Viessmann Vitodens 200-W B2HA-60 Fault codes & diagnostics

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

System Natural GasLPG 2015-present
Download the Viessmann Vitodens 200-W B2HA-60 manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
4 Emergency 38 High 24 Medium 16 Low
Just want the most common Viessmann Vitodens 200-W B2HA-60 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

The boiler has lost contact with a temperature sensor that controls one of your heating zones, so that zone may not heat properly, though the rest of your system should still work.

Medium Engineer only £90-200

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. Check that the boiler's control panel is showing this as a single fault and that the other heating zones and hot water are still working.
  3. If the code returns, stop and book a Gas Safe registered engineer, as the sensor and its wiring sit inside the boiler and controls.

Parts commonly replaced

  • Flow temperature sensor for heating circuit 3
  • Sensor cable or wiring connector
  • Mixer extension module (e.g. MM2 or EM-M1 board)

Related codes

Full explanation for 4C
9A

The boiler has lost contact with a small sensor used by the solar hot water panels on the roof, so it cannot read their temperature, but your heating and hot water will usually still work normally.

Low Engineer only £90-180

What to check first

  1. Turn off the boiler at its isolator switch or fused spur and wait 5 minutes.
  2. If you can see the solar sensor cable where it connects to the boiler without removing any panels or casing, check that the plug is pushed in fully and the cable is not damaged.
  3. Restore power, reset the boiler, and check whether the code clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 9A
9b

Your boiler's solar panel temperature sensor is reporting a broken or disconnected connection, so the boiler cannot read the solar panels properly, but your heating and gas-fired hot water should still work.

Low Engineer only £100-200

What to check first

  1. Note the exact code shown on the boiler display and check whether your home has solar thermal panels connected to the boiler.
  2. Turn the boiler off at its on/off switch and at the fused spur, wait 30 seconds, then switch it back on to see if the code clears.
  3. Check the boiler's display for heating and hot water operation, and if the code returns, book a Gas Safe registered engineer.

Parts commonly replaced

  • Collector return temperature sensor
  • Sensor cable or wiring harness
  • Solar control module (only if the fault persists)

Related codes

Full explanation for 9b
9C

The boiler has lost track of the temperature inside the hot water tank, so it may not be able to heat your stored 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, wait 60 seconds, then turn it back on to see whether the code clears.
  2. Check that the hot water cylinder sensor cable is firmly connected where it is visible, without removing any boiler casing.
  3. If the code returns or hot water is still missing, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or plug connector

Related codes

Full explanation for 9C
9E

The boiler has noticed that the solar water heating part of the system isn't circulating properly, so the solar panels may not be adding heat to your hot water, though your main heating and boiler should still work normally.

Medium Engineer only £120-350

What to check first

  1. Check the boiler display for any other fault codes and note them down.
  2. Look at the solar station pressure gauge (usually near the cylinder or in the airing cupboard). When the system is cold, the needle should sit roughly between 1.5 and 2 bar.
  3. Reset the boiler once using its reset button or menu. If the code returns, leave the solar system alone and book a Gas Safe or heating engineer with solar experience.

Parts commonly replaced

  • Solar circulation pump
  • Solar flow rate sensor
  • Solar station
  • Solar expansion vessel
  • Solar safety valve
  • Solar fluid (glycol)

Related codes

Full explanation for 9E
9F

Your boiler has lost contact with the solar heating controls, so the solar part of your system may not be working, but your boiler should still heat your home and water as normal.

Low Engineer only £150-350

What to check first

  1. Note down the full code shown on the boiler display and check whether heating and hot water still work normally.
  2. If you have a solar system, check the pressure gauge on the solar circuit; it should read roughly 1 to 2 bar when cold. Do not open the boiler casing or touch any wiring.
  3. Contact a Gas Safe registered engineer or a Viessmann-approved solar installer to check the solar control unit, its wiring and the collector sensor.

Parts commonly replaced

  • Solar control module (Vitosolic)
  • Collector temperature sensor
  • Solar circuit wiring or plug connector

Related codes

Full explanation for 9F
10

The boiler's sensor that measures the outside air temperature has a fault, so the boiler is guessing it's freezing outside and may heat your home more than it needs to, though it should still provide heating and hot water.

Low Engineer only £100-180

What to check first

  1. Turn the boiler off at its on/off switch and wait a few minutes.
  2. Look at the outside temperature sensor on the external wall (usually on the north or shaded side) and check its cable for visible damage, chafing, or water getting in.
  3. Turn the boiler back on and check whether the code clears; if it returns, book a Gas Safe registered engineer to test or replace the sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 10
18

Your boiler has lost the signal from its outdoor temperature sensor, so it is guessing the weather and may run harder than necessary, but it should still heat your home and water.

Low Engineer only £90-180

What to check first

  1. Switch off the boiler at its on/off switch and wait five minutes.
  2. Check the thin cable running from the outside sensor (usually on a north or shaded wall) for obvious damage, cuts, or chewed wiring.
  3. Check that the cable connection at the boiler's control panel is secure, then switch the boiler back on and see if the code clears.
  4. If the code returns, leave the boiler on and book a Viessmann-approved or Gas Safe engineer to test or replace the outside sensor and cable.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or lead

Related codes

Full explanation for 18
19

Your boiler has lost contact with its outside weather sensor, so it is heating your home as if it were freezing outside, which may make it run warmer or longer than necessary but it should still provide heating and hot water.

Low DIY-safe £120-250

What to check first

  1. Check that the boiler's wireless base station (the small box connected to the boiler) has power and that its indicator light is on.
  2. Check the batteries in the wireless outside temperature sensor and replace them if they are low, following the manufacturer's instructions.
  3. Make sure the outside sensor is mounted on a shaded north or north-facing wall, away from windows, vents and direct sunlight, and that nothing is blocking the signal path between the sensor and the base station.
  4. Switch the boiler off at the control panel for a few minutes, then switch it back on so it re-establishes communication with the sensor.
  5. If the code returns, contact a Gas Safe registered engineer to test the wireless signal and replace the sensor or base station if needed.

Parts commonly replaced

  • Wireless outside temperature sensor
  • Wireless base station (KM-BUS)
  • Wireless repeater
  • Sensor batteries

Related codes

Full explanation for 19
20

The boiler's sensor that measures the temperature of the water leaving it for the heating system has a fault, so the boiler is working from estimated readings instead, which usually still keeps your home heated but should be checked by an engineer soon.

