C

Your boiler has lost contact with a temperature sensor that controls the flow of hot water to one of your heating zones, so that zone may not heat evenly or may run at the wrong temperature, while the rest of your system should keep working.

Medium Engineer only £120-250

What to check first

  1. Note the exact code and any other messages on the boiler display before doing anything else.
  2. Switch the boiler off at its on/off switch, wait two minutes, then switch it back on to see whether the code clears.
  3. Check that other heating zones and hot water are working, and note which rooms or radiators are affected.
  4. If the code returns, leave the boiler alone and book a Gas Safe registered engineer, giving them the code and the affected zone.

Parts commonly replaced

  • Flow temperature sensor for heating circuit 3
  • Sensor lead or wiring harness
  • Mixer motor
  • Mixer extension module

Related codes

Full explanation for 4C
9A

Your boiler has stopped heating your hot water because it has lost the signal from a temperature sensor, but your heating may still work and this is usually a repairable fault for a Gas Safe engineer.

High Engineer only £120-220

What to check first

  1. Note the exact fault code shown on the boiler display and check whether the central heating still works, so you can tell the engineer what is affected.
  2. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on to see if the code clears. If it returns, leave it alone and book an engineer.
  3. Check that the hot water is not switched off on your thermostat or timer controls, and that the boiler is set to hot water mode.

Parts commonly replaced

  • Cylinder or collector temperature sensor
  • Sensor lead and plug connector

Related codes

Full explanation for 9A
9b

The boiler has lost the signal from the sensor that measures the hot water tank's temperature, so it may not be heating your water properly, though your heating should usually still work.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to see if the fault clears.
  2. Check that the cylinder sensor cable is firmly plugged into the boiler's control board, and that the cable has no visible damage where it runs out of the boiler.
  3. If the code returns, leave the boiler off or on standby and contact a Gas Safe registered engineer, as the sensor usually needs replacing.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor lead or wiring harness

Related codes

Full explanation for 9b
9C

Your boiler has lost contact with the sensor that checks how hot your hot water tank is, so it may not heat your hot water until the sensor is reconnected or replaced.

High Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait a few minutes.
  2. Check that the cable plug from the cylinder sensor is pushed firmly into the boiler's socket, and look for any visibly loose, cut or damaged wiring near the cylinder.
  3. Turn the power back on and reset the boiler once. If code 9C returns, stop and call a Gas Safe registered engineer, as the sensor or its wiring will need testing or replacing.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or wiring harness
  • Sensor plug connector

Related codes

Full explanation for 9C
9E

The boiler has detected that fluid isn't circulating properly through the solar panels on the roof, so it has stopped using them to heat water, though the boiler itself will usually keep running for heating and hot water.

Medium Engineer only £120-400

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check the solar system pressure gauge (usually on the solar station or boiler) and note the reading; it should typically sit between 1.0 and 2.0 bar when the system is cold.
  3. If the pressure is clearly low or the gauge is at zero, do not attempt to top it up or open any valves yourself; contact a Viessmann-registered heating engineer instead.

Parts commonly replaced

  • Solar circulation pump
  • Solar flow sensor or flow meter
  • Solar heat transfer fluid (glycol)
  • Solar circuit expansion vessel
  • Solar station non-return valve

Related codes

Full explanation for 9E
9F

Your boiler has detected a problem with the solar hot water add-on, so it is not using solar energy to heat your water, but your normal heating and hot water should still work.

Medium Engineer only £200-450

What to check first

  1. Note the code 9F on the display and check whether your heating and hot water are still working normally.
  2. Switch the boiler off at its on/off switch, wait 5 minutes, then switch it back on to see whether the code clears.
  3. Check that the solar pump and sensor cables are firmly connected, but do not open the control panel or touch any wiring inside the boiler casing.
  4. If the code returns, leave the solar system switched off and book a Gas Safe registered engineer to inspect the control module or Vitosolic controller.

Parts commonly replaced

  • Vitosolic solar controller
  • Solar control module
  • Solar collector or cylinder temperature sensor
  • Solar circuit wiring harness

Related codes

Full explanation for 9F
10

The boiler has lost track of the outside weather, so it is heating your home as if it were freezing outside, which can make it run hotter than needed, though heating and hot water should still work.

Low Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch, then turn it back on after about a minute to see whether the code clears.
  2. Check the cable running from the outside temperature sensor (usually on an outside wall, north or shaded side) for visible damage, chewed or split insulation, or water getting into the connections.
  3. If the cable or sensor looks damaged, do not repair it yourself. Book a Gas Safe registered engineer to test and replace the sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor cable or connector

Related codes

Full explanation for 10
18

Your boiler's outdoor weather sensor appears to be faulty, so it is heating your home as if it were freezing outside, which may make it run hotter and use more gas than needed, but it is not a safety risk.

Low Engineer only £90-170

What to check first

  1. Switch the boiler off at its isolator switch or fused spur, wait 60 seconds, then switch it back on to clear the code.
  2. Check the outdoor sensor, usually fixed to a north or north-facing wall, for obvious damage, a loose cable, or water getting in, without opening the boiler casing.
  3. If the code returns, leave the boiler on and book a Gas Safe registered engineer to test or replace the outside temperature sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 18
20

The boiler has noticed a fault with one of its temperature sensors in the heating circuit, but it is still working by estimating the water temperature, so your heating and hot water should carry on, though it may not run as precisely as normal.

Low Engineer only £120-220

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait 2 minutes
  2. Switch it back on and check whether the code clears and the boiler runs normally
  3. If the code returns, leave the boiler on and book a Gas Safe registered engineer to check and replace the sensor

Parts commonly replaced

  • System flow temperature sensor (low loss header sensor)
  • Sensor cable or plug connector

Related codes

Full explanation for 20
28

Your boiler has lost the reading from one of its water temperature sensors, so it is still running but using a less precise way of controlling the heating, and an engineer should check it when convenient.

Low Engineer only £120-220

What to check first

  1. Check the system pressure gauge on the boiler. It should read between 1 and 1.5 bar when cold, and top it up only if the manufacturer's filling instructions are followed.
  2. Switch the boiler off at its power supply, wait 30 seconds, then switch it back on and check whether the fault code clears.
  3. If the code returns, leave the boiler running and book a Gas Safe registered engineer to check the flow sensor and its wiring.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor cable or connector
  • Control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 28
Showing 11–20 of 82
30

Your boiler has shut itself down as a safety precaution because its burner is blocked, most likely due to a faulty temperature sensor sending it false readings, and it needs a qualified engineer to check it.

