Viessmann Vitodens 200-W B2HA-35 Fault codes & diagnostics

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

System Natural GasLPG 2013-present
GC Number 41-819-35
Download the Viessmann Vitodens 200-W B2HA-35 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.
3 Emergency 39 High 24 Medium 17 Low
Just want the most common Viessmann Vitodens 200-W B2HA-35 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 83 documented codes

Tap any card for details

4C

Your boiler has lost track of the water temperature in one part of your heating system, so that section may not heat up properly until a engineer checks it.

Medium Engineer only £120-220

What to check first

  1. Note the exact code shown on the boiler display and check whether heating and hot water are still working.
  2. Check the boiler pressure gauge and top up with the filling loop only if it reads below 1 bar, following the boiler manual.
  3. Switch the boiler off at the isolator or mains, wait 30 seconds, then switch it back on to see whether the fault clears.
  4. If the code returns, book a Gas Safe registered engineer and tell them the code is 4C.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor lead or wiring harness
  • Mixing valve actuator motor

Related codes

Full explanation for 4C
9A

Your boiler has lost contact with a small temperature sensor that is used for a solar hot water system, so it cannot read that part of the system, but your heating and hot water will usually still work.

Low Engineer only £100-200

What to check first

  1. Turn the boiler off at the switch, wait 10 minutes, then turn it back on to see whether the code clears.
  2. Note the code and any message on the display, then check whether your home has a solar thermal panel system before doing anything else.
  3. If the code returns, contact a Gas Safe registered engineer or a Viessmann-approved heating engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Collector temperature sensor (solar)
  • Sensor lead or cable

Related codes

Full explanation for 9A
9b

A temperature sensor inside the boiler has a broken or disconnected wire, so the boiler can't read the water temperature correctly, which may stop it heating until a qualified engineer reconnects or replaces it.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the boiler's pressure gauge and top it up to the recommended level (usually between 1 and 1.5 bar) if it is low, following the manufacturer's guide.
  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

  • Flow or return temperature sensor (NTC)
  • Sensor lead or wiring harness
  • Main PCB (only if the sensor is fine but the board fails to read it)

Related codes

Full explanation for 9b
9C

Your boiler has lost track of how hot the water in your hot water cylinder is, so it may not heat that water properly, although your central heating will usually carry on working.

Medium Engineer only £120-200

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 that the cylinder sensor cable is firmly plugged in at both the sensor and the boiler's control board, without touching any other connections.
  3. Switch the boiler back on and reset it once. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 9C
9E

The boiler's solar hot water circuit is not circulating fluid properly, so the solar panels may not be adding heat to your hot water, though your main heating and gas supply can usually carry on working.

Medium Engineer only £120-350

What to check first

  1. Check the solar system pressure gauge on the boiler or solar station; with the system cold it should normally read around 1.5 to 2 bar.
  2. Turn the boiler off at the switch, wait about 10 minutes, then turn it back on and see whether the code clears.
  3. If the code returns or the pressure is low, stop there and book a Gas Safe or heating engineer who can check the solar circuit, pump and flow sensor.

Parts commonly replaced

  • Solar circulation pump
  • Solar flow sensor
  • Solar circuit fluid (glycol)
  • Solar temperature sensor
  • Solar expansion vessel

Related codes

Full explanation for 9E
9F

Your boiler's solar panel controls have lost contact or aren't working correctly, but your heating and hot water should still be working as normal.

Low Engineer only £180-400

What to check first

  1. Turn off the boiler at its on/off switch and wait 10 minutes, then switch it back on to see whether the code clears.
  2. Check that the boiler is still showing heating and hot water working normally on its display.
  3. If the code returns, leave the solar controls alone and book a Gas Safe registered engineer, as the fault usually needs a module or sensor replaced.

Parts commonly replaced

  • Vitosolic solar control module
  • Solar collector temperature sensor
  • Solar cylinder temperature sensor
  • Solar circuit pump

Related codes

Full explanation for 9F
10

The boiler's outside temperature sensor appears to be giving a faulty reading, so the boiler is adjusting heating as if it were much colder outside than it really is, which is usually not dangerous but may make your home run warmer or use more gas than necessary.

Low Engineer only £120-220

What to check first

  1. Note down the fault code and check that the boiler still has adequate pressure on its display or gauge.
  2. Switch the boiler off at its isolator or mains switch, wait 10 minutes, 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 to inspect and replace the outside sensor and its cable.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable and connector

Related codes

Full explanation for 10
18

Your boiler is still working, but it cannot read the outside temperature correctly, so it is guessing it is freezing and may heat your home more than it needs to.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its on/off switch and wait 2 minutes, then switch it back on to see if the code clears.
  2. Check the thin cable running from the outside sensor (usually on a north or shaded wall) for visible damage, chew marks, or a loose connection at the sensor.
  3. If the code returns, leave the boiler running and book a Gas Safe registered engineer to check the outside sensor and its wiring.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor cable or connector

Related codes

Full explanation for 18
19

Your boiler has stopped receiving the outside temperature reading from its wireless sensor, so it is assuming it is freezing outside and may heat your home more than it needs to, though it should still keep working.

Low DIY-safe £80-200

What to check first

  1. Check the batteries in the wireless outside temperature sensor and replace them with fresh ones if they are low or flat.
  2. Make sure the sensor is mounted on a shaded north or north-facing wall, is not covered by plants or debris, and is not blocked by metal objects or thick walls between it and the boiler area.
  3. Press the reset button on the boiler or switch it off at the isolator for about 30 seconds, then switch it back on and see whether code 19 clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Wireless outside temperature sensor
  • Batteries for outside sensor
  • Wireless receiver or boiler control module

Related codes

Full explanation for 19
20

The boiler has lost its reading of how hot the water leaving it is, so it is working by estimate, which usually means heating may run less efficiently but it is normally still safe and can often keep working.

Low Engineer only £90-180

What to check first

  1. Check the boiler pressure gauge reads between 1.0 and 1.5 bar, topping up only if the manufacturer's filling instructions say it is safe to do so.
  2. Switch the boiler off at the fused spur and on again after 30 seconds to clear the code, then see whether it returns.
  3. Note whether heating and hot water are still working, and book a Gas Safe registered engineer if the code comes back or performance drops.