Low Engineer only £100-200

What to check first

  1. Write down the code shown on the display and check the boiler pressure gauge, which should read between 1 and 1.5 bar when cold.
  2. If the pressure is low, top it up using the filling loop only if you know how, then turn the boiler off at the switch, wait 30 seconds, and switch it back on to clear the code.
  3. If the code returns, stop and book a Gas Safe registered engineer, as the sensor and its wiring need testing and replacing safely.

Parts commonly replaced

  • Low loss header flow temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 20
Showing 11–20 of 82
28

The boiler has lost the signal from one of its temperature sensors, so it is controlling the heating less precisely, but it can usually keep running while this is looked at.

Low Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, wait 30 seconds, then switch it back on to see if the code clears.
  2. Check the boiler display for any other fault codes and note them down for the engineer.
  3. If the code returns, leave the boiler on if it is heating normally and book a Gas Safe registered engineer to check the sensor and its wiring.

Parts commonly replaced

  • Low loss header flow temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 28
30

The boiler has shut itself down because its burner is being blocked by a faulty temperature sensor, so it is not producing heat or hot water until a qualified engineer checks it.

High Engineer only £150-250

What to check first

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

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor wiring harness or plug connector
  • Control circuit board (PCB), if the short circuit has caused damage

Related codes

Full explanation for 30
38

Your boiler has shut itself down as a safety precaution because it has lost track of the water temperature, so it needs a qualified engineer to check the sensor and its wiring.

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 only if you know how, otherwise leave this to an engineer.
  2. Press the reset button on the boiler, or use the reset option on the control panel, and wait 5 minutes to see if the boiler restarts.
  3. If the fault returns straight away, stop and call a Gas Safe registered engineer. Do not open the casing or try to repair or replace the sensor yourself.

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor wiring lead or connector
  • Flow temperature sensor

Related codes

Full explanation for 38
38:≠0

Your boiler has stopped its burner from working properly, so it may not be producing heat or hot water until a qualified engineer checks it.

High Engineer only £120-400

What to check first

  1. Check that your gas supply is on by confirming your gas hob or another gas appliance still works, and check that the boiler pressure gauge reads between 1 and 1.5 bar.
  2. Press the reset button on the boiler control panel once, wait about 10 minutes, and see whether the boiler restarts.
  3. If the fault code returns or the boiler will not restart, stop trying to fix it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (PCB)
  • Ionisation (flame sensor) electrode
  • Ignition electrode
  • Gas valve
  • Fan

Related codes

Full explanation for 38:≠0
40

The boiler has detected a faulty temperature sensor on one of your heating zones, so it is limiting that zone's heating while the rest of the system should still work.

Medium Engineer only £120-220

What to check first

  1. Note the fault code and check whether heating and hot water are still working elsewhere in the house.
  2. Turn the boiler off at its isolator or power switch, wait 30 seconds, then switch it back on to see if the code clears.
  3. Check the sensor cable and plug on the mixer circuit for obvious damage or loose connections, without opening any gas components.
  4. If the code returns, contact a Gas Safe registered engineer to test and replace the sensor.

Parts commonly replaced

  • Heating circuit 2 flow temperature sensor
  • Sensor cable or plug (if damaged)

Related codes

Full explanation for 40
44

The boiler has noticed a fault with the temperature sensor for one of your heating zones, so it has partly shut down the valve controlling that zone, which may mean that part of your heating runs less effectively until it is repaired.

Medium 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 clear the code.
  2. Check the controller display to see whether the code returns, and note whether the affected heating zone is still warm.
  3. If the code returns, leave the boiler as it is and book a Gas Safe registered engineer to inspect the sensor and its wiring.

Parts commonly replaced

  • Flow temperature sensor (heating circuit 3)
  • Mixer extension module
  • Sensor wiring harness

Related codes

Full explanation for 44
48

The boiler has lost the signal from one of its temperature sensors, so it is limiting heating to the radiators while it waits for a repair, and it may run less efficiently or not heat as well as usual.

Medium Engineer only £110-220

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, without removing any covers, that the boiler's controls and sensor plugs appear to be pushed in fully and look undamaged.
  3. Restore power, switch the boiler back on, and press the reset button on the front panel once. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor lead or wiring harness
  • Mixer or control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 48
50

The boiler's sensor that measures the temperature of your hot water cylinder is sending a faulty reading, so the boiler may not heat your stored hot water properly until it's repaired.

High Engineer only £100-200

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 seconds, then switch it back on to see if the fault clears.
  2. Check that the hot water cylinder is still heating, and note whether the fault code returns after a few hours of use.
  3. Make a note of the exact code and any other messages on the boiler display, and when the fault started, to pass on to the engineer.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor wiring harness
  • Boiler control PCB (only if the sensor and wiring test good)

Related codes

Full explanation for 50
58

Your boiler has lost the signal from the sensor that tells it how hot your hot water cylinder is, so it may not be heating your stored hot water properly, though your central heating may still work.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at the control panel and at the isolator switch, then wait 10 minutes before switching it back on.
  2. Check whether the boiler's hot water or cylinder settings have been changed, and note if the boiler still shows code 58 after restarting.
  3. Call a Gas Safe registered engineer if the code returns, as the sensor, its cable or its connection at the boiler will need checking and may need replacing.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or lead
  • Sensor plug or connector

Related codes

Full explanation for 58
90

The boiler has detected a faulty temperature sensor that is giving false readings, so it has likely stopped to protect itself, and a Gas Safe engineer should check it.

High Engineer only £150-300

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait 30 seconds.
  2. Turn the power back on and press the reset button or hold the reset control as shown in the manual to clear the fault.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer. Do not open the casing or disconnect sensor wiring yourself.

Parts commonly replaced

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

Related codes

Full explanation for 90
Showing 21–30 of 82
91

The boiler has detected a faulty temperature reading from one of its internal sensors, which may stop it from heating your home or providing hot water until a qualified engineer checks it.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait about 5 minutes.
  2. Turn the power back on and check whether the boiler restarts and the fault code clears.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, giving them the code 91 and the model B2HA-49.

Parts commonly replaced

  • Temperature sensor
  • Sensor wiring harness
  • Control PCB (rare)

Related codes

Full explanation for 91
92

Your boiler has detected a fault with a sensor that monitors the solar panels on your roof, so it may not be using solar energy to heat your water, but your heating and hot water should still work normally.

Low Engineer only £100-200

What to check first

  1. Note the exact code on the boiler display and check whether heating and hot water are still working.
  2. Turn the boiler off at its power switch, wait five minutes, 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, mentioning the solar sensor fault.

