Viessmann Vitodens 200-W B2KB-26 Fault codes & diagnostics

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

Combi Natural GasLPG 2016-present
Download the Viessmann Vitodens 200-W B2KB-26 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.
7 Emergency 54 High 18 Medium 10 Low
Just want the most common Viessmann Vitodens 200-W B2KB-26 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 89 documented codes

Tap any card for details

9A

The boiler has lost the signal from a sensor that measures the temperature of the solar panels on the roof, so it can't use free solar energy to help heat your water, but your normal heating and hot water should still work.

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 for about 5 minutes, then switch it back on.
  2. Check the display to see if fault code 9A has cleared.
  3. If the code returns, leave the boiler as it is and arrange for a Gas Safe registered engineer or a Viessmann-approved installer to check the collector sensor and its cable.

Parts commonly replaced

  • Solar collector temperature sensor
  • Sensor cable or lead
  • Sensor plug connector

Related codes

Full explanation for 9A
9b

The boiler has lost the signal from one of its small temperature sensors, which is usually linked to the solar water heating part of the system, so it is a fault to get checked but is unlikely to leave you without heating or hot water.

Low Engineer only £120-220

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 10 minutes.
  2. Check that the solar sensor cable is pushed fully into its socket on the boiler control unit and that the cable is not visibly cut, crushed or burnt.
  3. Restore power, switch the boiler back on and see whether the code clears. If it returns, leave the boiler as it is and book a Gas Safe registered engineer.

Parts commonly replaced

  • Collector return temperature sensor
  • Sensor cable or connector
  • Control unit (rarely required)

Related codes

Full explanation for 9b
9C

The boiler has lost its connection to the sensor that tells it how hot the stored hot water is, so it may stop heating your water until the sensor is checked and reconnected.

High Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch and wait a few minutes.
  2. Check that the wire plug from the cylinder sensor is firmly connected at the boiler and that the cable is not visibly damaged, pinched or loose.
  3. Turn the boiler back on and press the reset button once to see if the fault clears.
  4. If the code returns, stop and call a Gas Safe registered engineer to test or replace the cylinder sensor.

Parts commonly replaced

  • Cylinder temperature sensor (NTC)
  • Sensor cable or wiring harness

Related codes

Full explanation for 9C
9E

The boiler's solar heating panel circuit isn't circulating fluid properly, so it has shut that part down as a safety precaution, though your main heating and hot water may still work.

Medium Engineer only £120-300

What to check first

  1. Check the boiler display for any pressure warning, and note whether the solar pressure gauge reads below about 1.0 bar.
  2. Switch the boiler off at the isolator, wait a few minutes, then switch it back on and see whether the code clears.
  3. If the code returns, leave the boiler off or as it is and contact a Gas Safe registered engineer, as the solar circuit contains hot, pressurised fluid that should not be opened or topped up without training.

Parts commonly replaced

  • Solar circuit pump
  • Solar flow sensor or flow meter
  • Solar temperature limiter
  • Solar heat transfer fluid (glycol)

Related codes

Full explanation for 9E
9F

Your boiler's solar heating controls have detected a fault, so the solar panels may not be working properly, but your central heating and hot water should still be running as normal.

Medium Engineer only £150-350

What to check first

  1. Check that the boiler and solar controller are receiving power and that no fuse has blown, then switch the boiler off at the isolator, wait 30 seconds, and switch it back on to see if the code clears.
  2. If the code returns, note any other codes on the display and avoid opening the control panel or touching wiring.
  3. Contact a Gas Safe registered heating engineer, or a Viessmann-approved installer, to inspect the solar control module, sensors and wiring.

Parts commonly replaced

  • Solar control module (Vitosolic)
  • Solar collector temperature sensor
  • Cylinder temperature sensor
  • Solar control module fuse

Related codes

Full explanation for 9F
10

Your boiler's outdoor temperature sensor appears to have an electrical fault, so it is working as if it is freezing outside, which may cause it to run warmer or less efficiently than needed, though heating and hot water usually still work.

Low Engineer only £90-180

What to check first

  1. Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on to see if the code clears.
  2. Check the outdoor sensor cable from the outside of the house for visible damage, chewed wires, or water getting into the connection, without opening the boiler casing.
  3. If the code returns, book a Gas Safe registered engineer to test or replace the outside temperature sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor cable or connector

Related codes

Full explanation for 10
18

Your boiler has lost the reading from its outside temperature sensor, so it is guessing the weather and still heating your home, though it may not adjust the heat as accurately as it should.

Low Engineer only £100-200

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or lead

Related codes

Full explanation for 18
19

Your boiler is still heating your home, but it has lost its wireless signal from the outdoor temperature sensor, so it is guessing the weather and may run hotter or cooler than necessary.

Low DIY-safe £80-180

What to check first

  1. Locate the wireless outdoor temperature sensor, usually mounted on an outside wall, often facing north or shaded.
  2. Check the sensor's batteries and replace them with new ones if they are low or flat.
  3. Make sure nothing is blocking the sensor or its line of sight to the boiler, and that it is not mounted near a large metal object or a new Wi-Fi or radio device.
  4. Switch the boiler off at its power supply for about a minute, then switch it back on and check whether the code clears.
  5. If the code returns, contact a Gas Safe registered engineer to check the wireless receiver and sensor pairing.

Parts commonly replaced

  • Wireless outdoor temperature sensor
  • Sensor batteries
  • Wireless receiver module

Related codes

Full explanation for 19
20

The boiler's sensor that measures how hot the water leaving it is has sent a faulty reading, so the boiler has shut itself down as a precaution and is probably not providing heating or hot water until it is checked.

High Engineer only £150-250

What to check first

  1. Turn off the boiler at its on/off switch and wait about 10 minutes.
  2. Switch the boiler back on and check whether it restarts.
  3. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 20
28

Your boiler has lost the signal from the sensor that measures how hot the water leaving it is, so it has stopped heating your home as a safety precaution, and a qualified engineer needs to replace or reconnect that sensor.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor
  • Sensor lead / wiring harness
  • Control PCB (if the sensor and wiring test good)

Related codes

Full explanation for 28
Showing 11–20 of 89
30

The boiler's sensor that measures how hot the water leaving it is has a wiring fault, so the boiler has likely shut itself down as a safety precaution and may not be providing heating or hot water until it's checked.

High 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 about 10 minutes.
  2. Turn the power back on and press the reset button on the boiler (if it has one) or follow the reset instructions in the user manual.
  3. Check that the system pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if the boiler's manual says it is safe to do so.
  4. If the code returns, leave the boiler off and call a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler flow temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Main PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 30
38

Your boiler has lost track of the water temperature reading it uses to stay safe, so it has shut itself down until a qualified engineer can check the connection or sensor.