Parts commonly replaced

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

Related codes

Full explanation for 20
Showing 11–20 of 83
28

The boiler has lost the sensor that measures the temperature of water leaving the heating system's central header, so it is controlling heat using less precise information, and it will usually keep running but may heat less accurately until a engineer replaces the sensor.

Low Engineer only £120-220

What to check first

  1. Check the boiler pressure on the display and top it up to between 1.0 and 1.5 bar if it is low, following the boiler manual.
  2. Switch the boiler off at the power supply for about 30 seconds, then switch it back on to see whether the code clears.
  3. If the code returns, leave the boiler on, note the code and any other messages, and book a Gas Safe registered engineer to check the sensor cable and replace the sensor if needed.

Parts commonly replaced

  • Flow temperature sensor (low loss header)
  • Sensor lead or cable
  • Sensor connector plug

Related codes

Full explanation for 28
30

Your boiler has stopped heating because a sensor that measures the water temperature has sent a faulty signal, so it has shut itself down as a safety precaution.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to see whether it restarts.
  2. 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 well below this, top it up using the filling loop as shown in the manual.
  3. Check that the boiler is not in a power cut or affected by a tripped fuse or RCD, and check that the room thermostat or programmer is set to call for heat.

Parts commonly replaced

  • Boiler water temperature sensor (NTC thermistor)
  • Sensor wiring harness or connector
  • Main PCB control board (if the sensor fault has damaged the board)

Related codes

Full explanation for 30
38

Your boiler has shut itself down because it cannot read the temperature of the water it is heating, so it is stopping the burner as a precaution.

High Engineer only £120-220

What to check first

  1. Check the boiler pressure gauge. It should read between 1 and 1.5 bar when the boiler is cold. If it is low, follow the filling instructions in your user manual.
  2. Press the reset button or use the reset option on the boiler display, wait about 10 minutes, and see whether the boiler restarts.
  3. If the fault returns or the boiler still will not heat or supply hot water, stop and call a Gas Safe registered engineer. Do not open the boiler casing or try to reconnect the sensor yourself.

Parts commonly replaced

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

Related codes

Full explanation for 38
40

The boiler has detected a faulty temperature reading for one of your heating zones, so it has closed the valve that controls that zone's water temperature, which may mean that zone heats less effectively until it is checked.

Medium Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch and at the isolator, wait 2 minutes, then turn it back on to see whether the code clears.
  2. Check the control panel for any other fault codes and note them down before calling an engineer.
  3. Look (without opening any covers) for obviously loose or damaged wiring on the boiler's control panel or near the mixer valve, and report anything you find rather than touching it.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor cable or wiring harness
  • Mixer extension module
  • Mixer valve motor

Related codes

Full explanation for 40
44

Your boiler has detected a fault with the temperature sensor controlling one of your heating zones, so that zone's valve has shut off and it may not heat properly, while the rest of your heating and hot water should still work.

Medium Engineer only £120-250

What to check first

  1. Note the code shown on the boiler display and check that the boiler still has pressure on its gauge (usually between 1 and 1.5 bar when cold).
  2. Turn the boiler off at its power switch for 30 seconds, then switch it back on to see whether the fault clears.
  3. If the code returns, leave the boiler on, avoid tampering with the wiring, and book a Gas Safe registered heating engineer to inspect the sensor and mixer circuit.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Mixer motor for heating circuit 3
  • Mixer extension module
  • Connecting cable or plug for the sensor

Related codes

Full explanation for 44
48

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

Medium Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 2 minutes before switching it back on.
  2. Check the display for the code again. If it returns, leave the boiler off and do not open the casing or touch the wiring.
  3. Check that your other heating zones and hot water still work, and note any cold radiators or unusual noises so you can tell the engineer.
  4. Book a Gas Safe registered heating engineer to check the sensor, its cable and the mixer valve.

Parts commonly replaced

  • Heating circuit 2 flow temperature sensor
  • Sensor lead or connector
  • Mixer motor or mixer extension module

Related codes

Full explanation for 48
50

The boiler's sensor that checks the hot water cylinder temperature is giving a faulty reading, so the boiler has stopped heating the water in the tank until it's checked.

High Engineer only £110-220

What to check first

  1. Switch the boiler off at its power supply, wait about 30 seconds, then switch it back on to see if the fault clears.
  2. Check the visible cable and plug leading to the cylinder temperature sensor for obvious damage, chafing or loose connections, without opening any gas components.
  3. If the fault returns, stop and call a Gas Safe registered engineer to test or replace the sensor.

Parts commonly replaced

  • Cylinder temperature sensor
  • Comfort sensor
  • Sensor cable or plug

Related codes

Full explanation for 50
51

The boiler's sensor that measures the temperature of the water leaving it is giving a faulty reading, so the boiler has stopped to protect itself and may not be providing heating or hot water until it is checked.

High Engineer only £120-250

What to check first

  1. Check the boiler pressure gauge and top up the system to between 1.0 and 1.5 bar if it is low, following the boiler manual.
  2. Turn the boiler off at the switch, wait 30 seconds, then turn it back on and press the reset button on the front if it is showing a lockout.
  3. If code 51 returns, stop attempting repairs and book a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness or plug
  • Main PCB (control board), in some cases

Related codes

Full explanation for 51
58

The boiler has lost contact with a temperature sensor that controls the hot water tank, so it may not heat stored hot water properly, although the central heating will often keep running.

Medium Engineer only £100-200

What to check first

  1. Switch the boiler off at its on/off switch and wait five minutes
  2. Switch it back on and check whether the code clears and hot water returns
  3. If the code returns, leave the boiler on for heating and book a Gas Safe registered engineer to check the sensor and its wiring

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or plug connector
  • Comfort sensor (if fitted)

Related codes

Full explanation for 58
59

Your boiler has lost track of how hot the water leaving it is, so it has shut itself down as a precaution, and a qualified engineer will need to replace a small sensor to get it running again.