Parts commonly replaced

  • Solar collector temperature sensor
  • Sensor cable or connector
  • Sensor pocket or immersion sleeve

Related codes

Full explanation for 92
93

The boiler's solar heating sensor that measures the temperature of water returning from the roof panels is reporting a fault, so the boiler has flagged it, but your central heating and hot water may still be working normally.

Low Engineer only £120-220

What to check first

  1. Check whether your heating and hot water are still working normally.
  2. Turn the boiler off at its on/off switch or isolator for about 10 minutes, then switch it back on to see if the code clears.
  3. If the code returns, contact a Gas Safe registered engineer, as the sensor or its wiring usually needs checking or replacing.

Parts commonly replaced

  • Collector return temperature sensor
  • Sensor wiring lead

Related codes

Full explanation for 93
94

Your boiler has detected an electrical fault with the sensor that measures the temperature of your hot water tank, so it may not be heating stored hot water properly, though your central heating may still work.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch, wait 30 seconds, then switch it back on to see whether the code clears.
  2. If the fault code returns, do not open the boiler casing. Book a Gas Safe registered engineer to test and replace the cylinder sensor if needed.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor wiring harness or plug
  • Control PCB (rare)

Related codes

Full explanation for 94
98

The boiler has detected a break in the wiring of one of its temperature sensors, so it can't read temperatures properly and may run less efficiently or switch off until a qualified engineer checks it.

Medium Engineer only £90-200

What to check first

  1. Check the boiler display for any other fault codes and note them down before doing anything else.
  2. Switch the boiler off at its on/off switch and at the fused spur, wait a few minutes, then switch it back on to see whether the code clears.
  3. Check that any visible sensor wiring, such as the outdoor sensor cable if it is accessible from outside, is not visibly damaged, chewed by animals, or pulled loose.
  4. If the code returns or the boiler stays off, contact a Gas Safe registered engineer to test and replace the faulty sensor.

Parts commonly replaced

  • Outdoor temperature sensor
  • Flow temperature sensor
  • Return temperature sensor
  • Sensor cable or wiring harness

Related codes

Full explanation for 98
99

Your boiler has lost track of a temperature reading it needs to run safely, so it has likely shut itself down to protect the system, and a qualified engineer will need to reconnect or replace the sensor.

High Engineer only £120-220

What to check first

  1. Note the exact fault code and any other messages shown on the boiler display.
  2. Check the boiler's pressure gauge and top up with the filling loop only if it reads below 1 bar and you know how to do this safely.
  3. Try one reset using the reset button or the control panel, wait 10 minutes, and see if the boiler restarts.
  4. If the fault returns, stop resetting it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flow or return temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Control PCB (only if the sensor circuit is damaged)

Related codes

Full explanation for 99
A3

The boiler has stopped its setup check because a sensor in the flue (the pipe that carries exhaust gases outside) isn't fitted in the right place, so a Gas Safe engineer needs to correct it before the boiler can run properly.

High Engineer only £80-180

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor clip or holder

Related codes

Full explanation for A3
A4

The boiler's water pressure has risen above its safe limit, so it has shut down to protect itself, and it needs a qualified engineer to find out why before it is used again.

Emergency Engineer only £150-350

Parts commonly replaced

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

Related codes

Full explanation for A4
A7

Your boiler's control panel has detected a fault with its own internal control unit, so it may have shut down and need a qualified engineer to check it.

High Engineer only £250-500

What to check first

  1. Turn off the boiler using its on/off switch and switch off the power supply at the fused spur or isolator for about 5 minutes.
  2. Switch the power back on, turn the boiler on, and check whether the A7 code clears and the boiler starts normally.
  3. If the code returns or the boiler stays off, stop trying to repair it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Programming unit (control unit)
  • Control unit display panel
  • Printed circuit board (PCB)

Related codes

Full explanation for A7
b0

Your boiler has stopped because its exhaust gas temperature sensor is sending a faulty signal, and it has shut down as a safety precaution until a engineer checks it.

High Engineer only £120-250

What to check first

  1. Check that the flue terminal outside the property is not blocked by leaves, snow, debris or nesting material, and clear it if it is safe to do so.
  2. Check that the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up using the filling loop only if the manufacturer's instructions allow it.
  3. Turn the boiler off at the power supply, wait 30 seconds, turn it back on, and press the reset button once. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Boiler control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for b0
Showing 31–40 of 82
b1

The boiler's control panel has lost its connection to the main boiler unit, so it may not respond to settings changes, but this is usually fixable and often resolved with a simple restart.

Medium DIY-safe £90-250 for diagnosis and a cable or connector repair; £400-700 if the control unit needs replacing

What to check first

  1. Turn the boiler off at its on/off switch and at the mains fuse or isolator, then wait 2 to 3 minutes.
  2. Switch the power back on and wait about 5 minutes for the boiler to fully start up.
  3. Check the display: if the b1 code has cleared and the boiler heats water and responds to the thermostat, no further action is needed.
  4. If b1 returns, stop and contact a Gas Safe registered engineer, as the cable or control unit may need inspecting.

Parts commonly replaced

  • LON communication cable
  • Programming unit (control panel)
  • Vitotronic control unit
  • Connector plug

Related codes

Full explanation for b1
b5

Your boiler has detected a problem inside its own control system, so it may have stopped working until it is checked by a heating engineer.

High Engineer only £150-400

What to check first

  1. Turn the boiler off at its on/off switch and at the isolator or fused spur for 2 minutes, then switch it back on to see if the fault clears.
  2. Check the system pressure gauge on the front of the boiler and top it up only if it sits below 1 bar, following the filling loop instructions in the manual.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the internal fault needs professional diagnosis.

Parts commonly replaced

  • Control unit (PCB)
  • Power supply board

Related codes

Full explanation for b5
b7

Your boiler's control unit can't read its setup card correctly, so it has stopped working until a engineer checks it, but this isn't usually dangerous.

High Engineer only £150-300

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait 30 seconds.
  2. Switch the power back on and press the reset button on the boiler, if there is one.
  3. If the code returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing or try to remove the coding card yourself.

Parts commonly replaced

  • Coding card
  • Control unit PCB

Related codes

Full explanation for b7
b8

The boiler's sensor that monitors the temperature of the hot exhaust gases leaving through the flue has lost its connection, so the boiler has stopped as a precaution until it is checked and repaired.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its isolator switch, wait about 10 minutes, then turn it back on to try one reset.
  2. Check that the flue terminal outside the property is clear of leaves, snow, nests or other blockages, and that nothing is obstructing the air inlet.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, quoting code b8 and the model Vitodens 200-W B2HA-49.