High Engineer only £150-300

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar. If it is low, top it up using the filling loop as your manual describes.
  2. Press the reset button once, or switch the boiler off at its supply for 10 minutes and back on. Wait 15 minutes to see if it restarts.
  3. If the fault code returns, do not keep resetting it or open the boiler casing. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler flow temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Burner assembly (if cleaning is not sufficient)

Related codes

Full explanation for 30
38

Your boiler has shut itself down because it cannot read its water temperature properly, so it is stopping the burner as a safety precaution and will likely need a qualified engineer to restore heating and hot water.

High Engineer only £120-220

What to check first

  1. Check the boiler display and pressure gauge: the pressure should sit between about 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top it up using the filling loop as shown in the manual, then stop.
  2. Turn the boiler off at its isolator or power switch, wait 10 minutes, switch it back on, and press reset once if the display shows a reset option. If the fault returns straight away, stop and book a Gas Safe engineer.
  3. Check that the thermostat or heating controls are not stuck or set to a very low temperature, and that there are no obvious leaks or water around the boiler.

Parts commonly replaced

  • Boiler water temperature sensor (flow sensor)
  • Sensor lead or wiring harness
  • Control PCB (only if the sensor and lead test good)

Related codes

Full explanation for 38
40

Your boiler has detected a problem with the temperature sensor that controls one of your heating zones, so that zone may not be heating properly, though your boiler is likely still working.

Medium Engineer only £120-250

What to check first

  1. Turn the boiler off at its power switch and wait 60 seconds.
  2. Switch it back on and see whether the code clears by itself.
  3. Check the pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's filling instructions are followed.

Parts commonly replaced

  • Flow temperature sensor (heating circuit 2)
  • Sensor cable or plug connector
  • Mixer motor (if the fault persists after sensor replacement)

Related codes

Full explanation for 40
44

One part of your heating, the zone controlled by a mixing valve, has lost its temperature reading, so the boiler has stopped adjusting that zone properly while the rest of your heating and hot water should still work.

Medium Engineer only £120-250

What to check first

  1. Switch the boiler off at its isolator or power switch and wait 10 minutes
  2. Switch it back on and check whether the code clears or returns
  3. Note which heating zone seems affected and whether the radiators in that zone heat up unevenly
  4. If the code returns, book a Viessmann-registered engineer rather than opening the boiler or disconnecting wiring

Parts commonly replaced

  • Flow temperature sensor for heating circuit 3
  • Mixing valve motor or mixer module
  • Sensor wiring harness or connector

Related codes

Full explanation for 44
48

One part of your heating system that controls water temperature for a separate heating zone, such as an upstairs or extension circuit, has lost its temperature reading, so the boiler is limiting that zone to protect itself while the rest of your heating and hot water should still work.

Medium Engineer only £120-220

What to check first

  1. Check the boiler pressure gauge and top up to between 1.0 and 1.5 bar if it is low, using the filling loop.
  2. Turn the boiler off at the switch, wait 10 minutes, then turn it back on and reset it using the reset button on the front panel.
  3. Check that your heating controls and thermostats are set to a reasonable temperature and that the affected zone is switched on.
  4. If the fault returns, contact a Gas Safe registered engineer, as the sensor or its wiring usually needs checking or replacing.

Parts commonly replaced

  • Flow temperature sensor
  • Heating circuit 2 mixer motor
  • Sensor wiring harness

Related codes

Full explanation for 48
50

Your boiler has stopped heating your hot water because it has detected a fault with the sensor that measures the water temperature in your cylinder, though your central heating may still work.

High Engineer only £120-260

What to check first

  1. Check the boiler display to confirm the code is 50 and whether heating still works.
  2. Turn the boiler off at its on/off switch, wait about 30 seconds, then turn it back on to see if the code clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the sensor and its wiring need to be tested and replaced safely.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor wiring or connector
  • Comfort sensor (where fitted)

Related codes

Full explanation for 50
51

Your boiler has stopped producing hot water because one of its internal temperature sensors is reporting a fault, which is usually straightforward for a qualified engineer to fix.

High Engineer only £150-300

What to check first

  1. Switch the boiler off at its power supply or fused spur, wait about 30 seconds, then switch it back on to see whether the fault clears.
  2. If code 51 returns, leave the boiler off and do not open the casing or disconnect any wires.
  3. Book a Gas Safe registered engineer to test and replace the faulty sensor if needed.

Parts commonly replaced

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

Related codes

Full explanation for 51
58

Your boiler has stopped heating your hot water because it can't read the temperature of the hot water cylinder, so it's best to get a Gas Safe engineer to check it soon.

High Engineer only £120-220

What to check first

  1. Check that your hot water is switched on at the boiler or controls and that the hot water temperature is set to a normal level.
  2. Turn the boiler off at its on/off switch and at the power supply, wait about 10 minutes, then switch it back on to see if the fault clears.
  3. Check the display for any other fault codes and note them down, along with the date and whether the heating still works, to pass on to the engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Comfort sensor
  • Sensor lead or cable harness

Related codes

Full explanation for 58
59

Your boiler has lost track of the temperature of the hot water it makes, so it has stopped heating your hot water. Your heating may still work, but a qualified engineer should check it.

High Engineer only £120-220

Parts commonly replaced

  • Outlet temperature sensor
  • Sensor lead or wiring harness

Related codes

Full explanation for 59
90

Your boiler has detected a problem with one of its internal temperature readings, so it has likely shut itself down as a safety precaution, and a qualified engineer will need to check the sensor.

High Engineer only £100-200

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, follow the filling instructions in your manual or ask an engineer to top it up.
  2. Turn the boiler off at its power switch, wait 10 minutes, then switch it back on and press the reset button if the display shows a lockout.
  3. If the code returns, stop trying to fix it yourself and book a Gas Safe registered engineer, mentioning fault code 90 and the short circuit.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor wiring harness
  • Boiler control PCB (if the sensor is not the cause)

Related codes

Full explanation for 90
Showing 21–30 of 82
91

Your boiler's internal thermometer, which tells it how hot the water is, has sent a faulty reading, so the boiler has shut itself down as a precaution and needs a qualified engineer to check the sensor.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 10 minutes.
  2. Switch the power back on and press the reset button on the boiler if the display shows it, then see whether the code returns.
  3. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if it is low and you know how to do so safely.
  4. If the code returns, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Sensor wiring harness
  • Control unit PCB (only if the sensor and wiring test good)

Related codes

Full explanation for 91
92

Your boiler has stopped heating your hot water because one of its temperature sensors is sending a faulty signal, so it cannot tell how warm the water is, but your central heating may still work.