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, following the boiler manual.
  2. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on to clear the fault once.
  3. If the fault returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring lead or harness
  • Sensor clip or pocket

Related codes

Full explanation for 59
Showing 21–30 of 83
90

Your boiler has detected a faulty temperature reading inside its system, so it has likely paused to protect itself, and a qualified engineer should check it.

High Engineer only £120-220

What to check first

  1. Check the boiler display for the fault code and confirm that the boiler pressure gauge reads between 1 and 1.5 bar, topping up only if the manufacturer's instructions say how.
  2. Press the reset button or use the reset option on the display once, then wait 10 minutes to see whether the heating and hot water return.
  3. If the fault returns straight away, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 90
91

The boiler's flow temperature sensor is sending a faulty signal, so the boiler has shut itself down as a safety precaution and will likely need a qualified engineer to check and replace the sensor.

High Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch and wait 10 minutes before switching it back on, as this sometimes clears a temporary fault.
  2. Check that the boiler is still under pressure on its gauge (usually between 1 and 1.5 bar when cold) and top it up only if the manual instructions say to do so.
  3. If the fault returns, stop and call a Gas Safe registered engineer rather than opening the boiler casing or disconnecting the sensor.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Control PCB (only if the sensor and wiring test good)

Related codes

Full explanation for 91
92

Your boiler has detected a faulty temperature sensor on its solar heating panel circuit, which usually means the solar part isn't working properly, though your main heating and hot water may still be working normally.

Low Engineer only £100-200

Parts commonly replaced

  • Viessmann solar collector temperature sensor (Vitosolar NTC sensor)
  • Sensor cable or plug connector

Related codes

Full explanation for 92
93

Your boiler's hot water tank thermometer seems to have an electrical fault, so the boiler may not be heating your hot water properly, though your heating may still work.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its power switch or fused spur, wait 10 minutes, then switch it back on to see whether the code clears.
  2. Check the boiler display for any further codes and note whether hot water is still working from the taps.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer to inspect the cylinder sensor and its wiring.

Parts commonly replaced

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

Related codes

Full explanation for 93
94

Your boiler has lost track of how hot the hot water tank is, so it may not be providing hot water from the tank until a qualified engineer checks the sensor.

High Engineer only £100-180

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur, then wait 5 minutes.
  2. Check that the cylinder sensor cable is firmly plugged into the boiler and that the wire isn't pinched, cut or damaged where it runs to the cylinder.
  3. Turn the power back on and reset the boiler using the reset button or the control panel. If the code returns, call a Gas Safe registered heating engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or connector
  • Control PCB (rarely required)

Related codes

Full explanation for 94
98

A temperature sensor inside the boiler appears to have a broken or disconnected wire, so the boiler can't read the water temperature correctly and may stop or run less efficiently until it is checked.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, then wait five minutes.
  2. Switch the power back on and press the reset button on the boiler or controls, if your model shows one.
  3. If the fault returns, do not open the boiler casing. Contact a Gas Safe registered engineer to inspect and replace the sensor or its wiring.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor lead or wiring harness

Related codes

Full explanation for 98
99

The boiler's sensor wiring has a break or loose connection, so it cannot read a temperature correctly, which usually means it will need a qualified engineer to check and replace the faulty sensor.

Low Engineer only £120-250

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur, then wait 10 minutes.
  2. Switch the power back on and press the reset button on the boiler control panel once.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer to inspect the sensor wiring and connectors.

Parts commonly replaced

  • Flow temperature sensor
  • Outside temperature sensor
  • Sensor wiring harness or connector

Related codes

Full explanation for 99
A3

The boiler has stopped its start-up checks because a sensor in the flue (the pipe that carries exhaust gases outside) is not fitted in the correct position, so a qualified engineer needs to reposition it before the boiler can run safely.

High Engineer only £80-150

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue gasket or seal

Related codes

Full explanation for A3
A7

Your boiler's main control panel has detected a fault with its internal programming, so it has most likely shut down and will need a Gas Safe engineer to check or replace it.

High Engineer only £250-550

Parts commonly replaced

  • Control unit (programming unit)
  • Coding card (if fitted)

Related codes

Full explanation for A7
b0

Your boiler's sensor that checks the temperature of the exhaust fumes has a wiring or electrical fault, so the boiler has likely shut itself down as a precaution until it's checked.

High Engineer only £150-300

What to check first

  1. Check the boiler's water pressure on its display or gauge. If it is below 1 bar, top it up using the filling loop as shown in the handbook, then stop.
  2. Switch the boiler off at its power supply, wait 10 minutes, switch it back on, and press the reset button once if the boiler has one.
  3. If the code returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for b0
Showing 31–40 of 83
b1

The boiler's control panel has lost its connection to the main boiler unit, so the system has likely shut down until it is reconnected or reset.

High Engineer only £90-250

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Switch the power back on and wait 2 to 3 minutes for the display to fully restart.
  3. If the code returns or the boiler stays off, do not open the casing. Book a Gas Safe registered engineer, as the connection cable or control unit may need checking or replacing.

Parts commonly replaced

  • Programming unit (control panel)
  • Connection cable between programming unit and control unit
  • Control unit PCB

Related codes

Full explanation for b1
b5

Your boiler's internal control system has detected a glitch and has likely paused itself as a precaution, which can often be cleared with a simple reset, but a repeat warning means it needs a qualified engineer to check it.

High DIY-safe £150-350

What to check first

  1. Check the boiler display for any other codes and note the pressure gauge reading, which should be between 1 and 1.5 bar when cold.
  2. Turn the boiler off at its on/off switch or the fused spur for about 1 minute, then switch it back on.
  3. If the display shows the fault again, press and hold the reset button (usually marked with a circle arrow symbol) for a few seconds to restart the boiler.
  4. If the code returns within a day or two, or you smell gas, turn the boiler off, do not attempt further resets, and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Fan assembly
  • Ignition electrode

Related codes

Full explanation for b5
b7

Your Viessmann boiler has detected a problem with its stored settings card, so it may not be able to run heating or hot water until a engineer checks it.

High Engineer only £180-350

What to check first

  1. Turn off the boiler at its power switch or fused spur and wait about 60 seconds.
  2. Turn the power back on and check whether the code b7 returns on the display.
  3. If the code clears, monitor the boiler for a day, but still book a Gas Safe engineer as the fault may come back.
  4. If the code remains, do not open the boiler casing. Contact a Gas Safe registered engineer to inspect or replace the coding card.