Parts commonly replaced

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

Related codes

Full explanation for b8
bA

The boiler has lost contact with the control unit that runs your second 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 power isolator or fused spur, wait 30 seconds, then switch it back on.
  2. Check the boiler display to see whether the bA code clears or returns after the restart.
  3. If the code returns, leave the boiler casing and wiring alone and call a Gas Safe registered engineer.

Parts commonly replaced

  • Viessmann extension kit for heating circuit 2 (mixer)
  • BUS communication cable
  • Mixer motor

Related codes

Full explanation for bA
bb

The boiler has lost contact with an add-on part that controls one of the heating zones (the third heating circuit with its mixing valve), so that zone may not heat properly, while the rest of the system should keep working.

Medium Engineer only £180-400

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait about 2 minutes, then turn it back on to see whether the code clears.
  2. Check the thermostat or controls display to see whether the heating and hot water still work, and note whether the fault returns.
  3. Do not open the boiler casing or the extension kit. If the code returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • Viessmann extension kit (heating circuit 3 module)
  • Communication (BUS) cable or connector
  • Heating circuit 3 mixer motor
  • Control unit (Vitotronic) in severe cases

Related codes

Full explanation for bb
bC

Your boiler has lost its link with the wireless room thermostat, so it may not be controlling the heating from the room temperature correctly, though it can usually still provide heat.

Medium DIY-safe £90-220

What to check first

  1. Check the batteries in the Vitotrol room thermostat and replace them if the display is dim or blank.
  2. Make sure the thermostat display is on and shows the correct time and temperature.
  3. Switch the boiler off at its power supply or isolator switch, wait 30 seconds, then switch it back on.
  4. Check that any wall-mounted thermostat wiring you can see from outside the boiler is secure, without opening the boiler casing.
  5. If the fault code returns, contact a Gas Safe registered engineer, as the fault may be the receiver or cable.

Parts commonly replaced

  • Vitotrol remote control batteries
  • Vitotrol remote control unit
  • Connecting cable or data lead
  • Wireless receiver or control module

Related codes

Full explanation for bC
bd

The wireless room controller has lost contact with your boiler, so the heating may not respond to its settings, though the boiler itself is usually still working.

Medium DIY-safe £120-300

What to check first

  1. Check the batteries in the Vitotrol room controller and replace them if they are low or flat.
  2. Switch the room controller off, wait about 30 seconds, then switch it back on and check whether the message clears.
  3. Make sure the controller is within range of the boiler and not blocked by thick walls, metal, or other interference, then check the boiler display again after a few minutes.
  4. If the fault remains, contact a Gas Safe registered engineer to check the controller's wiring, the receiver and the boiler's control connections.

Parts commonly replaced

  • Vitotrol room controller
  • Vitotrol radio receiver module
  • Control connection cable
  • Boiler control board (rarely required)
Full explanation for bd
bE

Your boiler's control unit has lost contact with the thermostat for one of your heating zones, so that zone may not be heating properly, but your boiler itself is likely still working and this is usually a fixable connection or battery problem.

Medium DIY-safe £90-250

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. Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to clear the error and see if it reappears.
  3. Check that the cable between the thermostat and the boiler's control unit is not loose or damaged, and that the thermostat is fitted in its base or mounted securely.
  4. If the code returns, contact a Gas Safe registered engineer to check the wiring, the mixer extension module and the thermostat.

Parts commonly replaced

  • Vitotrol remote control unit
  • Mixer extension module (EM-M or similar)
  • Connecting cable between thermostat and control unit
  • Batteries for remote control

Related codes

Full explanation for bE
bF

Your boiler's control system has lost reliable communication with one of its internal modules, so it may shut down or stop heating and hot water until a qualified engineer checks and replaces the faulty part.

High Engineer only £200-400

Parts commonly replaced

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

Related codes

Full explanation for bF
Showing 41–50 of 82
C1

Your boiler has lost its connection to a separate add-on control box, so it may not be controlling some of your heating or hot water properly, but this is usually fixable by a heating engineer.

Medium Engineer only £90-250

What to check first

  1. Turn off the boiler at its on/off switch, wait 2 minutes, then turn it back on to see if the code clears.
  2. Check the boiler's display for any other codes that appear alongside C1 and note them down for the engineer.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the connection and wiring inside the boiler need checking by a professional.

Parts commonly replaced

  • EA1 extension module
  • Communication (bus) cable
  • Control unit circuit board
  • Wiring connector or plug

Related codes

Full explanation for C1
C2

Your boiler's control system has lost contact with its solar hot water controller, so the solar part may not work, but your central heating and hot water from the boiler are usually still fine.

Low DIY-safe £150-500

What to check first

  1. Switch the boiler off at its power isolator or fused spur, then wait about 60 seconds before switching it back on.
  2. Check that the cable linking the boiler to the solar controller (Vitosolic) is firmly plugged in at both ends, without opening any boiler casing.
  3. If the code returns after a restart, leave the boiler running and contact a Viessmann-registered heating engineer to check the solar control module and communication cable.

Parts commonly replaced

  • Vitosolic solar controller
  • Solar control module
  • BUS communication cable
  • Control unit circuit board

Related codes

Full explanation for C2
C3

Your boiler's control system has lost contact with one of its add-on modules, so some heating functions may not be working properly, though the boiler itself is likely still safe to use.

Medium Engineer only £150-350

What to check first

  1. Switch the boiler off at the isolator or fused spur and wait 2 minutes, then switch it back on to see if the fault clears.
  2. Check that the power supply to the boiler is steady and that no other devices on the same circuit are causing a power interruption.
  3. If the code returns, stop and contact a Gas Safe registered engineer, as the module connections and wiring need to be checked inside the unit.

Parts commonly replaced

  • AM1 extension module
  • BUS communication cable
  • Vitotronic control unit

Related codes

Full explanation for C3
C4

Your boiler has lost clear communication with its external heating controller or thermostat, so it may not respond properly to your heating settings, though it can often still run.