High Engineer only £120-250

What to check first

  1. Note the fault code 92 and check whether the boiler's pressure gauge is between 1 and 2 bar.
  2. Switch the boiler off at its isolator or power socket, wait 30 seconds, then switch it back on to see if the fault clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Cylinder temperature sensor
  • Collector temperature sensor
  • Sensor wiring harness or connector
  • Control unit circuit board (less common)

Related codes

Full explanation for 92
93

Your boiler has detected a faulty hot water cylinder temperature sensor, so it may not be controlling your stored hot water correctly, but this is usually fixable by a qualified engineer.

Medium Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch, wait 5 minutes, then switch it back on to see if the fault clears.
  2. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up with the filling loop only if you know how to do so safely.
  3. Check whether hot water and heating are still working, and note any other warning codes shown on the display before contacting an engineer.

Parts commonly replaced

  • Cylinder temperature sensor (NTC)
  • Sensor wiring harness or cable
  • Boiler control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for 93
94

Your boiler has stopped producing hot water because it has detected a fault with the sensor that reads the temperature in your hot water cylinder, though your central heating may still work normally.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait 30 seconds, then switch it back on to see if the fault clears.
  2. Check whether hot water returns after the boiler restarts and runs for a few minutes.
  3. If the fault returns, do not open the boiler casing. Contact a Gas Safe registered engineer to inspect the cylinder sensor and its wiring.

Parts commonly replaced

  • Cylinder temperature sensor (NTC)
  • Sensor cable or plug connector
  • Wiring harness

Related codes

Full explanation for 94
98

Your boiler has lost track of its water temperature reading, so it has most likely stopped itself as a safety precaution, and a qualified engineer will need to reconnect or replace the faulty sensor.

High Engineer only £120-220

What to check first

  1. Check the boiler display for any other messages, then note down the exact code shown.
  2. Check that the system pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's filling instructions are followed.
  3. Press the reset button once on the boiler (or use the reset option on the display), wait 10 minutes, and see whether it restarts.
  4. If the fault returns, stop attempting repairs and book a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor lead or wiring harness
  • Control unit PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for 98
99

Your boiler has detected a problem with one of its temperature readings, so it may shut down or behave unpredictably until a qualified engineer checks the sensor and its wiring.

Low Engineer only £120-220

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait about one minute.
  2. Switch the power back on and let the boiler restart by itself.
  3. Check whether the boiler is working normally and whether the code 99 has cleared.
  4. If the code returns or the boiler stays off, contact a Gas Safe registered engineer rather than opening the casing.

Parts commonly replaced

  • Control mode temperature sensor
  • Sensor lead or wiring harness
  • Control unit (only if the fault persists after the sensor is replaced)

Related codes

Full explanation for 99
A7

The boiler's central control unit has developed a fault, so it may shut down or stop working until it is checked and repaired by a qualified engineer.

High Engineer only £250-450

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait 30 seconds.
  2. Switch it back on and check whether the A7 code clears and the boiler restarts.
  3. If the code returns or the boiler still does not work, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Programming unit (control unit)
  • Control unit fuse or PCB

Related codes

Full explanation for A7
b0

Your boiler has lost track of how hot its exhaust gases are, which usually stops it from running safely, so it has likely shut down until a qualified engineer checks the sensor.

High Engineer only £150-300

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 minutes, then switch it back on to see whether the code clears.
  2. Check that the boiler's flue terminal outside the property is not blocked by leaves, nests, debris or snow, and clear any blockage you can safely reach.
  3. If the code returns, stop trying to fix it and call a Gas Safe registered engineer. Do not open the boiler casing or touch the flue sensor yourself.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Main control PCB (if wiring or board fault is found)

Related codes

Full explanation for b0
b1

The boiler's control panel has lost its connection to the main boiler unit, so the system can't communicate properly and may stop heating or hot water until it is reset or repaired.

High DIY-safe £100-300

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator switch, then wait 5 minutes.
  2. Turn the power back on and check whether the display clears the b1 code and the boiler restarts.
  3. If the code returns, check that the cable between the control panel and the boiler is not loose or damaged, then switch off and reseat the plug if it is easy to reach, and do not open the boiler casing.
  4. If the fault remains after one reset, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Vitotronic control unit (programming unit)
  • Connecting data/BUS cable
  • Main control circuit board

Related codes

Full explanation for b1
b5

Your boiler's internal control system has detected a fault and has stopped working as it should, but this is usually something a Gas Safe engineer can diagnose and fix.

High Engineer only £200-450

What to check first

  1. Check the boiler pressure gauge on the front panel. It should read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top it up using the filling loop, following the boiler manual.
  2. Switch off the boiler at its on/off switch and at the fused spur or isolator supplying it. Wait 5 minutes, then switch it back on and see if the fault clears.
  3. If the code returns, or the boiler will not restart, call a Gas Safe registered engineer. Do not open the casing or attempt electrical repairs.

Parts commonly replaced

  • Control unit (PCB)
  • Display module
  • Ignition or control board
  • Fuse

Related codes

Full explanation for b5
Showing 31–40 of 82
b7

Your boiler's settings card, which tells it how it should be set up, is either missing or not reading correctly, so it has stopped working until a Gas Safe engineer checks it.

High Engineer only £150-300

What to check first

  1. Turn off the boiler at its on/off switch and at the household fuse or isolator, then wait 5 minutes.
  2. Switch the power back on and see whether the boiler restarts and the b7 code clears.
  3. If b7 returns or the boiler still does not work, stop and book a Gas Safe registered engineer. Do not open the boiler casing or remove the coding card yourself.

Parts commonly replaced

  • Coding card
  • Control unit circuit board

Related codes

Full explanation for b7
b8

Your boiler's safety sensor that monitors the temperature of the exhaust gases has lost its connection, so the boiler has stopped to stay safe until it is checked.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its isolator switch or fused spur, wait about 10 minutes, then switch it back on and see if the fault clears.
  2. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, following the manufacturer's instructions.
  3. From outside, check that the flue terminal is clear of leaves, nesting material, snow or other debris, and that nothing is blocking the airflow.
  4. If the fault returns, do not keep resetting the boiler. Contact a Gas Safe registered engineer to inspect the flue sensor and its wiring.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor wiring lead or harness
  • Control PCB (only if the sensor and wiring test good)

Related codes

Full explanation for b8
bA

The boiler's control system has lost contact with the add-on unit that manages the second heating zone, so that zone may not heat properly, while the rest of the system should keep working.