Parts commonly replaced

  • Coding card
  • Control unit

Related codes

Full explanation for b7
b8

The boiler has lost its link to a safety sensor that checks the exhaust fumes, so it has shut itself down as a precaution and needs a qualified engineer to reconnect or replace the sensor.

High Engineer only £150-280

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes.
  2. Turn it back on and press the reset button once, if your model has one.
  3. Check that the flue terminal outside the property is not blocked by leaves, nests or debris, and call a Gas Safe engineer if the code returns.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor lead/wiring harness
  • Control unit connection plug

Related codes

Full explanation for b8
bA

Your boiler has lost its connection to the add-on control unit that runs one of your heating zones, so that zone may not heat properly, though the rest of your heating and hot water may still work.

Medium Engineer only £120-250

What to check first

  1. Turn the heating off at the boiler's control panel, wait about 10 minutes, then switch it back on to see whether the fault clears by itself.
  2. Check the boiler display for any further fault codes and note them down for the engineer.
  3. Confirm that the boiler and heating controls are plugged in and that the boiler has not lost power, without opening any covers or touching the wiring.

Parts commonly replaced

  • Viessmann extension kit module for heating circuit 2 with mixer
  • BUS communication cable
  • Mixer motor or control module
  • Control unit connection board

Related codes

Full explanation for bA
bb

Your boiler's control system has lost contact with the add-on unit that runs one of your heating zones, so that zone may not heat properly while the rest of your system carries on working.

Medium Engineer only £100-250

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Switch the power back on and wait about 10 minutes for the controls to restart fully.
  3. Check the display: if the code has cleared and the heating zone works, keep an eye on it; if the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Viessmann extension kit (EA1 or similar)
  • Viessmann mixer extension kit (MW1 or MW2)
  • LON communication module
  • LON bus connecting cable

Related codes

Full explanation for bb
bC

The boiler has lost contact with its room thermostat or heating controller for the main heating zone, so it may not be following your temperature settings properly, though it can often still run.

Medium DIY-safe £120-250

What to check first

  1. Check that the room controller (Vitotrol) display is switched on and, if it is battery-powered, replace the batteries.
  2. Make sure the controller's cable or wireless connection is secure, then switch the boiler off at its isolator or fused spur for 30 seconds and back on.
  3. Wait about 10 minutes, then check whether the code has cleared. If it returns, call a Gas Safe registered engineer and do not open the boiler casing yourself.

Parts commonly replaced

  • Vitotrol room controller
  • Connecting BUS cable
  • Control unit circuit board

Related codes

Full explanation for bC
bd

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

Medium Engineer only £120-300

What to check first

  1. Switch the boiler off at its on/off switch and at the mains supply, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check the thermostat for this heating circuit has working batteries or power, and that its display is lit.
  3. Look at the thermostat's wiring connections on the wall, if you can see them safely, to confirm no cables are loose or damaged. Do not open the boiler casing or the control panel.
  4. If the code returns, leave the boiler on and book a Gas Safe registered engineer to check the thermostat, its cabling and the mixer module.

Parts commonly replaced

  • Vitotrol remote control (heating circuit 2)
  • Connecting BUS cable between thermostat and boiler
  • Mixer extension module for heating circuit 2

Related codes

Full explanation for bd
bE

The boiler has lost its connection to the thermostat that controls one of your heating zones, so that zone may not be heating to the temperature you set, while the rest of your heating and hot water should keep working.

Medium DIY-safe £150-350

What to check first

  1. Check the batteries in the wireless room thermostat (Vitotrol) and replace them if they are low.
  2. Make sure the thermostat is within range of the boiler and that nothing large or metal is blocking the signal.
  3. Switch the boiler off at its on/off switch, wait 2 minutes, switch it back on, and see whether the code clears.
  4. If the code returns, or the thermostat is wired rather than wireless, book a Gas Safe registered engineer rather than opening the boiler or disturbing the wiring.

Parts commonly replaced

  • Vitotrol remote control unit (heating circuit 3)
  • Thermostat batteries
  • Control bus or connection cable
  • Vitotronic extension module for the mixer circuit

Related codes

Full explanation for bE
bF

The boiler's control system is having trouble talking to one of its internal communication parts, so it may stop working properly until an engineer checks it.

High Engineer only £180-400

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator supplying it.
  2. Wait about five minutes, then switch the power back on.
  3. Check the display. If the code returns or the boiler still won't heat or supply hot water, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for bF
Showing 41–50 of 83
C1

Your boiler's main control unit has lost contact with an add-on module that manages part of your heating system, so it may not be working as intended, though it is not dangerous.

Medium Engineer only £100-250

What to check first

  1. Switch the boiler off at its on/off switch and at the wall or fused spur, wait two minutes, then switch it back on.
  2. Check the display to see whether the C1 code returns after restarting.
  3. Write down any other codes shown and note whether your heating and hot water are still working, then contact a Gas Safe registered engineer.

Parts commonly replaced

  • Vitotronic extension module EA1
  • Bus communication cable or connector
  • Control unit circuit board
  • Power supply fuse

Related codes

Full explanation for C1
C2

Your boiler has lost its connection to the solar heating controls, so it has stopped checking in with them, but your main heating and hot water should still work normally.

Low DIY-safe £90-250

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator for about 2 minutes, then switch it back on and check whether the code clears.
  2. Check that the solar controller or Vitosolic display (if you have one) has power and is switched on.
  3. If the code returns, stop and contact a Gas Safe registered engineer, as the fault may be a damaged or loose connecting cable that needs checking inside the boiler.

Parts commonly replaced

  • Vitosolic solar controller or solar control module
  • Communication (BUS) cable between boiler and solar controller
  • BUS connector or plug

Related codes

Full explanation for C2
C3

The boiler has lost its connection to a small add-on part that controls one of the heating circuits, so that part of the heating may not work properly until a qualified engineer checks it.