Medium DIY-safe £90-200

What to check first

  1. Turn off the boiler at its on/off switch and at the mains supply, wait 2 minutes, then turn it back on to reset the communication.
  2. Check that the batteries in your room thermostat or controller are fresh and replace them if needed.
  3. Look at the wiring between the boiler and the room thermostat or controller, and make sure no wires are loose or damaged. Do not open the boiler casing or touch any internal gas components.
  4. If the code returns, leave the boiler on and contact a Gas Safe registered engineer to check the OpenTherm connection and controller.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • OpenTherm connection cable or terminal block
  • Viessmann OpenTherm extension module

Related codes

Full explanation for C4
Cd

Your boiler's remote monitoring module isn't talking to Viessmann's system, which usually means a connection or signal problem rather than a fault with the heating itself.

Low DIY-safe £120-300

What to check first

  1. Check that the Vitocom 100 module has power, using the indicator light on the unit. If it is off, check the plug and socket and make sure it is switched on.
  2. Make sure the module's GSM aerial cable is firmly connected and that the aerial is not hidden behind metal or in a cupboard where the mobile signal is weak.
  3. Confirm the SIM card is active with your provider and has data available. A SIM that has run out of credit or data can cause this error.
  4. Switch the boiler off at its isolator or power switch, wait five minutes, then switch it back on. Check whether the Cd code clears.
  5. If the code returns, contact a Viessmann-registered engineer. Do not open the boiler casing or the gas side yourself.

Parts commonly replaced

  • Vitocom 100 GSM module
  • GSM aerial and extension cable
  • SIM card
  • Communication (BUS) connection cable

Related codes

Full explanation for Cd
CF

Your boiler's internal parts are having trouble talking to each other, so it may switch itself off or run in a limited way until the connection is restored.

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, wait 5 minutes, then switch it back on.
  2. Check the boiler display for any further codes after restarting and note them down.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • LON communication module
  • LON communication cable or plug
  • Control unit (Vitotronic) board

Related codes

Full explanation for CF
d6

Your boiler has detected a problem with a signal coming in from an external device or connection, which may stop heating from working as it should, but this is not usually a gas safety issue.

Medium 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, then wait 5 minutes.
  2. Turn the power back on and press the reset or OK button on the boiler panel if prompted.
  3. Check that any external controls, timers, or safety devices linked to the boiler are plugged in or connected properly, then check whether the code returns.
  4. If the code returns or heating still does not work, stop and call a Gas Safe registered engineer, as the connections inside the boiler need checking.

Parts commonly replaced

  • EA1 extension module
  • External connection wiring or plug
  • Input DE1 connection cable
  • External control or safety device

Related codes

Full explanation for d6
d7

Your boiler has spotted a problem with one of the external control signals connected to its add-on extension module, which may stop it responding correctly to your controls.

Medium Engineer only £120-300

What to check first

  1. Note the code shown on the boiler display, then turn the boiler off at its isolator switch and wait 30 seconds before switching it back on.
  2. Check that any external device wired to the boiler's extension module, such as a time switch or remote control, has power and is not visibly damaged or loose.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the wiring and extension module need professional testing.

Parts commonly replaced

  • EA1 extension module
  • Wiring harness or connector
  • External time switch or control device (if faulty)

Related codes

Full explanation for d7
d8

Your boiler has received an unexpected signal from a connected external device or extension unit, which may stop it heating or running properly until the signal is checked.

Medium Engineer only £120-300

What to check first

  1. Note the exact code and any other messages on the boiler display, then switch the boiler off at its isolator or fuse and wait 30 seconds.
  2. Switch the boiler back on and check whether the code returns or the boiler restarts normally.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as checking the wiring or extension unit needs a qualified technician.

Parts commonly replaced

  • EA1 extension module
  • Control unit circuit board
  • Connection cable or wiring harness
  • External fault or demand device

Related codes

Full explanation for d8
dA

Your boiler has lost the signal from the sensor that reads the room temperature, so it cannot fine-tune the heating to your room, though it will usually keep heating your home.

Low Engineer only £90-180

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator supplying it, then wait 10 minutes.
  2. Switch the power back on and check whether the code clears by itself or after pressing the reset button on the boiler's control panel.
  3. Look at the room sensor or room controller on your wall for obvious damage, loose wires or moisture, and make sure nothing is covering it.
  4. If the code returns, stop and call a Gas Safe registered engineer, as the fault usually needs the sensor or its wiring replaced.

Parts commonly replaced

  • Room temperature sensor (room controller)
  • Sensor connection cable
  • Boiler control unit circuit board (only if the fault persists after sensor replacement)

Related codes

Full explanation for dA
Showing 51–60 of 82
db

The boiler's temperature sensor for the second heating zone, usually the one in a room, is giving a faulty reading, so that zone may not heat correctly, though the rest of the system should still work.

Medium Engineer only £120-220

What to check first

  1. Power the boiler off at its switch and wait a few minutes, then switch it back on to see if the code clears.
  2. Check the room sensor on the wall for visible damage, and make sure its cable is not trapped, pinched or loose at the wall end.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer to test and replace the sensor.

Parts commonly replaced

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

Related codes

Full explanation for db
dC

The boiler's temperature sensor for one of the heating zones controlled by a mixer valve is reporting a faulty signal, so that zone may not heat correctly, though the rest of the system will usually keep working.

Low Engineer only £90-180

What to check first

  1. Switch the boiler off at its power supply and wait 2 minutes, then switch it back on to clear the fault.
  2. Check the wiring from the room sensor or room thermostat for this heating zone for obvious damage, such as a cut or pinched cable, without opening the boiler casing.
  3. If the code returns, contact a Gas Safe registered engineer to test and replace the sensor or check the wiring.

Parts commonly replaced

  • Room temperature sensor for heating circuit 3
  • Mixer circuit extension module
  • Sensor connection cable

Related codes

Full explanation for dC
dd

The boiler's room thermostat sensor, which tells it how warm your home is, appears to have lost its connection, so the heating may not be controlling the temperature properly, though it should still work.

Low Engineer only £120-220

What to check first

  1. Turn the heating off at the boiler's control panel and wait a few minutes.
  2. Check that the room sensor or room thermostat is plugged in or wired to the boiler correctly and that its cable is not visibly damaged or loose.
  3. If the room sensor is wireless, replace its batteries and make sure it is within range of the boiler.
  4. Turn the heating back on and see whether the code clears. If it returns, contact a Gas Safe registered engineer to test the sensor and wiring.

Parts commonly replaced

  • Room temperature sensor
  • Room sensor cable or wiring harness
  • Room sensor batteries (wireless models)

Related codes

Full explanation for dd
dE

The boiler has lost the signal from the thermostat sensor in one room that controls the heating for part of the home, so it may not heat that area correctly until it's checked.