High Engineer only £120-220

What to check first

  1. Check the boiler's display and note whether it shows any other codes alongside 38.
  2. Check that the boiler has power and that the controls are switched on.
  3. Look at the boiler's pressure gauge. If it is below 1 bar, top it up using the filling loop as shown in the manual, then stop.
  4. Try one reset by pressing the reset button once. If code 38 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler water temperature sensor (flow sensor)
  • Sensor wiring harness or connector
  • Control PCB (only if the fault persists after sensor replacement)

Related codes

Full explanation for 38
50

The boiler's hot water cylinder temperature sensor, or its comfort sensor, is sending a faulty signal, so the boiler may not heat your hot water correctly until it is checked.

Medium Engineer only £120-220

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 cylinder sensor cable and any comfort sensor cable are securely plugged in and not visibly damaged, pinched or chafed.
  3. If the code returns, stop and contact a Gas Safe registered engineer, as the sensor or its wiring will need testing and replacing.

Parts commonly replaced

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

Related codes

Full explanation for 50
51

Your boiler has detected a faulty temperature sensor that measures the water leaving the boiler, so it has stopped heating as a safety precaution and will need a qualified engineer to check and replace the sensor.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur for 5 minutes, then switch it back on to see if the fault clears.
  2. Check the boiler's display for any other fault codes and note them down for the engineer.
  3. If the fault returns, do not keep resetting the boiler. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet temperature sensor
  • Sensor wiring harness
  • Control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 51
58

The boiler has lost contact with one of its temperature sensors, so it can't read the water or room temperature correctly and may stop heating your hot water or controlling the rooms properly until it's checked.

Medium Engineer only £100-220

What to check first

  1. Turn the boiler off at its on/off switch, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. With the boiler switched off at the isolator, check that the sensor cable plugs are firmly pushed into their sockets and that no wires look loose, burnt, or damaged.
  3. If the code returns, do not open the boiler casing. Contact a Gas Safe registered engineer to test and replace the faulty sensor.

Parts commonly replaced

  • Cylinder temperature sensor
  • Comfort (room) sensor
  • Sensor cable or lead harness
Full explanation for 58
59

The boiler's sensor that measures how hot the water leaving it is has lost its connection, so the boiler can't read the temperature and has stopped to stay safe.

High Engineer only £120-220

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait 10 minutes.
  2. Turn the power back on and press the reset button on the boiler, or use the reset option on the display.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the sensor and its wiring sit inside the boiler and should not be opened by a homeowner.

Parts commonly replaced

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

Related codes

Full explanation for 59
70

Your boiler has stopped because it has detected that the gas supply pressure is not high enough to start safely, so it is not heating or providing hot water until the gas supply is checked.

High Engineer only £100-250

What to check first

  1. Check whether other gas appliances, such as the hob or fire, are working. If none are, the problem may be with the gas supply rather than the boiler.
  2. Find your gas meter and check that the emergency control valve is fully open. Do not force it or touch any other part of the meter.
  3. Wait about 10 minutes, then turn the boiler off at the switch and back on once. If it locks out again, stop and call a Gas Safe registered engineer.
  4. If you smell gas at any point, leave the property immediately, do not use switches or flames, and call the National Gas Emergency Service on 0800 111 999 from outside.

Parts commonly replaced

  • Gas pressure switch
  • Gas valve
  • Gas inlet filter

Related codes

Full explanation for 70
71

Your boiler has detected that the electricity supply coming into your home is weaker than it should be, so it has paused to protect its components until the power returns to normal.

Medium Engineer only £90-200

What to check first

  1. Check whether other appliances or lights in your home are dimming, flickering or behaving oddly, which suggests a wider supply problem.
  2. Turn the boiler off at its isolator switch or fused spur, wait 30 seconds, then turn it back on to see whether the fault clears.
  3. If the fault returns or other appliances are affected, contact your electricity network operator (the number is on your electricity bill or the 105 non-emergency line) to report a low voltage supply.
  4. If the fault keeps coming back, book a Gas Safe registered engineer or qualified electrician to check the boiler's mains connections and measure the supply voltage.

Parts commonly replaced

  • Mains supply cable or terminal connections
  • Voltage stabiliser or surge protector (in some installations)
  • Main PCB (only if the supply is confirmed normal and the fault persists)

Related codes

Full explanation for 71
73

Your boiler has detected a communication fault with one of its connected add-on parts, so it is stopping itself from firing up to heat your home or water until an engineer can check it.

High Engineer only £120-260

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and clears the code.
  3. Check that the boiler's gas supply valve is open and that the system water pressure is between 1 and 1.5 bar on the gauge.
  4. If the fault returns, do not keep resetting the boiler. Call a Gas Safe registered engineer.

Parts commonly replaced

  • H1 extension module (accessory relay board)
  • Control unit PCB
  • Connection wiring loom

Related codes

Full explanation for 73
74

Your boiler has shut itself down as a safety precaution because it has detected an electrical fault that needs a qualified engineer to inspect before it can safely restart.

High Engineer only £100-300

Parts commonly replaced

  • Safety temperature limiter (STB)
  • Wiring harness or earth connection
  • Main control PCB

Related codes

Full explanation for 74
Showing 21–30 of 89
75

Your boiler has detected an electrical wiring fault that it considers unsafe, so it has shut itself down to protect you, and it needs a qualified engineer to find and fix the cause.

High Engineer only £150-400

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 5 minutes.
  2. Check that the area around the boiler and its wiring is dry and free of visible water, damage, or scorching.
  3. Restore power and try one reset using the reset button on the boiler. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB (circuit board)
  • Mains wiring harness or internal connector cable
  • Earth bonding lead

Related codes

Full explanation for 75
76

Your boiler has stopped because its gas pressure safety sensor has detected an electrical fault, so it has shut down to protect your home until a qualified engineer checks it.

High Engineer only £150-300

What to check first

  1. Check the boiler pressure gauge and top it up to the level in the manual (usually around 1.0 to 1.5 bar) if it is low.
  2. Press the reset button once, wait 10 minutes, and see whether the boiler restarts.
  3. Check that other gas appliances, such as your hob, are working, and that the boiler room or cupboard is ventilated.
  4. If the fault returns or you smell gas, turn off the gas supply at the meter, open windows, and call the National Gas Emergency number 0800 111 999.

Parts commonly replaced

  • Gas pressure switch
  • Gas pressure switch wiring harness
  • Gas valve connector

Related codes

Full explanation for 76
80

Your boiler has stopped because it isn't receiving enough gas to run safely, so it has shut itself down until the supply is checked.