Medium Engineer only £150-350

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 30 seconds.
  2. Check that the cables between the boiler's control unit and the extension module (AM1) are firmly plugged in, without touching any internal parts.
  3. Turn the power back on, wait a few minutes, and see whether the C3 code clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • AM1 extension module
  • Control unit communication (BUS) cable
  • Vitotronic control unit board

Related codes

Full explanation for C3
C4

Your boiler has lost its connection to a separate heating control unit, such as the room thermostat or a weather controller, so it may not be following its usual settings, but it is not a safety hazard.

Medium DIY-safe £80-200

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait two minutes.
  2. Check that the wires on the room thermostat or external control unit are fully pushed into their terminals and look undamaged, without opening the boiler casing.
  3. Restore power, then press and hold the reset button on the boiler (or use the reset option on the display) and check whether the C4 code clears.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • Thermostat wiring or connector cable
  • Vitotronic control PCB (less common)

Related codes

Full explanation for C4
C5

The boiler's internal pump controller has lost contact with the main control unit, so the boiler may stop heating or running properly until it is checked by an engineer.

High Engineer only £180-420

What to check first

  1. Check the boiler display for any other error codes and note them down for the engineer.
  2. Check the central heating system pressure on the gauge. If it is below 1 bar, top it up using the filling loop as shown in the manual, then stop.
  3. Turn the boiler off at its isolator or power switch for 30 seconds, then turn it back on and see whether the code clears. If it returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Variable speed circulation pump
  • Pump wiring harness or connector
  • Main PCB control board

Related codes

Full explanation for C5
C6

Your boiler has lost its connection to the pump that controls one of your heating zones, so that zone may not heat properly while the rest of your heating and hot water 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, wait two minutes, then switch it back on to see whether the fault clears.
  2. Check the display for any other fault codes, and note whether heating circuit 1 and hot water still work.
  3. If the fault returns, contact a Gas Safe registered engineer. Do not open the boiler casing or the external pump wiring yourself.

Parts commonly replaced

  • External variable speed heating circuit pump
  • Connecting cable or BUS communication lead
  • Mixer extension module (Vitotronic extension kit)

Related codes

Full explanation for C6
C7

Your boiler has lost contact with the pump that moves hot water around your radiators, so heating on this part of the system may not be working, though the fault is usually fixable by a qualified engineer.

High Engineer only £120-350

What to check first

  1. Check the boiler display for any other fault codes and note them down before doing anything else.
  2. Turn the boiler off at its on/off switch and at the fused spur or isolator supplying it, wait 5 minutes, then switch it back on to see if the code clears.
  3. Check that the room thermostat or controls are switched on and set to call for heat, and that the boiler is not in a standby or holiday mode.
  4. If the code returns, stop and call a Gas Safe registered engineer, as the pump, its wiring or the control board will need checking with proper test equipment.

Parts commonly replaced

  • External variable speed heating circuit pump
  • Pump connection cable or wiring harness
  • Control board or PCB interface module

Related codes

Full explanation for C7
C8

The boiler has lost contact with the pump that serves one of your heating zones (the one with a mixing valve), so that zone may not heat properly while the rest of your home and your hot water should carry on working.

Medium Engineer only £150-400

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, wait 2 minutes, then switch it back on to see whether the code clears.
  2. Check that the boiler's controls display is not showing other fault codes alongside C8, and note any extra codes to give to the engineer.
  3. Confirm whether the other heating zones and hot water are working normally, so you know how far the fault reaches.
  4. If the code returns, do not open the boiler casing or the mixing valve area. Book a Gas Safe registered engineer and tell them the code is C8.

Parts commonly replaced

  • External variable speed heating circuit pump
  • Mixer extension module
  • Pump connection cable or plug
  • Vitotronic control unit (rarely needed)

Related codes

Full explanation for C8
Cd

Your boiler is having trouble talking to its optional internet and mobile connection unit, which is used for remote monitoring, so the heating and hot water may still work normally while this is fixed.

Low DIY-safe £120-300

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or mains supply, wait 2 minutes, then switch it back on.
  2. Check that the Vitocom 100 unit has its power adapter firmly plugged in and that its signal light is showing.
  3. Check the Vitocom 100's cable connection to the boiler, but do not open the boiler casing. If the fault returns, contact a Viessmann-approved engineer.

Parts commonly replaced

  • Vitocom 100 GSM module
  • KM BUS connection cable
  • Vitocom 100 power adapter
Full explanation for Cd
CF

Your boiler's controls have temporarily lost their connection to each other, so it may not respond properly to your thermostat or settings until this is fixed.

Medium DIY-safe £150-350

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait about 5 minutes.
  2. Turn the power back on and wait a few minutes to see whether the fault code clears and the boiler resumes normal operation.
  3. If the code returns, check that any cables plugged into the boiler's control panel or the room thermostat are fully pushed in, then restart. Do not open the boiler casing; if the fault persists, call a Gas Safe registered engineer.

Parts commonly replaced

  • LON communication module
  • LON communication cable
  • Control unit (Vitotronic)

Related codes

Full explanation for CF
Showing 51–60 of 83
d6

Your boiler has received a warning signal from a connected external device or control, so it may pause or limit its operation until the signal is checked.

Medium Engineer only £90-250

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 the boiler pressure gauge is between 1 and 1.5 bar, and top up only if the manufacturer's filling instructions are clearly followed.
  3. Look for any external controls or devices wired to the boiler, such as a safety switch or external controller, and confirm they are switched on and not obviously damaged.

Parts commonly replaced

  • Extension module EA1
  • External input wiring harness
  • External safety or control device connected to DE1

Related codes

Full explanation for d6
d7

Your boiler has received a warning signal from an external device connected to its extension module, which usually means something outside the boiler needs checking, though the boiler may still be working normally.

Medium DIY-safe £100-250

What to check first

  1. Turn off the boiler at its power switch or fused spur and wait 30 seconds.
  2. Check any external device wired to the boiler's extension module, such as a room thermostat, alarm contact, or external control, for loose wires or obvious damage, without opening the boiler casing.
  3. Turn the power back on and press the reset button on the boiler or controller. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Extension module EA1
  • Connecting cable or wiring to input DE2
  • External control or switching device connected to DE2

Related codes

Full explanation for d7
d8

Your boiler has detected a problem with one of the wired connections that link it to an add-on control unit, so it may not be controlling heating or hot water as well as it should, but this is usually fixable by a heating engineer.