Medium Engineer only £120-220

What to check first

  1. Switch off the boiler at its isolator switch and wait 2 minutes
  2. Check the thin cable running from the room thermostat sensor to the boiler control unit for visible damage or loose connections
  3. Switch the boiler back on and see whether the code clears; if it returns, call a Gas Safe registered engineer

Parts commonly replaced

  • Room temperature sensor
  • Sensor connection cable
  • Heating circuit 2 control wiring connector

Related codes

Full explanation for dE
dF

The boiler has lost track of the room temperature reading for one of its heating zones, which is usually a wiring or sensor issue and not an immediate danger, though that zone may not heat correctly until it's fixed.

Medium Engineer only £120-220

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 room temperature sensor cable for heating circuit 3 is firmly connected at both ends and is not damaged, kinked, or trapped.
  3. Restore power, press the reset button on the boiler controls, and see whether the fault clears.
  4. If the code returns, stop and contact a Gas Safe registered engineer, as the sensor or its connection usually needs replacing.

Parts commonly replaced

  • Room temperature sensor
  • Sensor connection cable
  • Heating circuit extension module

Related codes

Full explanation for dF
E0

The boiler's control panel has lost its connection to an add-on part that helps it communicate with the rest of your heating system, so it may not be working as it should, but this is not usually a sign of immediate danger.

Medium Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 2 minutes, then turn it back on to reset the system.
  2. With the power off, check that any cables and plugs linking the boiler to external modules, thermostats or the controller are fully pushed in and not damaged.
  3. If the code returns after the reset, do not keep resetting it. Contact a Gas Safe registered engineer to check the communication wiring and any external module.

Parts commonly replaced

  • LON communication module
  • LON bus cable or connector plug
  • Vitotronic control unit (in some cases)

Related codes

Full explanation for E0
E1

Your boiler's flame sensor is reading a stronger signal than expected while it runs its startup check, so it has shut down as a precaution and will need a qualified engineer to look at it.

High Engineer only £120-300

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then switch it back on to see if the fault clears.
  2. Check the pressure gauge on the front of the boiler is between 1.0 and 1.5 bar, and top up using the filling loop only if it is low.
  3. If the fault returns, note the code and call a Gas Safe registered engineer. Do not open the boiler casing or attempt to adjust or clean internal parts yourself.

Parts commonly replaced

  • Ionisation/ignition electrode
  • Burner gasket
  • Control PCB

Related codes

Full explanation for E1
E2

The boiler has stopped because it isn't moving enough hot water round the system, which is a protective safety shutdown rather than a sign of immediate danger.

High Engineer only £100-400

What to check first

  1. Check the pressure gauge on the front of the boiler. With the system cold, it should read between 1.0 and 1.5 bar. If it is below 1.0 bar, top it up using the filling loop, then close the loop valves.
  2. Make sure all radiator and heating valves are fully open, and that any thermostatic radiator valves are not stuck closed.
  3. Bleed any radiators that are cold at the top, as trapped air can restrict flow. Then wait a few minutes and reset the boiler once using the reset button or the controls.
  4. If E2 returns, stop and call a Gas Safe registered engineer. The likely causes, such as a blocked system filter, a faulty circulation pump or a faulty flow switch, need specialist diagnosis and repair.

Parts commonly replaced

  • Circulation pump
  • System filter or magnetic filter
  • Flow switch
  • Expansion vessel

Related codes

Full explanation for E2
E3

Your boiler is running a routine self-check to fine-tune how it burns gas, and it has flagged that it is not getting enough heat away from the burner, which is often fixed by a simple reset and a quick check of the heating system.

Low DIY-safe £0-150

What to check first

  1. Check the pressure gauge on the front of the boiler when the system is cold; it should read between 1.0 and 1.5 bar. If it is low, top it up using the filling loop as shown in the manual, then close the loop valves.
  2. Bleed any radiators that are cold at the top or feel cooler than the rest, using a radiator key, and check the pressure again afterwards.
  3. Press the reset button marked R on the boiler control panel and let the boiler run its calibration without interruption for a few minutes. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion controller sensor
  • Gas valve (rarely)
  • Circulation pump (rarely)

Related codes

Full explanation for E3
E4

Your boiler's internal control power has developed a fault, so it has likely shut itself down and may not be providing heating or hot water until a qualified engineer checks it.

High Engineer only £150-350

What to check first

  1. Check that the boiler has mains power, including the isolator switch and any fused spur, and that the boiler's display is lit.
  2. Switch the boiler off at the isolator or fused spur, wait 5 minutes, then switch it back on and see whether the code clears.
  3. If E4 returns or the boiler still won't work, stop and call a Gas Safe registered engineer. Do not open the casing or attempt internal repairs.

Parts commonly replaced

  • 24 V power supply unit
  • Main control PCB (printed circuit board)
  • PCB mains fuse

Related codes

Full explanation for E4
Showing 61–70 of 82
E5

Your boiler has shut itself down because it couldn't detect a steady flame when it lit, which is a safety measure and usually fixable by a qualified engineer.

High Engineer only £120-260

What to check first

  1. Check the pressure gauge on the boiler front. It should read between 1 and 1.5 bar. If it is low, top it up using the filling loop as shown in the manual.
  2. Press the reset button once (or switch the boiler off at its isolator, wait 10 minutes, then switch it back on) and allow it to attempt ignition again.
  3. Make sure the gas supply tap to the boiler is fully open and that the boiler and its flue outlet are not blocked by debris or vegetation.
  4. If the boiler lights and runs normally, monitor it. If the E5 code returns or the boiler locks out again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for E5
E6

Your boiler has detected that the water pressure in your central heating system has dropped below its safe level, so it has likely stopped heating until the pressure is restored.

High DIY-safe £0-250, depending on whether a leak needs fixing

What to check first

  1. Make sure the boiler is switched off and the water in the system has cooled down before you begin.
  2. Check the pressure gauge on the front of the boiler. With the system cold, it should read about 1.0 to 1.5 bar. If it is below 1.0 bar, the system needs topping up.
  3. Using the filling loop (the flexible hose with two valves, usually under the boiler), slowly open the valves to add water until the gauge reads about 1.2 to 1.5 bar. Close the valves and disconnect the hose.
  4. Restart the boiler and watch the gauge over the next few days. If the pressure keeps falling, there is probably a leak, so call a Gas Safe registered engineer.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
  • Pipe joint or radiator valve seal

Related codes

Full explanation for E6
E7

Your boiler has shut itself down because it is not detecting the gas flame properly when it starts up, which is a common issue that a Gas Safe engineer can usually fix.