Medium Engineer only £150-350

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait two minutes.
  2. Switch the power back on and wait several minutes for the boiler to restart and reconnect its components.
  3. Check whether the fault code clears and whether the second heating zone responds to its thermostat or timer. If the code returns, contact a Gas Safe registered engineer, as the cabling and control unit need a professional check.

Parts commonly replaced

  • Viessmann extension kit (EM-M or equivalent) for heating circuit 2
  • BUS communication cable between boiler control unit and extension kit
  • Mixer motor for heating circuit 2
  • Vitotronic control unit (only if the fault is traced to it)

Related codes

Full explanation for bA
bb

The boiler's controls have lost contact with the add-on unit that manages your third heating zone, so that zone may not heat properly, while the rest of the system usually keeps working.

Medium Engineer only £150-350

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait 60 seconds.
  2. Switch the power back on and wait several minutes for the boiler to fully restart.
  3. Check the display: if code bb has cleared, monitor the heating for a day. If it returns, call a Viessmann-registered engineer.

Parts commonly replaced

  • Viessmann heating circuit 3 extension kit (mixer extension module)
  • LON communication cable or connector
  • Vitotronic control unit (uncommon)

Related codes

Full explanation for bb
bC

The boiler has lost contact with the wireless room thermostat that controls the heating, so it may not be heating your home the way it normally does, but this is usually a simple communication fault and not a safety danger.

Medium DIY-safe £80-250

What to check first

  1. Check the batteries in the room thermostat (Vitotrol remote control) and replace them if they are low or flat.
  2. Make sure the thermostat is within range of the boiler's receiver and that nothing metal or a new electrical device sits between them.
  3. Turn the boiler off at its on/off switch, wait about 30 seconds, then turn it back on and see if the code clears.
  4. If the fault remains, check the thermostat's connection cable at the thermostat and at the boiler's control terminals, but do not open the boiler casing. Contact a Viessmann-registered engineer if the code returns.

Parts commonly replaced

  • Vitotrol remote control unit
  • Remote control batteries
  • Wireless receiver or BUS connection cable
  • Control board connector

Related codes

Full explanation for bC
bd

The boiler has lost contact with the thermostat controlling your second heating zone, so that zone may not be heating properly, though the rest of the system should still work.

Medium Engineer only £120-250

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur for about two minutes, then switch it back on.
  2. Check the display on the Vitotrol remote control for a low battery or blank screen, and replace the batteries if needed.
  3. Check that the cable between the remote control and the boiler's control unit is not loose or damaged, without opening the boiler casing.
  4. If the code returns, leave the boiler running and book a Gas Safe registered engineer to check the wiring and the mixer module.

Parts commonly replaced

  • Vitotrol remote control
  • BUS communication cable
  • Heating circuit mixer extension module
  • Control unit PCB

Related codes

Full explanation for bd
bE

The boiler has lost contact with the thermostat controlling one of your heating zones, so that zone may not heat as it should, but the rest of your heating and hot water should still work.

Medium Engineer only £120-280

What to check first

  1. Check that the thermostat for the affected heating zone has power and its display is lit.
  2. Look at the cable running from the thermostat to the boiler or wall box, and make sure it is not visibly damaged or loose at either end.
  3. Turn the boiler off at its isolator switch for about a minute, then turn it back on and see whether the code clears.
  4. If the code returns, contact a Gas Safe registered engineer, as the fault usually needs the wiring or control module checked with a service tool.

Parts commonly replaced

  • Vitotrol remote control (heating circuit 3)
  • Connecting cable between remote control and boiler control unit
  • Mixer extension module (LON/BUS module)

Related codes

Full explanation for bE
bF

The boiler's internal parts are having trouble talking to each other, so it may stop working or behave unpredictably until a qualified engineer checks the connections or replaces a part.

High Engineer only £150-350

What to check first

  1. Note the exact fault code shown on the boiler display and any other messages.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait about 2 minutes, then switch it back on.
  3. Check whether the boiler restarts and whether the code clears. If it returns, leave the boiler off and call a Gas Safe registered engineer.

Parts commonly replaced

  • LON communication module
  • LON connection cable
  • Vitotronic control unit

Related codes

Full explanation for bF
C1

Your boiler's control parts have lost contact with each other, so it may stop working properly until a heating engineer checks the connections.

High DIY-safe £150-350

What to check first

  1. Turn off the heating and hot water controls, then switch the boiler off at its isolator or fused spur and wait about 60 seconds.
  2. Switch the power back on and let the boiler restart fully, then check whether the C1 code has cleared.
  3. If the code returns, look only at the plug connections on the boiler's extension module (EA1) and make sure they are fully seated. Do not open the boiler casing or touch any gas components. If the fault persists, call a Gas Safe registered engineer.

Parts commonly replaced

  • EA1 extension module
  • Communication (bus) cable
  • Boiler control unit PCB

Related codes

Full explanation for C1
C2

Your boiler has lost contact with its solar heating controller, which usually means your main heating and hot water can still work, but the solar part of the system isn't being managed properly until a qualified engineer checks the connection.

Low Engineer only £100-350

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur, then wait 5 minutes.
  2. Check that the communication cable between the boiler and the solar controller (Vitosolic or solar module) is firmly plugged in at both ends, without opening any sealed or live electrical covers.
  3. Restore power, wait for the boiler to restart fully, and check whether the C2 code has cleared. If it returns, book a heating engineer.

Parts commonly replaced

  • Solar control module
  • Vitosolic solar controller
  • BUS communication cable

Related codes

Full explanation for C2
Showing 41–50 of 82
C3

The boiler's add-on control unit has lost its connection to the main boiler, so it may stop working properly until the link is restored, but this is usually a fixable wiring or part issue.

Medium Engineer only £90-250

What to check first

  1. Switch off the boiler at its isolator switch or fused spur and wait 2 minutes.
  2. Switch the power back on and check whether the code clears or returns on the display.
  3. If the code returns, contact a Gas Safe registered engineer to check the communication cable and plug connections between the boiler and the extension module.