Medium Engineer only £150-350

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 any external controls, thermostats or devices plugged into the boiler's external connection points are fitted securely and that their cables are not damaged or loose. Do not open the boiler casing.
  3. Restore power, switch the boiler back on and check whether the code clears. If it returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • EA1 extension module
  • Connection or wiring harness
  • External control device or sensor (if faulty)

Related codes

Full explanation for d8
dA

The boiler's control system is receiving an invalid reading from the room temperature sensor for your main heating circuit, so it may not be controlling room temperatures as precisely as it should, though heating will often still run.

Low Engineer only £110-220

What to check first

  1. Turn off the boiler at its on/off switch and check that the room sensor (usually on an inside wall, or a wireless room unit) is not damaged, loose, or pinched.
  2. Check that the sensor cable connections at the wall and at the boiler control unit are secure, with no bare wires touching each other.
  3. Turn the boiler back on and check whether the fault code clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor (or wireless room unit)
  • Sensor cable or connector
  • Control unit (rarely required)

Related codes

Full explanation for dA
db

The boiler's thermostat sensor for one of your heating zones is reporting a fault, so that zone may not heat properly, though the rest of your heating and hot water should still work.

Medium Engineer only £120-220

What to check first

  1. Switch off the boiler at its power isolator or fused spur and turn off the mains power.
  2. Check the room sensor on the affected heating zone for visible damage, loose wires, or cable that has been crushed or pinched.
  3. Restore power, reset the boiler from the control panel, and see whether the fault clears.
  4. 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)
  • Sensor connection cable
  • Mixer circuit extension module

Related codes

Full explanation for db
dC

The boiler has lost the signal from the room thermostat sensor for one of its heating zones, so that zone may not heat to the right temperature, while the rest of the system should still work.

Low Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the room sensor cable at the sensor end and at the wall for any visible damage, pinched wires or loose connections.
  3. If the fault returns, leave the boiler on and contact a Gas Safe registered engineer to test the room sensor and the mixer circuit wiring.

Parts commonly replaced

  • Room temperature sensor (heating circuit 3)
  • Extension module (EM-M or equivalent)
  • Sensor connection cable

Related codes

Full explanation for dC
dd

Your boiler has lost contact with its room temperature sensor, so it can't check how warm your rooms actually are, though it should still provide heating using its other settings.

Low Engineer only £90-180

What to check first

  1. Switch off the boiler at its on/off switch and wait a few minutes.
  2. Check that the cable from the room sensor or room control unit is firmly plugged in at both ends, and look for any visibly damaged or pinched wiring.
  3. Switch the boiler back on and see whether the code clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor connection cable
  • Control unit (rarely required)

Related codes

Full explanation for dd
dE

The boiler has lost the signal from the room thermostat sensor for its second heating zone, so that zone may not heat to the right temperature, though the rest of the system should keep working.

Low Engineer only £110-200

What to check first

  1. Turn the boiler off at its on/off switch and wait a few minutes, then switch it back on to see if the code clears.
  2. Check that the wall-mounted room sensor for heating circuit 2 is visible, undamaged and not covered by furniture or curtains.
  3. If the code returns, contact a Gas Safe registered engineer to check the sensor wiring and replace the sensor if needed.

Parts commonly replaced

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

Related codes

Full explanation for dE
dF

The boiler's control unit can't read the room thermostat sensor for one of the heating zones, so that zone may not heat to the right temperature, though the rest of the system should keep working.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at the control panel, wait 30 seconds, then turn it back on to clear the fault.
  2. Check that the room sensor for heating circuit 3 is not covered, damaged, or placed behind a curtain or radiator.
  3. If the code returns, contact a Gas Safe registered engineer to test the sensor and its wiring.

Parts commonly replaced

  • Room temperature sensor
  • Sensor connection cable

Related codes

Full explanation for dF
E0

Your boiler has lost its connection to an external accessory, such as a thermostat or add-on control unit, so it may not be controlling your heating or hot water as it should.

Medium Engineer only £120-260

What to check first

  1. Turn off the boiler at its on/off switch and the mains supply, wait 2 minutes, then turn it back on to see if the code clears.
  2. Check that any external controls or thermostats connected to the boiler are switched on and have power.
  3. If the code returns, note the number shown and contact a Gas Safe registered engineer, as the connection wiring inside the boiler needs checking.

Parts commonly replaced

  • LON communication module
  • LON bus cable or plug connector
  • Vitotronic control unit
  • External extension or accessory module

Related codes

Full explanation for E0
Showing 61–70 of 83
E1

The boiler's flame sensor is picking up a stronger electrical signal than expected while it checks itself, so it has stopped to stay safe until a Gas Safe engineer looks at it.

High Engineer only £150-300

What to check first

  1. Check that the boiler has water pressure, ideally between 1.0 and 1.5 bar on the gauge. Top it up only if the manufacturer's filling instructions say it is safe to do so.
  2. Press the reset button on the boiler once, wait about 30 minutes, and see whether it restarts. Do not repeat resets more than two or three times.
  3. If the boiler still shows E1, or if you smell gas, turn off the gas supply at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Ionisation electrode
  • Electrode gasket or seal
  • Burner gasket
  • Main PCB (in rare cases)

Related codes

Full explanation for E1
E3

Your boiler has paused because it could not finish a self-check of how it burns gas, which usually happens when it cannot get rid of its heat fast enough, and a simple reset often gets it running again.

High DIY-safe £90-180

What to check first

  1. Check the boiler pressure gauge. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop, then close the loop valves.
  2. Make sure all radiator valves and any heating zone valves are open, and turn off any thermostatic valves that might be blocking flow, so the boiler has somewhere to send its heat.
  3. Press the reset button marked R once and wait 10 to 15 minutes while the boiler runs its calibration. If the fault returns or the boiler stays locked out, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan
  • Gas valve
  • Control PCB

Related codes

Full explanation for E3
E4

Your boiler's internal control power has dropped below what it needs, so it has probably shut down and stopped providing heating and hot water until the power problem is sorted.