High Engineer only £100-250

What to check first

  1. Check that the gas supply is on, including the gas meter and any emergency control valve, and that no other gas appliances have stopped working at the same time.
  2. Look at the boiler's pressure gauge with the system cold. If the needle is below 1 bar, top up the pressure using the filling loop as shown in the manual, then stop once it reaches about 1.5 bar.
  3. Press the reset button on the boiler once and wait a few minutes to see if it restarts. If the code returns, stop and call a Gas Safe registered engineer, as the flame sensor needs cleaning or replacing and this must not be done by a homeowner.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Burner gasket or seal
  • Earth lead and electrode connector

Related codes

Full explanation for E7
E8

The boiler's flame sensor is reading the flame's signal at an abnormal level, so it has shut itself down as a precaution. This is usually caused by a dirty or misaligned sensor, and a qualified engineer can normally fix it quickly.

High Engineer only £100-250

What to check first

  1. Check the boiler pressure gauge and top it up to about 1.0-1.5 bar if it is low, following the manufacturer's instructions.
  2. Make sure the flue outlet outside the property is not blocked by leaves, snow, nests or debris, and that the boiler has not been covered or obstructed.
  3. Turn the boiler off at the isolator switch for 30 seconds, switch it back on, and reset it once. If E8 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for E8
EA

The boiler's flame sensor isn't reading the flame strength it expects during its self-check, so it has stopped to stay safe and will need a qualified engineer to look at it.

High Engineer only £100-250

What to check first

  1. Check that the gas supply is on, including the meter and the boiler's isolator valve, and that no other gas appliances have recently been turned off.
  2. Check the system pressure on the boiler's display or gauge. If it is below 1 bar, top it up following the manual, then wait a few minutes.
  3. Press the reset button once on the boiler, wait 10 minutes, and see whether it restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for EA
Eb

Your boiler is recalibrating how it mixes gas and air for burning, which sometimes needs a reset, but if it keeps showing this code, a qualified engineer should check it.

Medium Engineer only £90-250

What to check first

  1. Check the boiler pressure on the display is between 1.0 and 1.5 bar, and top it up only if the instructions in your manual say to do so.
  2. Make sure the flue outlet outside your home is clear of leaves, nests, snow or other obstructions.
  3. Press the reset button (marked R) on the boiler and wait about 10 minutes to see whether the boiler restarts and the code clears.
  4. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gauze gasket
  • Ignition electrode
  • Gas valve (occasionally)

Related codes

Full explanation for Eb
EC

Your boiler has lost some of its stored settings during its self-check, so it has stopped working to protect itself until an engineer can reset and recalibrate it.

High Engineer only £150-450

What to check first

  1. Check the boiler pressure gauge and confirm it is between 1.0 and 1.5 bar, topping up only if the boiler manual says to.
  2. Switch the boiler off at its power supply, wait 5 minutes, then switch it back on to try a simple reset.
  3. If the code returns, stop trying and book a Gas Safe registered engineer, as the fault needs reprogramming or a part replacement.

Parts commonly replaced

  • Main PCB (control board)
  • Boiler control unit
  • Fan assembly

Related codes

Full explanation for EC
Ed

Your boiler's internal control system has detected a fault and may have shut itself down as a precaution, which usually needs a qualified engineer to diagnose and repair.

High Engineer only £200-450

What to check first

  1. Turn the boiler off using its on/off switch, then switch off the power at the fused spur or isolator for about 60 seconds.
  2. Switch the power back on and turn the boiler on, then check whether the Ed code clears and the boiler restarts.
  3. If the code returns or the boiler stays off, do not try to repair it yourself. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (Vitotronic PCB)
  • Main circuit board
  • Ignition electrode
  • Ionisation electrode

Related codes

Full explanation for Ed
EE

Your boiler has stopped because it could not light its burner safely, so it has shut itself down to protect your home and it will not heat your water or radiators until a qualified engineer checks it.

High Engineer only £120-350

What to check first

  1. If you smell gas, open windows, leave the property, do not use switches or flames, and call the National Gas Emergency Service on 0800 111 999 from outside.
  2. Check that other gas appliances in your home, such as your hob, are working, and ask neighbours or your energy supplier whether there is a local gas supply problem.
  3. Check that your gas meter emergency control valve and the boiler's isolation valve are fully open.
  4. Check the boiler pressure gauge and top it up only if it sits below 1 bar, following the manufacturer's filling instructions.
  5. Press the reset button once, wait 10 minutes, then try again. If the fault code returns, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Ignition module
  • Ionisation electrode
  • Gas valve (gas train)

Related codes

Full explanation for EE
EF

The boiler lit its burner but the flame went out again within seconds, so it has shut itself down as a safety precaution and will need a reset and a check to find out why the flame isn't staying lit.

High Engineer only £90-250

What to check first

  1. Check that the gas supply is on by confirming the gas meter isn't showing a problem and that other gas appliances work. Note that this does not mean you should touch the gas valve or burner.
  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. Make sure the condensate pipe outside is not frozen or blocked, and that the condensate trap is not blocked.
  4. Switch the boiler off at the isolator or fused spur, wait 5 minutes, switch it back on, and press the reset button on the boiler once.
  5. If the fault returns within a few attempts, stop resetting it and book a Gas Safe registered engineer, as the flame sensor, gas valve or ignition system will need inspection.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Burner gasket
  • Gas valve
  • Fan or fan speed sensor

Related codes

Full explanation for EF
Showing 71–80 of 82
F0

Your boiler's internal control system has detected a fault and may have stopped working, but this is often resolved by a simple restart, and a qualified engineer can fix it if it returns.

High DIY-safe £180-400

What to check first

  1. Check the boiler's pressure gauge and make sure it reads between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop only if you know how, otherwise leave this to an engineer.
  2. Turn off the boiler at its electrical isolator switch or fused spur, wait at least 5 minutes, then switch it back on.
  3. Press and hold the reset button or select the reset option on the display, if your model has one, and check whether the code clears and the boiler restarts normally.
  4. If the code F0 returns within a few hours or days, stop trying to reset it and book a Gas Safe registered engineer to check the control unit and wiring.

Parts commonly replaced

  • Control unit (PCB)
  • Boiler display panel
  • Wiring harness or connector

Related codes

Full explanation for F0
F1

Your boiler has switched itself off because the hot exhaust gases leaving it are running hotter than its safety limit, which usually means the flue is partly blocked or the boiler needs a professional check.