High Engineer only £80-180

What to check first

  1. Check whether other gas appliances, such as the cooker or a gas fire, are working. If none of them are, the problem is likely with the gas supply rather than the boiler.
  2. If you have a prepayment gas meter, check that it has credit and that the emergency control valve is fully open. Contact your gas supplier if you are unsure.
  3. Check that the gas meter isolation valve is fully open. Do not touch any other valves or fittings on the gas pipework.
  4. Wait a few minutes, then reset the boiler once using the reset button on the front panel. If the code returns, stop and call a Gas Safe registered engineer.
  5. If you smell gas at any point, leave the property, do not use switches or flames, and call the National Gas Emergency number on 0800 111 999 straight away.

Parts commonly replaced

  • Gas pressure switch
  • Gas valve
  • Gas inlet filter

Related codes

Full explanation for 80
80/EE

Your boiler has stopped because it is not receiving the gas it needs to start, so it cannot currently provide heating or hot water.

High Engineer only £150-350

What to check first

  1. Check that the gas supply is switched on at the gas meter and the boiler's own gas isolation valve, and that other gas appliances such as the hob are working.
  2. Check the boiler's pressure gauge and water pressure, then try one manual reset using the reset button or the control panel, waiting a minute before trying again.
  3. If the fault returns, or you smell gas at any time, turn off the gas supply at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999 or a Gas Safe registered engineer.

Parts commonly replaced

  • Gas valve (gas train)
  • Gas filter
  • Gas pressure switch
  • Control PCB

Related codes

Full explanation for 80/EE
81

Your boiler is not lighting its burner when it starts up, so it has stopped working and is not providing heating or hot water until a qualified engineer can check it.

High Engineer only £120-300

What to check first

  1. Check that other gas appliances, such as your cooker or hob, are working, to confirm the gas supply is on.
  2. Check the boiler's pressure gauge. It should read between 1 and 1.5 bar. If it is low, top it up using the filling loop only if you know how to do so safely.
  3. Press the reset button once, wait 10 minutes, and see whether the boiler restarts. If it faults again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Ignition cable
  • Ignition unit or transformer
  • Gas valve
  • Main PCB (control board)

Related codes

Full explanation for 81
81/EE

Your boiler tried to light its burner but couldn't keep a flame going, so it has shut down as a safety measure and is probably not providing heating or hot water until it's checked.

High Engineer only £150-350

What to check first

  1. If you smell gas at any point, leave the property, do not use electrical switches or flames, and call the National Gas Emergency number on 0800 111 999 from outside.
  2. Check the boiler's pressure gauge. It should read between 1 and 1.5 bar when the boiler is cold. If it is low, follow the top-up instructions in your user manual.
  3. Confirm your gas supply is working by checking that your gas hob or other gas appliances light, and that the gas meter's emergency control valve is open. Then press the reset button on the boiler and wait a few minutes before trying again. If the fault returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Ignition electrode
  • Ignition module (ignition unit)
  • Gas valve
  • Flame sensor (ionisation electrode)

Related codes

Full explanation for 81/EE
82

Your boiler tried to light its burner but couldn't detect a steady flame, so it has stopped to stay safe, and it will need a check to find out why.

High Engineer only £100-300

What to check first

  1. Check that the gas supply to the boiler is turned on, including the isolation valve near the boiler and the meter tap.
  2. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top it up using the filling loop only if it is low.
  3. Press the reset button once and wait 10 minutes. If the fault returns or the boiler locks out again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Gas pressure switch
  • Control PCB

Related codes

Full explanation for 82
82/EE

The boiler started up to heat your home but could not detect a steady gas flame, so it has stopped as a safety precaution and will need a reset and a check of the burner.

High Engineer only £90-220

What to check first

  1. Check that the gas supply is on: confirm your gas meter emergency control valve and any boiler isolation valve are open, and that other gas appliances such as the hob are working.
  2. Check the boiler pressure gauge and top it up to about 1.0 to 1.5 bar if it is low, using the filling loop, as low pressure can stop the boiler starting.
  3. Press the reset button on the boiler control panel once and wait 10 to 15 minutes to see whether it restarts normally.
  4. If the code returns or you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999 straight away.
  5. If the boiler still shows 82/EE after one reset, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation and ignition electrode assembly
  • Electrode gasket or seal
  • Gas valve (if the gas supply or valve is faulty)
  • Gas filter

Related codes

Full explanation for 82/EE
83

Your boiler has detected a flame when it should not be burning, so it has stopped as a safety precaution, and a Gas Safe engineer should check it before it is used again.

High Engineer only £120-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 10 minutes.
  2. Switch the boiler back on and press the reset button on the front panel once, then check whether it restarts normally.
  3. If the fault code returns, or the boiler will not stay on, stop trying and call a Gas Safe registered engineer. Do not attempt to open or adjust the gas valve or the burner.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Gas valve
  • Printed circuit board (PCB)
  • Gas/air mixture unit

Related codes

Full explanation for 83
84

The boiler has detected a higher-than-safe level of carbon monoxide inside itself and has shut down as a safety precaution, so it needs a qualified engineer to check it before it is used again.

Emergency Engineer only

Parts commonly replaced

  • Heat exchanger
  • Burner
  • Burner gasket and seals
  • Fan assembly
  • Gas valve
  • Flue and air intake components

Related codes

Full explanation for 84
Showing 31–40 of 89
85

Your boiler has detected a problem with the sensor that measures the temperature of your hot water cylinder, so it may not be heating your hot water properly, but your central heating and the boiler itself are usually still safe to use.

Medium Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch, wait about 30 seconds, then switch it back on to see if the fault clears.
  2. Check the boiler's display and note whether the code returns and whether hot water and heating are still working.
  3. Check the boiler's water pressure on its gauge and, if it is below 1 bar, top it up as shown in the manual.
  4. If the code returns, contact a Gas Safe registered engineer, as the sensor and its wiring need professional testing.

Parts commonly replaced

  • Cylinder temperature sensor (DHW sensor)
  • Sensor cable or plug connector
  • Control unit (rarely required)

Related codes

Full explanation for 85
90

The boiler's internal thermometer has reported a faulty reading, so it may switch itself off or run less efficiently until the sensor is checked and repaired.

Medium Engineer only £120-260

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check that the boiler's water pressure is between 1 and 1.5 bar on the gauge, topping up only if the manufacturer's filling instructions allow it.
  3. If the fault returns, note the exact code and any other messages on the display and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for 90
91

The boiler's control system has detected a temperature sensor reporting a faulty electrical reading, so it has likely stopped to protect itself and needs 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 isolate its electrical supply at the fused spur.
  2. Wait a few minutes, then check that the plug on the temperature sensor cable is firmly connected and that the cable is not visibly damaged or pinched.
  3. Restore power, switch the boiler on and press the reset button once. If the fault returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Temperature sensor cable or plug harness

Related codes

Full explanation for 91
92

The boiler has detected a fault with a temperature sensor in the solar panel circuit, which is usually not stopping your heating or hot water, so it's worth getting it checked but not an emergency.