High Engineer only £150-350

What to check first

  1. Switch off the boiler at its on/off switch and at the consumer unit or fused spur, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and clears the E4 code.
  3. Check that the boiler's supply fuse or spur fuse has not blown and that the boiler is not on a faulty socket or extension lead, then call a Gas Safe registered engineer if the code returns.

Parts commonly replaced

  • Power supply unit (24 V)
  • Main control PCB
  • Boiler fuse

Related codes

Full explanation for E4
E5

Your boiler has switched itself off because its flame sensor isn't reading the gas flame correctly, which is a common, usually fixable problem that a heating engineer can normally sort.

High Engineer only £120-250

What to check first

  1. Check that your gas supply is on by confirming the gas meter isn't showing a fault and that other gas appliances work.
  2. Check the boiler pressure gauge is between 1 and 1.5 bar, topping up only if the manufacturer's filling instructions are followed.
  3. Press the reset button once, wait 10 minutes, and if the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flame ionisation electrode
  • Electrode lead or ignition cable
  • Gas valve
  • Control PCB

Related codes

Full explanation for E5
E7

Your boiler has stopped because it cannot detect a steady gas flame when it starts up, which is a safety check, so it has shut itself down until a qualified engineer looks at it.

High Engineer only £120-250

What to check first

  1. Check the boiler display to confirm the code reads E7 and that the boiler is not showing any other warnings.
  2. Check the system 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, following the manual.
  3. Make sure your gas meter and supply are working by checking that other gas appliances, such as the hob, light properly. If they do not, contact your gas supplier, not a plumber.
  4. Press the reset button once, wait 10 minutes, and see if the boiler restarts. If E7 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Ignition cable
  • Gas valve (in some cases)

Related codes

Full explanation for E7
E8

The boiler has shut itself down because it cannot detect its gas flame properly, which is a safety feature, and a qualified engineer can usually fix this without much fuss.

High Engineer only £100-200

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes.
  2. Check that the gas supply tap to the boiler is fully open and that the pressure gauge reads between 1 and 1.5 bar when the system is cold.
  3. Turn the boiler back on and press the reset button once. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Ignition electrode
  • Burner gasket
  • Control PCB

Related codes

Full explanation for E8
EA

Your boiler has shut itself down because its flame sensor isn't reading the gas flame correctly, which usually means it needs cleaning or a check by a qualified engineer.

High Engineer only £90-250

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then turn it back on to try a reset.
  2. Check that the gas supply is turned on at the meter and that other gas appliances are working.
  3. Check the pressure gauge on the front of the boiler reads between 1 and 1.5 bar, and top up with the filling loop only if the manufacturer's guide tells you to.
  4. Make sure nothing is blocking the flue outlet outside your home, then call a Gas Safe registered engineer if the code returns.

Parts commonly replaced

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

Related codes

Full explanation for EA
Eb

Your boiler has paused while it sets up its internal settings for burning gas safely, and it needs a reset and a check by a qualified engineer to finish the job.

High Engineer only £120-260

What to check first

  1. Check the gas supply is on by confirming your gas meter valve is open and other gas appliances work.
  2. Check the flue outlet outside your home is not blocked by leaves, nests, snow or debris.
  3. Press the reset button (marked R) on the boiler once, then wait 10 minutes for it to restart.
  4. If the fault returns, stop and call a Gas Safe registered engineer rather than repeating the reset.

Parts commonly replaced

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

Related codes

Full explanation for Eb
EC

Your boiler has detected a settings error while it was starting up its self-check, so it has paused to protect itself and may need an engineer to reset its configuration.

High Engineer only £100-300

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or circuit breaker, wait 30 seconds, then switch it back on.
  2. Check the boiler display again to see if the EC code has cleared.
  3. If the code returns or the boiler will not restart, leave it off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Coding card
  • Burner control unit

Related codes

Full explanation for EC
Ed

The boiler's internal control system has detected a fault and has likely paused itself, so it may not be providing heating or hot water until it is reset or repaired.

High DIY-safe £200-450

What to check first

  1. Switch the boiler off using its on/off switch, then turn off the mains power supply to the boiler, such as a fused spur or isolator switch nearby.
  2. Wait at least 5 minutes, then switch the power back on and turn the boiler back on.
  3. Check whether the boiler restarts and the display no longer shows 'Ed'. If the fault returns, stop resetting it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (printed circuit board)
  • Power supply unit
  • Ignition unit

Related codes

Full explanation for Ed
Showing 71–80 of 83
EE

The boiler tried to switch on but could not detect a flame, so it has stopped as a safety precaution and will need a qualified engineer to find out why.

High Engineer only £100-400

What to check first

  1. Check that your gas supply is on: confirm the gas meter has credit if it is prepayment, and that the gas isolation valve on the pipe near the boiler is open (handle parallel to the pipe).
  2. Check that other gas appliances, such as the hob or a gas fire, are working. If they are not, contact your gas network emergency line on 0800 111 999 rather than your boiler engineer.
  3. Press the reset button on the boiler once, wait a few minutes, and see whether it starts. If it fails again, stop and book a Gas Safe registered engineer. Do not keep resetting it.

Parts commonly replaced

  • Ignition and ionisation electrode
  • Gas valve (gas train)
  • Gas filter
  • Gas pressure switch
  • Control unit PCB

Related codes

Full explanation for EE
EF

The boiler started to light but the flame went out almost straight away, so it has shut down and is not heating your home or water until a qualified engineer checks it.

High Engineer only £120-280

What to check first

  1. Check that the gas supply is on by confirming the gas meter and any isolation valve are open and that other gas appliances are working.
  2. Check that the boiler has power and the display is on, then press the reset button once and wait about 10 minutes for it to try again.
  3. Make sure the flue outlet outside is not blocked by leaves, debris or snow, and that nothing is stored around the boiler.
  4. If the code returns or the boiler will not stay on, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Gas valve
  • Gas-air mixing unit seal or gasket

Related codes

Full explanation for EF
F0

The boiler's internal control system has detected a fault and has likely stopped working until it is reset or repaired, so heating and hot water may be unavailable.