Parts commonly replaced

  • Extension AM1 module
  • BUS communication cable
  • Control PCB connector or wiring harness

Related codes

Full explanation for C3
C4

Your boiler's control system is having trouble talking to a connected accessory, such as a room thermostat or control module, so some smart controls may not work properly, though the boiler may still heat your home.

Medium Engineer only £90-220

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait about 60 seconds before turning it back on.
  2. Check that the room thermostat, weather compensator or other external controller is switched on and has working batteries if it has them.
  3. Look at the wiring plugs and cables between the boiler and any external controller to check they are firmly connected, undamaged and not loose.
  4. If the code returns after these checks, contact a Gas Safe registered engineer, as the fault may be in the control module, wiring or the accessory itself.

Parts commonly replaced

  • OpenTherm extension module
  • Vitotronic control unit
  • Connection cable or wiring harness
  • Room thermostat or weather compensator

Related codes

Full explanation for C4
C5

Your boiler has lost its ability to talk properly to one of its internal water pumps, so it has stopped to protect itself and will likely need a qualified engineer to restore heating and hot water.

High Engineer only £180-550

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 the filling instructions in your user manual to top it up.
  2. Switch the boiler off at its isolator or fused spur for about 30 seconds, then switch it back on to try a reset.
  3. If the fault code returns, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Variable speed circulation pump
  • Pump electrical connector or wiring loom
  • Main control PCB (circuit board)

Related codes

Full explanation for C5
C6

Your boiler has lost its connection to the pump that circulates heating for one of your zones, so that zone may heat unevenly or not at all while the rest of the system keeps working.

Medium Engineer only £150-400

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, wait 2 minutes, then switch it back on to clear the fault.
  2. Check the boiler display for any other fault codes and note them down before calling an engineer.
  3. Make sure the heating is set to the correct zone and thermostat settings, and that the other radiators or zones are still heating.
  4. If the code returns, leave the boiler on for the engineer to inspect rather than repeatedly resetting it.

Parts commonly replaced

  • External variable speed heating circuit pump
  • Pump connection cable or plug
  • Vitotronic control or extension module (LON/mixer module)
  • Mixer circuit wiring harness

Related codes

Full explanation for C6
C7

Your boiler has lost contact with the pump that circulates hot water to one of your heating zones, so that zone may not be heating properly, and a qualified engineer should check it soon.

High Engineer only £120-350

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then wait two minutes.
  2. Switch the power back on and check whether the code clears or returns after a few minutes.
  3. If the code returns, leave the boiler off and call a Gas Safe registered engineer, as the fault usually needs the pump wiring or control module checked.

Parts commonly replaced

  • External variable speed heating circuit pump
  • Pump connection cable or wiring harness
  • Control module or PCB (EM1 or main board)
  • Pump supply fuse

Related codes

Full explanation for C7
C8

Your boiler has lost its connection to the pump that heats one of your extra heating zones, so that zone may not warm up properly while the rest of the system keeps working.

Medium Engineer only £150-350

What to check first

  1. Check the boiler display for any other fault codes and note them down before doing anything else.
  2. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait about 10 minutes, then switch it back on to see if the communication fault clears.
  3. If the code returns, leave the boiler alone and book a Gas Safe registered engineer, mentioning code C8 and heating circuit 3 with mixer.

Parts commonly replaced

  • Variable speed heating circuit pump
  • Communication (LON/BUS) cable
  • Extension module for heating circuit 3
  • Mixer motor

Related codes

Full explanation for C8
Cd

Your boiler has lost its connection to an external communication module, which usually affects remote monitoring or controls rather than the heating and hot water themselves, so it may still be working normally.

Low DIY-safe £80-250

What to check first

  1. Turn the boiler off at its isolator switch or fused spur and wait about 60 seconds, then switch it back on.
  2. Check that the communication cable running from the boiler to the Vitocom 100 unit is firmly plugged in at both ends, without touching any wiring inside the boiler casing.
  3. If the code clears and the boiler runs normally, keep an eye on it. If Cd returns, book a Gas Safe registered engineer to check the Vitocom 100 module and the KM BUS wiring.

Parts commonly replaced

  • Vitocom 100 communication module
  • KM BUS connection cable

Related codes

Full explanation for Cd
CE

The boiler's main controller has lost contact with an add-on module it relies on, which usually means a loose connection or a wiring fault that needs checking, though it may not stop the heating or hot water completely.

Medium Engineer only £120-300

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. Check that the boiler's control panel and any extension module have their cables firmly seated, without forcing or unplugging anything inside the casing.
  3. Restore power, switch the boiler back on, and see whether the CE code clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Extension module (e.g. EA1 or AM1)
  • Communication (LON) cable or plug
  • Control unit circuit board

Related codes

Full explanation for CE
CF

Your boiler's main controls have lost contact with a communication module, so it may not respond correctly to your thermostat or timer, and a qualified engineer should check it.

Medium Engineer only £150-350

What to check first

  1. Check whether your heating and hot water are still working and note any other codes shown on the boiler display.
  2. Turn off the boiler at its power isolator or fused spur, wait 30 seconds, then switch it back on to see if the code clears.
  3. If the code returns, leave the boiler alone and book a Gas Safe registered engineer, as the communication module and its wiring need professional testing.

Parts commonly replaced

  • LON communication module
  • LON communication cable
  • Vitotronic control unit

Related codes

Full explanation for CF
d6

Your boiler has received a fault signal from an external device or safety input connected to it, which is likely stopping it from heating your home or producing hot water until it is checked.

High Engineer only £90-250

What to check first

  1. Note the code shown on the boiler display and check that the boiler is not showing any other fault codes.
  2. Check whether any external controls or safety devices connected to the boiler, such as a remote switch or external thermostat, are switched on and working correctly.
  3. Turn the boiler off at its power supply for 30 seconds, then switch it back on to see whether the code clears.
  4. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the fault usually needs wiring or module checks.

Parts commonly replaced

  • EA1 extension module
  • External fault input wiring or connector
  • External safety or control device connected to DE1

Related codes

Full explanation for d6
Showing 51–60 of 82
d7

Your boiler's control system has detected a fault signal from one of the external devices connected to it, which can stop the heating from working until a qualified engineer checks it.

High Engineer only £90-300

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on to see if the code clears.
  2. Check that the boiler's system pressure gauge reads between 1 and 2 bar, and top it up only if the instructions in your manual say how to do so safely.
  3. Look at any external controls or devices wired to the boiler (such as a remote switch or safety device) and make sure nothing is obviously loose or damaged, then contact a Gas Safe registered engineer if the code returns.