Low Engineer only £120-260

What to check first

  1. Note the exact code on the boiler display and check whether your home has solar thermal panels connected to this boiler.
  2. Switch the boiler off at the control panel, wait 30 seconds, then switch it back on to see if the code clears.
  3. If the code returns, leave the boiler casing closed and contact a Gas Safe registered engineer or Viessmann-approved installer to inspect the solar collector sensor and its cable.

Parts commonly replaced

  • Solar collector temperature sensor
  • Sensor cable or plug connector
  • Solar control module (if the sensor circuit is faulty)

Related codes

Full explanation for 92
93

The boiler has spotted a problem with one of its solar heating sensors, which is usually a minor fault that doesn't stop your heating or hot water from working.

Low Engineer only £100-180

What to check first

  1. Turn off the boiler at its on/off switch and wait about 10 minutes, then switch it back on to see if the code clears.
  2. Check whether your home has a solar thermal panel system; if it doesn't, the boiler may be reading a sensor that isn't fitted, so ask your engineer to check the setting.
  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

  • Collector return temperature sensor
  • Sensor wiring harness or plug
  • Solar control module (only if the fault persists after the sensor is replaced)

Related codes

Full explanation for 93
94

Your boiler's sensor that measures the hot water tank temperature is sending a faulty signal, so it may not be heating your stored hot water properly, though your central heating may still work.

Medium Engineer only £120-220

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 10 minutes.
  2. Restore power, then press the reset button on the boiler or follow the reset procedure in the manual to clear the code.
  3. If the fault returns, stop and call a Gas Safe registered engineer, as the sensor and its wiring need testing and replacing by a qualified person.

Parts commonly replaced

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

Related codes

Full explanation for 94
95

The boiler has detected more carbon monoxide than it should produce, so it has shut itself off as a safety precaution.

Emergency Engineer only £90-350

Parts commonly replaced

  • Burner
  • Heat exchanger
  • Flue or air intake components
  • Gas valve
  • Fan

Related codes

Full explanation for 95
96

The boiler's carbon monoxide safety sensor has reported a fault, so the boiler has most likely shut itself down as a precaution, and it needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £150-300

Parts commonly replaced

  • CO limiter sensor
  • Main PCB (control board)
  • Flue gas sensor assembly

Related codes

Full explanation for 96
98

The boiler has detected a break in the wiring to one of its temperature sensors, so it can't read the temperature correctly and may stop or reduce heating until it's repaired.

Medium Engineer only £120-220

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 fault clears.
  2. Check that the boiler's thermostat or room controller is correctly connected and its wiring looks undamaged.
  3. Note the full fault code and any other messages on the display, and call a Gas Safe registered engineer if the fault returns.

Parts commonly replaced

  • Flow or return temperature sensor
  • Sensor lead or wiring harness
  • Outdoor temperature sensor (if fitted)

Related codes

Full explanation for 98
99

Your boiler has lost the signal from one of its temperature sensors, so it can't read the water temperature correctly. This is usually a wiring problem rather than a sign of danger, but it should be checked by a Gas Safe engineer.

Low Engineer only £90-200

What to check first

  1. Turn the boiler off at its power supply and wait 5 minutes.
  2. Switch the power back on and check whether the code clears or returns after a few minutes.
  3. If the code returns, stop and book a Gas Safe registered engineer. Do not open the boiler casing or disconnect any sensor wiring yourself.

Parts commonly replaced

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

Related codes

Full explanation for 99
Showing 41–50 of 89
A3

Your boiler has stopped its setup check because a part that measures the temperature of the exhaust gases is not fitted in the right place, so it won't start up until a qualified engineer corrects it.

High Engineer only £80-150

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor seal or gasket

Related codes

Full explanation for A3
A7

The boiler's main control unit has detected a fault with its own programming, so it has likely stopped working and needs a qualified engineer to check or replace it.

High Engineer only £250-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait 10 minutes.
  2. Switch the power back on and check whether the fault code clears and the boiler restarts.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the control unit usually needs checking or replacing.

Parts commonly replaced

  • Control unit (programming unit)
  • Control unit display panel
  • E-BUS connection lead

Related codes

Full explanation for A7
A9

The boiler's internal water pump appears to be blocked, so it may not be moving hot water around your heating system, and it's worth having it checked soon.

High 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, then wait about 10 minutes.
  2. Switch it back on and check whether the fault clears or the boiler restarts normally.
  3. Check the system pressure gauge on the front of the boiler; it should read roughly 1 to 1.5 bar when cold. If it is very low, top it up using the filling loop as your boiler manual describes.
  4. If the code returns, stop and call a Gas Safe registered engineer, as the pump usually needs to be cleaned or replaced.

Parts commonly replaced

  • Internal circulation pump
  • Pump rotor or cartridge
  • Pump motor

Related codes

Full explanation for A9
B0

The boiler has stopped because a safety sensor that checks the temperature of the fumes leaving through the flue is sending a faulty signal, so it has shut itself down until a qualified engineer can check it.

High Engineer only £150-250

What to check first

  1. Switch the boiler off at the isolator switch or fused spur, wait about 10 minutes, then switch it back on to see if it restarts.
  2. Check that the flue terminal outside the property is clear of leaves, nests, snow or other debris, and that nothing is blocking the airflow around it.
  3. If the code returns, or you smell gas, see soot or scorching around the boiler, turn the boiler off, do not keep resetting it, and call a Gas Safe registered engineer. If you suspect a gas leak, call the National Gas Emergency number 0800 111 999 straight away.

Parts commonly replaced

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

Related codes

Full explanation for B0
b1

The boiler's control panel has lost contact with the main boiler unit, so you may not be able to change settings from the panel, although the boiler may still be able to heat your home and water.

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 for about one minute, then switch it back on.
  2. Wait a few minutes for the boiler to restart, then check whether the b1 code has cleared from the display.
  3. If the code returns, stop trying to fix it yourself and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Programming unit (operating panel)
  • Communication cable between control unit and programming unit
  • Control unit circuit board

Related codes

Full explanation for b1
b5

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

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 your user manual.
  2. Turn the boiler off at its on/off switch, wait 10 minutes, then switch it back on to see whether the fault clears.
  3. If the code returns or the boiler will not restart, stop trying to reset it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Main control unit (PCB)
  • Ignition electrode
  • Fan assembly
  • Gas valve

Related codes

Full explanation for b5
b7

Your boiler's settings memory card isn't being read correctly, so the boiler has stopped itself as a safety precaution and will need a qualified engineer to check it.