High Engineer only £200-450

What to check first

  1. Check the boiler's water pressure on the display or gauge; if it is below about 1 bar, top it up using the filling loop as shown in the manual.
  2. Switch off the boiler at its isolator or fused spur, wait 10 minutes, then switch it back on to try a reset.
  3. If the F0 code returns, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (printed circuit board)
  • Power supply unit
  • Display or control panel module

Related codes

Full explanation for F0
F1

Your boiler has shut itself down as a safety precaution because the hot exhaust gases leaving it are running hotter than they should, so please don't ignore it and arrange a Gas Safe engineer to check it soon.

Emergency Engineer only £90-300

What to check first

  1. Turn the boiler off at its on/off switch and leave it off for a few minutes.
  2. From outside the property, check the flue outlet on the wall or roof for visible blockages such as nests, leaves, or debris, but do not climb ladders or touch the flue if you are unsure.
  3. If the outlet looks clear, turn the boiler back on and press the reset button once; if the fault returns or the boiler shuts down again, stop and call a Gas Safe registered engineer.
  4. If you notice a gas smell, hear a hissing sound, feel unwell, or have a carbon monoxide alarm sounding, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

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

Related codes

Full explanation for F1
F2

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

High Engineer only £120-350

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold, so top it up using the filling loop if it is low.
  2. Make sure all radiator valves and any system isolation valves are fully open, and bleed any cold radiators that feel cooler at the top.
  3. Wait at least 15 minutes for the boiler to cool, then press the reset button once. If the fault returns, stop and call a Gas Safe registered engineer, as repeated resets can hide a fault that needs repair.

Parts commonly replaced

  • Temperature limiter (STB)
  • Central heating circulation pump
  • Flow temperature sensor
  • Heat exchanger (uncommon, usually only if scaled or damaged)

Related codes

Full explanation for F2
F3

Your boiler has detected a flame when it should not be burning, so it has stopped as a precaution and needs a qualified engineer to check it safely.

High Engineer only £80-450

What to check first

  1. Switch the boiler off at its on/off switch and at the isolator or fused spur if you can reach it safely.
  2. Check for any smell of gas. If you smell gas, open windows and doors, avoid switches and flames, leave the property, and call the National Gas Emergency number on 0800 111 999 from outside.
  3. If there is no gas smell, leave the boiler off and do not keep resetting it, then book a Gas Safe registered engineer to inspect the flame sensor and gas valve.

Parts commonly replaced

  • Flame ionisation electrode
  • Gas valve
  • Burner control PCB
  • Burner gasket

Related codes

Full explanation for F3
F8

The boiler's gas shut-off valve didn't close quickly enough when it was supposed to stop the burner, so it has shut down as a safety precaution and needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £180-380

What to check first

  1. Turn the boiler off at its on/off switch and do not keep trying to restart it.
  2. Turn off the gas supply at the meter or the isolation valve next to the boiler if you can reach it safely.
  3. Open a window and ventilate the room, and check that your carbon monoxide alarm is working.
  4. If you smell gas, leave the property immediately and call the National Gas Emergency Service on 0800 111 999 from a safe location.
  5. Otherwise, book a Gas Safe registered engineer to inspect and repair the gas valve system.

Parts commonly replaced

  • Gas valve assembly
  • Gas valve actuator/solenoid coil
  • Control PCB
  • Ignition electrode

Related codes

Full explanation for F8
F9

The boiler's internal fan is not starting when it needs to, so it has stopped and is unable to heat your home or water safely until it is checked by an engineer.

High Engineer only £200-400

What to check first

  1. Check that the boiler has power: confirm it is switched on at the wall and that the display is lit.
  2. Check the water pressure gauge on the front of the boiler. It should read between 1 and 2 bar when cold; if it is low, top it up using the filling loop as shown in the manual.
  3. Press the reset button once (or use the reset option on the display) and wait 10 minutes. If the fault returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan connecting cable
  • Fan control PCB (main board)

Related codes

Full explanation for F9
FA

The boiler's fan has not slowed down and stopped properly when it should have, so the boiler has shut itself down as a safety precaution, and a qualified engineer will need to check it.

High Engineer only £200-450

What to check first

  1. Check the boiler's pressure gauge. 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 the manual.
  2. Make sure the flue outlet outside your home is not blocked by leaves, nests, snow or debris.
  3. Press the reset button on the front of the boiler once, wait 10 minutes, and see if it restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for FA
FC

Your boiler has shut itself down as a safety precaution because part of its gas supply or exhaust route isn't working properly, so it needs a qualified engineer to check it before it's used again.

Emergency Engineer only £150-450

Parts commonly replaced

  • Gas valve or gas train assembly
  • Modulation coil
  • Fan assembly
  • Flue terminal or flue blockage clearance

Related codes

Full explanation for FC
Showing 81–83 of 83
Fd

Your boiler cannot read the settings card that tells it how to operate, so it may not heat your home or water until a qualified engineer checks it.

High Engineer only £120-250

What to check first

  1. Note the codes shown on the boiler display (Fd and b7) and check the boiler's manual for any other messages.
  2. Switch the boiler off at its power isolator or fused spur, wait 30 seconds, then switch it back on to see if the fault clears.
  3. If the code returns, stop and book a Gas Safe registered engineer, since the coding card sits inside the boiler's control unit and should not be handled by non-qualified people.

Parts commonly replaced

  • Coding card
  • Vitotronic control unit

Related codes

Full explanation for Fd
FE

Your boiler has stopped working because its internal control settings have a fault, so it needs a qualified engineer to check and repair it before heating and hot water can return.

High Engineer only £120-450

Parts commonly replaced

  • Boiler coding card
  • Main PCB (printed circuit board)

Related codes

Full explanation for FE
FF

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

High Engineer only £250-450

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator that supplies it, then wait 5 minutes.
  2. Turn the power back on and wait for the boiler to restart. Check the display for the code again.
  3. If the code returns or the boiler stays off, stop and call a Gas Safe registered engineer. Do not open the casing or attempt internal repairs.

Parts commonly replaced

  • Main control board (PCB)
  • Power supply unit
  • Ignition electrode assembly

Related codes

Full explanation for FF
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