Parts commonly replaced

  • Vitotronic extension module EA1
  • Input wiring or connection cable to DE2
  • External fault switch or connected device
  • Control unit (uncommon)

Related codes

Full explanation for d7
d8

Your boiler has received a warning signal from a device or wiring connected to it, which may cause it to pause or stop heating until a qualified engineer checks it.

Medium Engineer only £90-220

What to check first

  1. Check the boiler display for any other fault codes and note them down.
  2. Turn the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then turn it back on.
  3. If the code returns, check that any external controls, thermostats or devices connected to the boiler are switched on and not damaged, then book a Gas Safe registered engineer.

Parts commonly replaced

  • External control or relay module
  • Wiring harness or connector
  • Room thermostat or external sensor
  • Control unit circuit board

Related codes

Full explanation for d8
dA

Your boiler's room thermostat sensor is sending a faulty signal, so the heating is running on outside-weather settings alone and may not hold your home at the exact temperature you set.

Low Engineer only £100-200

What to check first

  1. Switch the boiler off at the wall switch and wait 5 minutes, then switch it back on to see if the fault clears.
  2. Look at the room sensor on the wall and check that its cable is not pinched, damaged, or loose at either end, without opening the sensor or the boiler casing.
  3. If the fault returns, leave the boiler on and contact a Gas Safe registered engineer to test or replace the room sensor.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or connection

Related codes

Full explanation for dA
db

The boiler's room thermostat for your second heating zone has stopped sending a reliable temperature reading, so that zone is now heating based only on outside weather readings and may not hold the exact room temperature you set.

Medium Engineer only £100-200

What to check first

  1. Switch the boiler off at its on/off switch, wait 2 minutes, then switch it back on to see if the fault clears.
  2. Check the room thermostat or sensor on the affected heating zone for visible damage, and look for any loose or pinched wiring that could be causing a short.
  3. If the fault returns or the wiring looks fine, contact a Gas Safe registered engineer to test and replace the room sensor.

Parts commonly replaced

  • Viessmann room temperature sensor or room unit
  • Sensor connection cable
  • Heating circuit 2 control module (rarely required)

Related codes

Full explanation for db
dC

The boiler has lost the signal from the room thermostat sensor for the third heating zone, so that zone is being controlled by outside temperature alone and may not heat as precisely as usual.

Medium Engineer only £90-180

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur, then wait five minutes.
  2. Find the wall-mounted room sensor or room thermostat for heating circuit 3 and check its cable for visible damage, pinched wires or loose connections at both ends.
  3. Restore power and see whether the dC code clears. If it returns, stop and call a qualified heating engineer, as the sensor or wiring will need testing and replacing.

Parts commonly replaced

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

Related codes

Full explanation for dC
dd

Your boiler has lost contact with its room temperature sensor, so it is heating your home based only on the outdoor temperature, which usually keeps you warm but may not match the room exactly.

Low DIY-safe £90-180

What to check first

  1. Turn off the power to the boiler at the fused spur or isolator switch and wait a few minutes.
  2. Check that the wires from the room thermostat or room sensor on the wall are firmly connected at both ends, and look for any visibly loose or damaged cable without opening the boiler casing.
  3. Restore the power, then reset the boiler using the reset button or the controls, and check whether the code clears.
  4. If the code returns, contact a Gas Safe registered engineer to test or replace the room sensor, as the boiler's internal wiring should not be opened by a homeowner.

Parts commonly replaced

  • Room temperature sensor or room thermostat
  • Sensor connecting cable
  • Control unit connector (occasionally)

Related codes

Full explanation for dd
dE

Your boiler has lost the room temperature reading for one of your heating zones, so it is heating that zone using outside weather readings instead, and it should still keep your home warm.

Low DIY-safe £120-£250

What to check first

  1. Check that the room temperature sensor for the second heating circuit (usually a small wall-mounted unit near a thermostat) is plugged in securely and has no visible damage to its cable.
  2. Make sure the sensor is not blocked by furniture, curtains, or a radiator, and is not in direct sunlight or a draught.
  3. Reset the boiler using the reset button or the menu on the control panel, wait around 10 minutes, and check whether the code clears. If it returns, contact a Gas Safe registered engineer to test or replace the sensor.

Parts commonly replaced

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

Related codes

Full explanation for dE
dF

Your boiler is still heating your home, but it has lost contact with the room sensor for one of its heating zones, so that zone may not reach the temperature you have set.

Low Engineer only £100-200

What to check first

  1. Check that the room temperature sensor for heating zone 3 is plugged in securely and that its cable is not damaged or loose.
  2. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on to see whether the fault clears.
  3. If the code returns, leave the boiler on and contact a Gas Safe registered engineer, as the sensor or its wiring may need replacing.

Parts commonly replaced

  • Room temperature sensor
  • Sensor connection cable
  • Mixer extension module

Related codes

Full explanation for dF
E0

The boiler's control system has lost contact with a connected accessory or control unit, so it may not be working as well as it should, and a Gas Safe engineer should check the connections.

Medium Engineer only £150-350

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait 2 minutes, then switch it back on to see if the code clears.
  2. Check the display for any other fault codes and note whether heating and hot water are still working.
  3. If the code returns, stop and contact a Gas Safe registered engineer, as the connections and control unit need professional inspection.

Parts commonly replaced

  • LON communication module
  • LON bus cable or connector plug
  • Control unit main PCB
  • Vitotronic control unit

Related codes

Full explanation for E0
E1

Your boiler has shut itself down for safety because its flame sensor is reading an unexpectedly strong signal during start-up, so it needs a qualified engineer to check the burner before it can heat your home again.

High Engineer only £120-250

What to check first

  1. Check the pressure gauge on the front of the boiler and top it up to the level in the manual (usually about 1.0 to 1.5 bar) if it is low, with the boiler cold.
  2. Make sure nothing is blocking the flue outlet outside and that there is clear space around the boiler.
  3. Turn the boiler off at the controls and at the mains, wait 5 minutes, then switch it back on and press the reset button once. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Gas valve (if faulty)

Related codes

Full explanation for E1
Showing 61–70 of 82
E3

The boiler is pausing while it sets up its gas-burning settings, and it needs a reset and a check that water is flowing freely through the heating system before it can run properly again.