High Engineer only £150-350

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or circuit breaker, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and whether the b7 code returns.
  3. If the code remains, do not open the boiler casing. Note the exact code and any other symbols shown, then book a Gas Safe registered engineer.

Parts commonly replaced

  • Coding card
  • Control unit (PCB)

Related codes

Full explanation for b7
B8

A sensor that checks the heat leaving your boiler through the flue has lost its electrical connection, so the boiler has likely shut itself down as a precaution until a qualified engineer can reconnect or replace 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 5 minutes.
  2. Turn the power back on and press the reset button on the boiler (or use the reset option on the display) once.
  3. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if it is below 1 bar and you know how.
  4. Check that the flue terminal outside the property is not blocked by leaves, snow, debris or a bird's nest, and clear it if it is safe to reach.
  5. If the code returns, stop resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor wiring lead or harness
  • Connector plug

Related codes

Full explanation for B8
bF

Your boiler's internal communication part isn't recognised as the correct type, so it can't talk properly to its control system, and a qualified engineer will need to fit or re-code the right part.

High Engineer only £180-350

Parts commonly replaced

  • LON communication module
  • LON bus connection cable
  • Vitotronic control unit (if the module cannot be re-coded)

Related codes

Full explanation for bF
C1

Your boiler has lost its connection to a separate add-on control unit, so the extra heating controls it manages may not be working, though the boiler itself may still run.

Medium 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, wait 5 minutes, then switch it back on to reset the communication link.
  2. With the power off, check that the cable between the boiler and the EA1 extension box is firmly plugged in at both ends, and look for any obvious damage to the cable. Do not open the boiler casing or the EA1 box.
  3. If the C1 code returns after the reset, or the cable is damaged, stop and book a Viessmann-registered heating engineer.

Parts commonly replaced

  • EA1 extension module
  • LON communication bus cable
  • Control unit connector or fuse

Related codes

Full explanation for C1
Showing 51–60 of 89
C2

Your boiler has lost its connection to the solar panel controller, so the solar part of the system may stop working, but your heating and hot water usually carry on as normal.

Low Engineer only £90-350

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 and check whether the code clears.
  2. Check that the solar controller (Vitosolic or solar control module) has power and that its indicator light is on, if you can see it safely from outside any casing.
  3. If the code returns, leave it and contact a Viessmann-registered engineer. Do not open the boiler casing or disturb the internal wiring.

Parts commonly replaced

  • Vitosolic solar controller
  • Solar control module
  • BUS communication cable
  • Boiler control PCB (rarely needed)

Related codes

Full explanation for C2
C3

The boiler's main controls are having trouble talking to an add-on control unit that manages part of your heating, so the system may run with reduced function until this link is restored.

Medium Engineer only £150-350

What to check first

  1. Switch the boiler off at its wall isolator or fused spur and wait about 5 minutes.
  2. Switch it back on and check whether the display clears the code or the boiler restarts normally.
  3. If the code returns, check that the boiler's power supply is stable and that any visible plugs or cables on the control panel are fully pushed in, then contact a Gas Safe engineer if it reappears.

Parts commonly replaced

  • AM1 extension module
  • Communication bus cable or connector
  • Control unit fuse

Related codes

Full explanation for C3
C4

Your boiler has lost its connection to the room thermostat or its control unit, so it may not be responding to heating requests correctly, but this is usually fixable and is not a safety risk.

Medium DIY-safe £80-250

What to check first

  1. Turn off the boiler at its isolator switch or fused spur, wait about 30 seconds, then turn it back on to reset the connection.
  2. Check that the room thermostat or controller has working batteries, and replace them if needed.
  3. Look at the wiring between the thermostat and the boiler for loose or damaged cables, but do not open the boiler casing.
  4. If the code returns after the reset, contact a Gas Safe registered engineer to check the OpenTherm connection and controller.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • Thermostat wiring or connecting cable
  • Boiler control board (PCB)

Related codes

Full explanation for C4
C5

Your boiler's internal pump control has lost contact with the main boiler system, so it may shut down or stop heating your home until it is checked.

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, then wait 5 minutes.
  2. Turn the power back on and wait a few minutes for the boiler to restart.
  3. Check the boiler display: if the C5 code has cleared and the heating and hot water are working, keep an eye on it. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Variable speed circulation pump
  • Pump electronics module
  • Main control PCB
  • Pump wiring harness

Related codes

Full explanation for C5
Cd

Your boiler is having trouble talking to its add-on internet and remote monitoring unit, but this usually doesn't stop your heating or hot water from working.

Low DIY-safe £120-250

What to check first

  1. Find the small Vitocom 100 unit (usually mounted near the boiler or on a wall nearby) and check its status light is on.
  2. Switch off the power to the Vitocom 100 at its plug or socket, wait 30 seconds, then switch it back on and wait a few minutes for it to restart.
  3. Check that the thin communication cable between the Vitocom 100 and the boiler is firmly plugged in at both ends, and look for any visible damage.
  4. Check the boiler display again. If the Cd code returns, contact a Viessmann-registered engineer, as the Vitocom 100 or its cable may need replacing.

Parts commonly replaced

  • Vitocom 100 GSM communication module
  • KM-BUS communication cable
  • Vitocom 100 power supply unit

Related codes

Full explanation for Cd
CF

The boiler's controls have lost contact with one of their internal communication parts, so the boiler may not respond properly until a heating engineer checks the connection.

Medium Engineer only £150-400

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or consumer unit, then wait 2 minutes.
  2. Switch the power back on and wait 5 minutes to see whether the boiler restarts and clears the code.
  3. If the code returns, do not open the boiler casing. Contact a Gas Safe registered engineer to check the communication cables and module.

Parts commonly replaced

  • LON communication module
  • LON connection cable or plug
  • Vitotronic control unit (if the fault persists)

Related codes

Full explanation for CF
d3

Your boiler's control system has lost contact with one of its internal parts, so it has stopped running to protect itself until the connection is checked.

High Engineer only £250-550

What to check first

  1. Switch the boiler off at the isolator switch or fused spur, wait about 10 seconds, then switch it back on to reset the fault.
  2. Check that the boiler's display shows no other fault codes and that the boiler pressure gauge reads between 1 and 1.5 bar.
  3. If the code returns, leave the boiler off and call a Gas Safe registered engineer, as the fan and its wiring must be inspected by a qualified person.