High Engineer only £100-350

What to check first

  1. Turn the boiler off at its power switch, wait about 10 minutes, then switch it back on and use the reset button on the front panel once.
  2. Look at the flue terminal on the outside wall from a safe distance for visible blockages such as leaves, nesting material, or debris, and do not poke anything into it.
  3. If the fault clears and the boiler runs normally, keep an eye on it. If F1 returns, or you smell gas or hear a CO alarm, turn the boiler off and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for F1
F2

Your boiler has switched itself off as a safety precaution because it got too hot, and it will need checking before it works normally again.

High Engineer only £150-300

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is low, follow the filling instructions in your manual to top it up.
  2. Make sure the heating system's valves and the boiler's pump are not blocked or switched off, and that radiators are not all closed off.
  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.

Parts commonly replaced

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

Related codes

Full explanation for F2
F3

The boiler has detected a flame when it should not be burning, 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 £150-350

What to check first

  1. Turn the boiler off at its on/off switch
  2. Turn off the gas supply at the meter using the emergency control valve
  3. Open a window and avoid using naval lights or flames near the boiler
  4. Call a Gas Safe registered engineer and do not repeatedly reset the boiler

Parts commonly replaced

  • Ionisation electrode (flame sensor)
  • Gas valve assembly
  • Control PCB

Related codes

Full explanation for F3
F6

The boiler has shut itself down because two of its internal temperature readings disagree too much, so it can't tell reliably how hot the water is and needs a qualified check before it can safely run again.

High Engineer only £120-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop, following the boiler manual.
  2. Make sure the boiler's power is on and that any heating controls and thermostats are not blocking it. Then press the reset button once, wait 10 minutes, and see whether the boiler restarts.
  3. If F6 returns straight away or the boiler shuts down again, stop resetting it and book a Gas Safe registered engineer. Repeated resets can make the fault harder to diagnose.

Parts commonly replaced

  • Boiler flow temperature sensor (NTC)
  • Boiler return temperature sensor (NTC)
  • Sensor wiring harness or connector

Related codes

Full explanation for F6
F8

The boiler has shut itself down as a safety precaution because the valve that controls its gas supply did not close quickly enough, so a Gas Safe engineer needs to check it before it is used again.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve assembly
  • Gas valve solenoid coil
  • Gas valve diaphragm

Related codes

Full explanation for F8
F9

Your boiler's fan isn't starting, so it can't safely run the burner, which means it has shut itself down and is likely not giving you heating or hot water until a qualified engineer looks at it.

High Engineer only £200-450

What to check first

  1. Check that the boiler has power: confirm the mains switch is on, the fuse is intact and the boiler's display is lit.
  2. Make sure the flue outlet outside is not blocked by leaves, debris, nesting material or snow, and that nothing is obstructing the air intake.
  3. Wait about 10 minutes, then press the reset button once on the boiler. If F9 returns straight away, stop and call a Gas Safe registered engineer rather than repeating the reset.

Parts commonly replaced

  • Fan assembly
  • Fan connecting cable or harness
  • Fan control PCB (main board)
  • Gas valve (if the fan fault has damaged it)

Related codes

Full explanation for F9
FA

Your boiler has shut itself down for safety because its burner and fan did not start up or settle into their normal resting state, so it is not producing heat or hot water until it is checked.

High Engineer only £150-400

What to check first

  1. Check the boiler pressure gauge and top it up to between 1 and 1.5 bar if it is low, following the filling loop instructions in the manual.
  2. Check that the gas supply tap to the boiler is fully open and that other gas appliances are working.
  3. Switch the boiler off at the isolator, wait 10 minutes, 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

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

Related codes

Full explanation for FA
FC

Your boiler has shut itself down for safety because its burner is not working correctly, which may be caused by a gas supply fault or a blocked flue, so please don't try to restart it repeatedly and arrange for a Gas Safe engineer to check it.

Emergency Engineer only £120-400

What to check first

  1. Check that the gas supply is on, including the gas meter and the boiler's own gas isolation valve, and that other gas appliances work.
  2. Look at the flue terminal outside (the pipe or grille on an outside wall) and make sure nothing is blocking it, such as leaves, nests or debris. Do not put anything into it.
  3. Check the boiler pressure gauge and top up only if it is below 1 bar, following the manufacturer's instructions.
  4. Press the reset button once. If the fault returns, stop and call a Gas Safe registered engineer. Never keep resetting it.
  5. If you smell gas, hear a hissing sound, or your carbon monoxide alarm sounds, turn off the gas at the meter, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Gas valve (gas train)
  • Modulating fan or modulation valve
  • Ignition electrode
  • Flame sensor
  • Flue terminal or flue components

Related codes

Full explanation for FC
Fd

Your boiler has stopped its burner from lighting or staying lit, so it has shut itself down for safety and will likely need a qualified engineer to check it and clear a related setting fault.

High Engineer only £120-350

What to check first

  1. Check that the gas supply to the boiler is on, including the meter and the boiler's isolation valve, and that other gas appliances are working.
  2. Check the pressure gauge on the front of the boiler. If it reads below about 1 bar, top it up using the filling loop as shown in the manual, then stop.
  3. Press the reset button or switch the boiler off at the isolator for about 30 seconds, then turn it back on once. If the fault returns or the boiler will not restart, leave it switched off and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for Fd
Showing 81–82 of 82
FE

The boiler has stopped its burner for safety and is waiting to be checked, so it may not be providing heating or hot water until an engineer looks at it.

High Engineer only £150-450

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is low, follow the filling instructions in the user manual.
  2. Press and hold the reset button (the circular button with the 'i' or reset symbol) for about 2 seconds to clear the fault. Wait 5 minutes and see if the boiler restarts.
  3. If the fault returns or the boiler does not restart, turn off the power supply to the boiler, wait 1 minute, switch it back on and try the reset once more. If it fails again, book a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Ionisation (flame sensor) electrode
  • Burner gasket
  • Gas valve
  • Fan
  • Main PCB or coding card

Related codes

Full explanation for FE
FF

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

High DIY-safe £100-300

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 the system is cold. If it is low, top it up using the filling loop only if you know how to do this safely, otherwise leave it to an engineer.
  2. Make sure nothing is blocking the boiler or its air vents, and that the gas supply and other appliances are working normally.
  3. Press the reset button marked R on the control panel once and wait a few minutes for the boiler to restart.
  4. If the fault code returns or the boiler does not restart, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for FF
Boiler cover options

Avoid the next call-out bill

Monthly cover means no unexpected bills when your boiler breaks down.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.