High DIY-safe £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 when the system is cold. If it is low, top it up using the filling loop as shown in the user manual, then close the filling loop valves.
  2. Bleed any radiators that feel cold at the top or are cold all over, using a radiator key, to remove trapped air that can block water flow.
  3. Make sure all radiator valves and any isolation valves are fully open so water can circulate freely.
  4. Press and hold the reset button marked R on the boiler control panel for a few seconds, then release it. Wait around 10 minutes for the boiler to complete its calibration.
  5. If the code returns, or the boiler still will not heat, stop and call a Gas Safe registered engineer. Do not open any gas components yourself.

Parts commonly replaced

  • Combustion fan
  • Gas valve
  • Flue gas sensor
  • Heat exchanger (if blocked)
  • Circulation pump

Related codes

Full explanation for E3
E4

Your boiler has shut itself down because its burner is not running correctly, so it may stop producing heat and hot water until a qualified engineer checks it.

High Engineer only £150-350

What to check first

  1. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top it up using the filling loop only if it is low.
  2. Wait about 10 minutes, then press and hold the reset button on the boiler (or use the reset option on the control panel) once to try restarting it.
  3. If the fault code returns, turn the boiler off at its on/off switch, make sure no gas or electrical work is attempted, and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve
  • Fan assembly
  • Ignition electrode
  • Flame sensor
  • Burner gasket
  • Control PCB

Related codes

Full explanation for E4
E5

Your boiler has shut itself down because its flame sensor is detecting a problem with the burner flame, which is usually caused by a dirty or faulty sensor rather than anything dangerous.

High Engineer only £120-280

What to check first

  1. Check that your gas supply is on by confirming other gas appliances, such as the hob, are working.
  2. Check that the boiler's flue outlet outside and any air vents are not blocked by leaves, snow, debris or nests.
  3. Turn the boiler off at its power switch, wait 30 seconds, then turn it back on and press the reset button once if one is shown.
  4. If the fault returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Burner or electrode gasket
  • Control PCB (printed circuit board)
  • Gas valve

Related codes

Full explanation for E5
E7

The boiler has shut itself down because its flame detector isn't sensing the gas flame strongly enough during start-up, so it can't safely light and heat your home or water until it's checked.

High Engineer only £90-250

What to check first

  1. Check that the gas supply to the boiler is turned on, including the gas meter valve and the boiler's own isolation valve.
  2. Check the boiler pressure gauge. If the needle is below 1 bar, top it up using the filling loop as shown in the manual, then check it stays steady.
  3. Press the reset button once (or use the reset option on the display), wait 10 minutes, and see whether the boiler restarts. If E7 returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ignition electrode
  • Gas valve
  • Gas/air mixture fan
  • Electrode gasket

Related codes

Full explanation for E7
E8

Your boiler has stopped because its burner flame isn't being detected correctly, so it is shutting down as a safety precaution and will usually need a qualified engineer to check it.

High Engineer only £120-300

What to check first

  1. Check that the boiler's gas supply is on, including the gas meter and any isolation valve, and that other gas appliances in the home are working.
  2. Check the boiler pressure gauge and top it up using the filling loop if it is below 1 bar, following the manufacturer's instructions.
  3. Press the reset button once (or use the reset option on the controls), wait a few minutes, and see whether the boiler restarts.
  4. If the code returns or the boiler will not restart, leave it switched off and call a Gas Safe registered engineer. Do not open the boiler casing or try to adjust gas components yourself.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Combustion chamber seal
  • Gas valve (in some cases)
  • Control PCB (in some cases)

Related codes

Full explanation for E8
EA

Your boiler's flame sensing check during startup has come back outside its normal range, so it has stopped to stay safe until a qualified engineer looks at it.

High Engineer only £100-250

What to check first

  1. Check that your heating and hot water are not working and that the boiler display shows the EA code.
  2. Check the boiler pressure on the display or gauge and top it up only if it is below the range shown in the manual (typically 1.0 to 1.5 bar when cold).
  3. Press the reset button once, wait 10 minutes, and see if the boiler restarts.
  4. If the fault returns or the boiler will not reset, stop and call a Gas Safe registered engineer. Do not open the casing or attempt any gas component work yourself.

Parts commonly replaced

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

Related codes

Full explanation for EA
Eb

The boiler has stopped itself while it tries to set its gas burning mix correctly, which usually means it needs a qualified engineer to check the flame sensing and flue before it can run safely again.

High Engineer only £120-250

What to check first

  1. Press the reset button once, wait about 10 minutes, and see whether the boiler restarts normally.
  2. Check that the flue outlet outside the property is not blocked by debris, nests, or snow, and that nothing is obstructing the air vents.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the fault involves gas settings and combustion checks.

Parts commonly replaced

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

Related codes

Full explanation for Eb
EC

Your boiler's control system has hit a settings error while checking itself, so it may have stopped or be unable to start up properly until a engineer restores the correct settings.

High Engineer only £120-300

What to check first

  1. Check the boiler's pressure gauge and top it up if it is below 1 bar, following the filling instructions in the manual.
  2. Turn off the boiler at its on/off switch and at the isolator or fused spur, wait 60 seconds, then switch it back on.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the settings or control unit will need professional attention.

Parts commonly replaced

  • Coding card
  • Control unit (PCB)

Related codes

Full explanation for EC
Ed

Your boiler's internal control system has detected a fault in its own electronics, which usually means it has shut down to protect itself, so heating and hot water may be off until it is reset or repaired.

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 circuit breaker supplying it, wait 5 minutes, then switch the power back on.
  2. Check the boiler display for a 'Reset' or 'Ed' message and press the reset button if the boiler offers one, then wait a few minutes for it to restart.
  3. If the code returns or the boiler will not restart, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (electronic control unit)
  • Boiler display/control panel
  • Power supply board

Related codes

Full explanation for Ed
EE

Your boiler is trying to light but cannot get the flame started, so it has shut itself down to stay safe, and this usually means you have no heating or hot water until a engineer checks it.

High Engineer only £150-350

What to check first

  1. Check that the boiler has power, the on/off switch is on, and the boiler's display is lit.
  2. Check that your gas supply is on: look at the gas meter and confirm your gas hob or other gas appliances work. If you smell gas, leave the property and call the National Gas Emergency number 0800 111 999 from outside.
  3. Wait 10 to 15 minutes, then press the reset button once on the boiler. If it fails to light again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Ignition module (control unit)
  • Ionisation (flame sensing) electrode
  • Gas valve
  • Ignition lead or plug connectors

Related codes

Full explanation for EE
Showing 71–80 of 82
EF

The boiler lights its burner but the flame goes out almost straight away, so it shuts down to stay safe and your heating and hot water may not work until a qualified engineer finds out why.