Parts commonly replaced

  • Fan unit
  • Control unit (PCB)
  • Fan wiring harness
  • Gas valve (if the fan fault has affected combustion)

Related codes

Full explanation for d3
d4

The boiler has stopped because it is not detecting the air it needs to burn gas safely, so it has shut itself down to stay safe.

High Engineer only £250-450

What to check first

  1. Check that the flue outlet on the outside wall is not blocked by leaves, nests, snow or debris, and clear any obstruction you can reach safely.
  2. Check the boiler pressure on the display and top it up if it is below about 1 bar, following the manufacturer's instructions.
  3. Switch the boiler off at the isolator, wait 5 minutes, switch it back on and press reset once. If the code returns, do not keep resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Gas/air mixing unit
  • Fan speed sensor
  • Flue seals

Related codes

Full explanation for d4
d5

Your boiler's exhaust fan appears to be blocked or obstructed, so the boiler has stopped to protect itself and will need a qualified engineer to check it.

High Engineer only £150-350

What to check first

  1. Turn the boiler off at the control panel and at the isolator switch, then wait 10 minutes.
  2. From outside, check the flue terminal (the pipe or grille on an outside wall) for leaves, nesting material, snow, or debris, and carefully clear any you can reach safely.
  3. Turn the boiler back on and try a reset. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan gasket or seal
  • Flue terminal cover or guard

Related codes

Full explanation for d5
d6

Your boiler's control system has detected a problem with one of its external connection inputs, which may stop part of your heating from working correctly until it's checked.

Medium Engineer only £120-300

What to check first

  1. Switch off the boiler at its isolator switch and wait 2 minutes.
  2. Check that any external controls, such as a room thermostat or timer, are plugged in and their wires are secure.
  3. Switch the boiler back on and see whether the code clears. If it returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • EA1 extension module
  • Control unit wiring harness
  • External control input cable

Related codes

Full explanation for d6
Showing 61–70 of 89
d7

Your boiler has detected a problem with one of its external connections, which may affect how it responds to outside controls, so it's worth having a qualified engineer take a look soon.

Medium Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge reads between 1 and 1.5 bar, and top it up only if the manual says to.
  2. Switch the boiler off at its power supply, wait 10 minutes, then switch it back on to see whether the code clears.
  3. If the code returns, stop trying to fix it and book a Gas Safe registered engineer.

Parts commonly replaced

  • EA1 extension module
  • Wiring harness or connector
  • External input device or volt-free contact

Related codes

Full explanation for d7
d8

Your boiler has detected a problem with one of the signals coming in from an add-on control box or an external controller, so it may not respond correctly to those settings until this is checked.

Medium Engineer only £120-300

What to check first

  1. Note the code shown on the boiler display and turn the boiler off at its isolator switch or fused spur for a few minutes.
  2. Turn the power back on and see whether the code clears on its own.
  3. If the code returns, leave the boiler off and call a Gas Safe registered engineer, as the fault usually involves wiring or the EA1 extension module inside the boiler.

Parts commonly replaced

  • EA1 extension module
  • Control wiring harness
  • External control input connector

Related codes

Full explanation for d8
E0

Your boiler's controls have lost contact with one of its add-on parts, such as an external module or room controller, so it may not be managing the heating as it should.

Medium Engineer only £100-300

What to check first

  1. Switch off the boiler at its isolator or fused spur and wait two minutes.
  2. Check that the cable plugs linking any external modules or room controllers to the boiler are fully pushed in and not visibly damaged, without opening any casing.
  3. Switch the power back on and see whether the code clears. If E0 returns, contact a Gas Safe registered engineer.

Parts commonly replaced

  • LON communication cable
  • LON extension module
  • Control unit circuit board
  • Room thermostat or remote control unit

Related codes

Full explanation for E0
E1

Your boiler has stopped because its burner isn't lighting or staying lit, so it can't produce heat or hot water until a engineer checks the flame and gas supply.

High Engineer only £90-350

What to check first

  1. Check that your gas supply is on, including the meter and any isolation valve, and that other gas appliances work.
  2. Check the boiler pressure gauge; if it reads below 1 bar, follow the manual to top it up, then look at the display again.
  3. Press the reset button once (or use the reset option in the display menu). If E1 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Gas valve
  • Fan
  • Gas/air ratio control

Related codes

Full explanation for E1
E2

Your boiler has stopped because too little water is moving through its heating system, so it can't safely run until the flow is checked and restored.

High Engineer only £120-300

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read about 1.0 to 1.5 bar when cold. If it is below 1.0 bar, top it up using the filling loop, following the boiler manual.
  2. Make sure all radiator valves and the thermostatic valves are open, and that no other valves on the heating circuit are closed.
  3. Turn the boiler off at the switch, wait a few minutes, then turn it back on and reset it. If the code returns, call a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump
  • Magnetic system filter or sludge filter
  • Flow sensor or flow switch
  • Diverter valve
  • Expansion vessel

Related codes

Full explanation for E2
E3

Your boiler has stopped because its burner settings need recalibrating, which usually means a Gas Safe engineer needs to check it, though a simple reset may get it running again for now.

High Engineer only £120-350

What to check first

  1. Check the pressure gauge on the front of the boiler and make sure it reads between 1.0 and 1.5 bar. If it is low, top it up using the filling loop only if you know how to do this safely.
  2. Press the reset button marked R once, then wait about 10 minutes for the boiler to restart and recalibrate.
  3. If the E3 code returns or the boiler will not fire again, leave it switched off and book a Gas Safe registered engineer to recalibrate the combustion controller.

Parts commonly replaced

  • Combustion fan
  • Gas valve
  • Ignition electrode
  • Combustion controller board

Related codes

Full explanation for E3
E4

Your boiler has lost a small internal power supply that its control system needs, so it may have shut itself down to stay safe, and a qualified engineer should check it.

High Engineer only £150-450

What to check first

  1. Check that the boiler still has mains power, including any switch or fused spur next to it and the fuse in the plug if it has one.
  2. Switch the boiler off at its on/off switch or isolator, wait 30 seconds, then switch it back on.
  3. If the boiler shows E4 again, leave it off and call a Gas Safe registered engineer, as the internal power supply needs professional testing.

Parts commonly replaced

  • 24 V power supply unit
  • Main control PCB (circuit board)
  • PCB fuse
  • Wiring harness or connector

Related codes

Full explanation for E4
E5

The boiler has stopped because its flame sensor isn't reading the flame correctly, so it has shut itself down as a safety precaution and may not be giving you heating or hot water until it's checked.

High Engineer only £120-250

What to check first

  1. Check that the gas supply is on, including the gas meter tap or emergency control valve, and that other gas appliances work.
  2. Check the boiler pressure gauge; if it reads below 1 bar, top it up using the filling loop as shown in the manual, then wait for the pressure to settle.
  3. Turn the boiler off at the switch, wait 10 minutes, then turn it back on and press the reset button once. If the fault returns quickly, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flame detection electrode (ionisation probe)
  • Ignition electrode
  • Electrode gasket
  • Control PCB

Related codes

Full explanation for E5
E7

Your boiler has stopped its burner because it detected a problem with how it is starting or staying lit, so it may not be heating your home or water until it is checked.

High Engineer only £120-300

What to check first

  1. Check that your gas supply is on, including the gas meter and the boiler's gas isolation valve, and that other gas appliances are working.
  2. Press the reset button on the boiler once (or use the reset option on the display), then wait about 10 minutes before trying again.
  3. If the code returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer. If you smell gas at any time, turn off the gas supply, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Gas pressure switch
  • Burner control unit
  • Fan

Related codes

Full explanation for E7
E8

The boiler has switched itself off because it is not detecting the gas flame properly, so it is stopping as a safety precaution and will likely need a reset and a check by an engineer.

High Engineer only £90-220

What to check first

  1. Check that the gas supply is on, including the meter and any isolation valve, and that other gas appliances work.
  2. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop if it is low.
  3. Press and hold the reset button for a few seconds (or use the reset option on the display) to restart the boiler once, then leave it to run.
  4. If the fault returns, stop resetting it and call a Gas Safe registered engineer to check and clean the flame sensing electrode and burner.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Burner gasket or seal
  • Gas valve
  • Control PCB

Related codes

Full explanation for E8
Showing 71–80 of 89
E9

Your boiler has stopped because it can't reliably confirm that its gas flame is staying lit, so it has shut itself down as a safety precaution.

High Engineer only £120-250

What to check first

  1. Switch the boiler off at its on/off switch, wait about 10 seconds, then switch it back on to try a reset.
  2. Check that the gas supply to the boiler is turned on and that other gas appliances are working.
  3. Check 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 say to.
  4. If the code returns, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Burner gasket
  • Gas valve
  • Burner control unit

Related codes

Full explanation for E9
EA

Your boiler has stopped because it has received a fault signal from one of its connected controls or devices, so heating and hot water may be off until a engineer checks the cause.

High Engineer only £90-220

What to check first

  1. Check the boiler display for any pressure warning and, if the pressure gauge reads below 1 bar, top it up using the filling loop as shown in the manual.
  2. Check that any room thermostat, programmer or external control is switched on and wired correctly, then turn the boiler off at its isolator switch for 30 seconds before turning it back on.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer rather than repeatedly resetting it.

Parts commonly replaced

  • External control or fault input connector
  • Wiring harness or connector plug
  • Control unit (Vitotronic)

Related codes

Full explanation for EA
Eb

Your boiler has stopped itself from running because its gas burner isn't setting up correctly, which is a safety precaution that usually has a fixable cause and is not an immediate danger if the boiler is switched off.

High Engineer only £90-250

What to check first

  1. Check that the gas supply is on and the boiler pressure gauge reads between 1 and 1.5 bar, topping up using the filling loop only if it is low.
  2. Make sure the flue outlet outside the house is clear of leaves, nests, snow or other debris, and that nothing is blocking the air vents.
  3. Press the reset button marked R on the boiler once, wait a few minutes, and see whether the boiler restarts. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for Eb
EC

Your boiler's control system has lost some of its stored settings during its self-check, so it has stopped working until a qualified engineer restores them.

High Engineer only £120-350

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator, wait 5 minutes, then turn it back on to see if the code clears.
  2. If the code returns, check the boiler pressure gauge and top up with the filling loop only if it is below 1 bar, then restart.
  3. If the code still shows, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Main control PCB
  • Fan assembly
  • Gas valve

Related codes

Full explanation for EC
Ed

Your boiler's main control computer has detected an internal fault, so it has likely shut itself down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £300-600

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait about 60 seconds.
  2. Switch the power back on and wait several minutes for the boiler to restart and show whether the fault clears.
  3. If the code returns or the boiler stays off, stop and book a Gas Safe registered engineer. Do not open the casing or attempt any repairs yourself.

Parts commonly replaced

  • Main control unit (PCB)
  • Control panel/display module

Related codes

Full explanation for Ed
EE

The boiler has paused because air has got into the gas supply pipe, which is common during setup and usually clears with a single reset, but it may need a gas engineer to purge the line if it keeps happening.

High Engineer only £80-150

What to check first

  1. Wait about 5 seconds after the EE code appears, then press the reset button R once on the boiler.
  2. Check that the boiler's gas supply valve is fully open and that other gas appliances in the home are working normally.
  3. If the code returns or the boiler will not restart after one reset, stop and call a Gas Safe registered engineer to purge the gas line and recommission the boiler.

Parts commonly replaced

  • Gas line purge and recommissioning (labour only, usually no parts)
  • Gas valve (only if purging does not resolve the fault)

Related codes

Full explanation for EE
EF

Your boiler lit its burner but the flame went out almost straight away, so it has stopped heating and may not be producing hot water until a qualified engineer checks it.

High Engineer only £120-250

What to check first

  1. Check that the gas supply is turned on at the meter and that other gas appliances work.
  2. Check the boiler pressure on the display and top it up to the recommended level if it is low.
  3. Press the reset button once, wait a few minutes, and see if the boiler restarts. Do not keep pressing it.
  4. Make sure the flue outlet outside is not blocked by debris, snow, or plants.
  5. If the fault returns or the boiler will not restart, turn it off and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for EF
F0

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

High DIY-safe £250-450

What to check first

  1. Turn off the boiler using its on/off switch, then switch off the spur or fused connection supplying it at the consumer unit or isolator.
  2. Wait at least 5 minutes so the control board fully powers down, then switch the power back on and turn the boiler on again.
  3. Check whether the boiler restarts and the fault code clears. If F0 returns within a few days or the boiler keeps shutting down, stop resetting it and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for F0
F1

Your boiler has switched itself off as a safety precaution because the exhaust gases leaving through the flue are running too hot, which is often caused by a partial blockage or airflow problem.

Emergency Engineer only £80-350

What to check first

  1. Turn the boiler off at its on/off switch and leave it off until a Gas Safe engineer has checked it.
  2. Check the outside flue terminal (the pipe exiting the wall) for visible blockages such as leaves, nests, or debris, but do not put anything inside it or dismantle it.
  3. Make sure your carbon monoxide alarm is working and ventilate the room, and call a Gas Safe registered engineer to inspect the flue and combustion system before using the boiler again.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue terminal or flue seals
  • Condensate trap
  • Combustion fan

Related codes

Full explanation for F1
F2

Your boiler has shut itself down as a precaution because it got too hot, and it will usually need to cool down and be reset before it will work again.