High Engineer only £150-350

What to check first

  1. Check the boiler pressure gauge and confirm it reads between 1.0 and 1.5 bar, topping up only if the manufacturer's instructions say how to do so.
  2. Check that other gas appliances in your home, such as the hob, are working, which helps rule out a gas supply interruption.
  3. Press the reset button on the boiler once, wait about 10 minutes, and see if it restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Gas valve
  • Flue or combustion fan
  • Gas pressure switch

Related codes

Full explanation for EF
F0

Your boiler has detected an internal electronic fault and has shut itself down as a precaution, so it may not be providing heating or hot water until it is checked.

High Engineer only £250-550

What to check first

  1. Check the boiler pressure gauge on the front panel. It should read between 1.0 and 1.5 bar when cold. If it is low, stop here and call an engineer, as topping up needs specific steps.
  2. Press and hold the reset button (or press the reset/OK button on the display) for a few seconds to try restarting the boiler once.
  3. Wait 5 minutes and check whether the boiler restarts and the F0 code clears. If the code returns, or the boiler will not restart, do not repeat resets. Book a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (control unit)
  • Power supply board
  • Display and control panel

Related codes

Full explanation for F0
F1

Your boiler has switched itself off because the hot gases leaving through the flue got hotter than it is designed to allow, which is a built-in safety feature doing its job.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes for it to cool.
  2. Check that the flue terminal outside your home is clear of leaves, nests, bird debris, or anything blocking it, and that nothing is stored or growing right next to it. Do not climb ladders or work at height to do this.
  3. Switch the boiler back on and press the reset button once. If the fault returns straight away or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue terminal or flue extension
  • Flue cleaning or blockage clearance

Related codes

Full explanation for F1
F2

Your boiler has shut itself down as a safety precaution because the water inside got too hot, and it will usually need a reset and a check of the system before it heats again.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at its control panel and leave it for 15 to 20 minutes so the water can cool.
  2. Check the system pressure gauge: it should read about 1 to 1.5 bar when cold. If it is low, you can top it up using the filling loop only if you are confident doing so; otherwise leave this to an engineer.
  3. Press the reset button on the boiler control panel, then check whether heating and hot water return. If the fault code comes back straight away or repeats, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Temperature limiter (safety thermostat)
  • Circulation pump
  • Flow temperature sensor
  • Expansion vessel

Related codes

Full explanation for F2
F3

The boiler has shut itself down because it detected a flame when it should not have been burning, which is a safety check and usually means a sensor or gas valve needs looking at by an engineer.

High Engineer only £120-300

What to check first

  1. Turn off the boiler at its on/off switch and check for any smell of gas. If you smell gas, leave the property, do not use electrical switches, and call the National Gas Emergency number on 0800 111 999 straight away.
  2. If there is no gas smell, check that the boiler pressure on the display is between about 1.0 and 1.5 bar, topping it up using the filling loop only if the manual instructs you to.
  3. Wait a few minutes, then press the reset button on the front panel once. If the fault returns, or the boiler will not restart, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flame ionisation electrode
  • Gas valve (combination valve)
  • Electronic control PCB

Related codes

Full explanation for F3
F8

The boiler's gas control valve isn't shutting off the gas quickly enough when it should, so the boiler has switched itself off as a safety precaution and needs a qualified engineer to check it.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve unit
  • Gas valve solenoid coil
  • Control PCB

Related codes

Full explanation for F8
F9

Your boiler's internal fan isn't starting, so it has shut itself down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £180-450

What to check first

  1. Check that the boiler has power and that its on/off switch and any external isolator are switched on.
  2. Wait a few minutes, then press the reset button on the boiler control panel once to try restarting it.
  3. If the F9 code returns, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan connecting cable or wiring harness
  • Main control PCB

Related codes

Full explanation for F9
FA

The boiler's fan has not stopped when it should, so the boiler has likely shut itself down as a safety precaution and may not be providing heating or hot water until it is checked.

High Engineer only £200-450

What to check first

  1. Check that the boiler has power and that the pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's instructions allow it.
  2. Check that the flue outlet and any air intake on the outside wall are clear of leaves, nests, snow or other debris.
  3. Wait about 10 minutes, then 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

  • Fan assembly
  • Main control PCB
  • Gas and air mixing unit seals

Related codes

Full explanation for FA
FC

Your boiler has detected a problem with its gas supply controls or its exhaust flue, so it has shut itself down as a safety precaution and needs a qualified engineer to check it before it is used again.

Emergency Engineer only £150-400

Parts commonly replaced

  • Gas valve (gas train)
  • Modulation valve actuator
  • Flue terminal or flue blockage clearance
  • Gas valve gasket or seals

Related codes

Full explanation for FC
Fd

Your boiler's control panel is reporting that a small memory card it needs to run safely is missing or not being read, so it has probably stopped working until an engineer checks it.

High Engineer only £150-300

What to check first

  1. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on.
  2. If code Fd returns, leave the boiler off and contact a Gas Safe registered engineer to check the coding card.

Parts commonly replaced

  • Coding card
  • Control unit (PCB)

Related codes

Full explanation for Fd
Showing 81–82 of 82
FE

The boiler has stopped working because its internal control settings or electronics have a fault, and it needs a qualified engineer to check and correct them.

High Engineer only £180-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait about 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and clears the FE code.
  3. Check the boiler pressure gauge and top up with the filling loop only if it is below 1 bar, following the manufacturer's instructions.
  4. If the FE code returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing or attempt to replace the coding card or PCB yourself.

Parts commonly replaced

  • Boiler coding card
  • Main PCB (control unit board)
  • Coding card connector or wiring harness

Related codes

Full explanation for FE
FF

Your boiler's internal control system has detected a fault and has likely stopped working until it is reset or repaired, so a qualified engineer should check it soon.

High Engineer only £300-650

What to check first

  1. 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 manual or ask an engineer to top it up.
  2. Turn the boiler off at its on/off switch and at the fused spur or circuit breaker, wait 2 minutes, then switch it back on.
  3. Press the reset button or select reset on the boiler, then wait a few minutes to see whether the fault clears. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main control board (PCB)
  • Power supply unit
  • Ignition or fan control module

Related codes

Full explanation for FF