High Engineer only £120-280

What to check first

  1. Turn the boiler off at the isolation switch and let it cool for 15 to 30 minutes.
  2. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the manual, then close the loop.
  3. Switch the boiler back on and press the reset button once. If the fault clears and does not return, keep an eye on it. If it shows F2 again, or the boiler will not stay on, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Safety temperature limiter (STB)
  • Central heating circulation pump
  • Flow temperature sensor

Related codes

Full explanation for F2
Showing 81–89 of 89
F3

The boiler has detected a flame when it should not be lit, so it has shut itself down as a safety precaution, and it needs checking by a Gas Safe engineer before it is used again.

Emergency Engineer only £120-400

What to check first

  1. Turn off the boiler at its on/off switch and, if you can safely reach it, turn off the gas supply at the meter or the isolation valve next to the boiler.
  2. Check the room for any smell of gas. If you can smell gas, leave the property, do not use switches or flames, and call the National Gas Emergency Service on 0800 111 999 from outside.
  3. If there is no gas smell, wait a few minutes, turn the gas back on, switch the boiler on, and press the reset button once. Note the code if it appears again.
  4. If the F3 code returns, stop resetting the boiler and book a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode (flame sensor)
  • Gas valve assembly
  • Control PCB (main circuit board)
  • Burner gasket

Related codes

Full explanation for F3
F8

Your boiler's gas control valve isn't shutting off fully when it should, which is a safety concern, so please switch the boiler off and get a Gas Safe engineer to check it promptly.

Emergency Engineer only £250-450

What to check first

  1. Turn the boiler off at its on/off switch and do not try to restart it.
  2. Turn off the gas supply at the emergency control valve, usually a lever near the gas meter or boiler, so the lever is at 90 degrees to the pipe.
  3. Open a window and avoid using naked flames or electrical switches near the boiler if you smell gas.
  4. If you smell gas at any time, leave the property, do not use electrical switches, and call the National Gas Emergency Service on 0800 111 999 from outside.
  5. Call a Gas Safe registered engineer to inspect and repair the gas valve. Do not attempt the repair yourself.

Parts commonly replaced

  • Gas valve (combined gas control valve)
  • Gas valve modulation actuator or coil
  • Main control PCB (circuit board)

Related codes

Full explanation for F8
F9

The boiler's internal fan is not running properly, so it keeps trying to start and then shuts itself down as a safety precaution, which usually means you'll have no heating or hot water until a engineer checks it.

High Engineer only £250-450

What to check first

  1. Check the boiler has power, including that the isolator switch and fuse are on and the display is lit.
  2. Check the boiler pressure gauge reads between 1.0 and 1.5 bar, and top up using the filling loop only if your boiler manual says to.
  3. Check the flue outlet outside is clear of leaves, nests or debris, then press the reset button once and wait 10 minutes to see if the fault returns.
  4. If F9 returns, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan connecting cable
  • Fan control PCB
  • Gas valve

Related codes

Full explanation for F9
FA

Your boiler's fan isn't spinning at the speed it needs to, so the boiler has shut itself down as a safety precaution until it can be checked.

High Engineer only £180-450

What to check first

  1. Check the boiler pressure gauge and make sure it reads between 1 and 1.5 bar. If it is low, top it up using the filling loop only if you know how to do this safely, otherwise leave it for the engineer.
  2. Check the flue terminal outside your home for any visible blockage such as leaves, debris or a bird's nest, and clear it only if it is safe to reach without a ladder.
  3. Switch the boiler off at the isolator or power switch, wait five minutes, then switch it back on once. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Control unit PCB
  • Fan power and signal cable

Related codes

Full explanation for FA
Fb

The boiler's burner has briefly gone out while running, so it has shut itself down as a safety precaution and may need a reset or a check by a qualified engineer.

High Engineer only £120-300

What to check first

  1. Check that other gas appliances in the home, such as the hob, are working, to confirm the gas supply is on.
  2. Check that the gas meter or emergency control valve is fully open and that you have not run out of credit on a prepayment meter.
  3. Check the flue terminal outside the wall is not blocked by leaves, snow, nests or debris, and clear it only if it is safe and easy to reach.
  4. Press the reset button on the boiler control panel once, wait a few minutes, and see whether it restarts. If it fails again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for Fb
FC

Your boiler has shut down because it has detected a fault with how it controls the gas supply or its exhaust route, which is a safety matter that needs a qualified engineer to check promptly.

Emergency Engineer only £200-450

Parts commonly replaced

  • Gas valve assembly
  • Modulation valve coil
  • Gas pressure switch
  • Flue terminal or flue blockage clearance
  • Fan assembly

Related codes

Full explanation for FC
Fd

Your boiler has stopped because its internal controls have detected a fault with how it lights and manages the gas flame, so it has shut itself down for safety.

High 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, wait 5 minutes, then switch it back on to try a reset.
  2. Check that the gas supply is on by confirming your gas meter valve and any boiler gas isolation valve are open, and check that other gas appliances are working.
  3. If the fault code returns, or the boiler will not restart after a reset, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (PCU or ignition module)
  • Ionisation (flame sensor) electrode
  • Ignition electrode
  • Ignition lead

Related codes

Full explanation for Fd
FE

Your boiler has stopped itself for safety because its burner is blocked or not working properly, so it needs a reset and a check by a qualified engineer.

High Engineer only £100-300

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar. If it is low, top it up using the filling loop as shown in the manual.
  2. Check that the boiler has power and that no other faults are showing on the display. Press the reset button once and wait 5 minutes.
  3. If the fault returns or the boiler will not restart, stop trying and call a Gas Safe registered engineer. Do not attempt to open or clean the burner yourself.

Parts commonly replaced

  • Burner
  • Ionisation (flame) sensor
  • Gas valve
  • Fan
  • Condensate trap
  • Burner gasket

Related codes

Full explanation for FE
FF

Your boiler has stopped its gas flame from working properly and has locked itself out as a safety precaution, so it may not be providing heating or hot water until a engineer checks it.

High Engineer only £120-300

What to check first

  1. Check the boiler display shows a pressure of between 1 and 2 bar, and top it up using the filling loop if it is low.
  2. Press the reset button or hold the reset control on the boiler for a few seconds, then wait 10 minutes to see if it restarts.
  3. Check that your gas supply is on by confirming the gas meter and the boiler's gas isolation valve are open, and call a Gas Safe registered engineer if the fault returns.

Parts commonly replaced

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

Related codes

Full explanation for FF
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