Boiler Fault Codes from all Leading Manufacturers

When there’s a fault with your boiler, an error code will generally be displayed on the front panel to help you figure out what the problem is, it also helps gas engineers like us to quickly diagnose and rectify issues.

A fault code can mean anything from a simple fix to a significant and potentially dangerous problem with the boiler, so it’s really important to report any error codes to a professional.

If your boiler is displaying a fault code, you can find out what it means using our boiler code tool below, use the search function for quick and easy locating.

CodeManufacturerMeaning
9AWorcester BoschBoiler has been fitted with incorrect Heat Control Module (HTM).
9UWorcester BoschFaulty or not properly connected code plug.
B7Worcester BoschControl board needs replacing.
C6Worcester BoschSystem is running too fast or slow.
C7Worcester BoschFan is not running or the airflow is blocked.
D1Worcester BoschSensor has become wet, damaged or has disconnected. Boiler will block for 300 seconds.
D1Worcester BoschBoiler is overheating.
E2Worcester BoschFlow sensor has shorted, become damaged or has disconnected.
E5Worcester BoschPrimary flow sensor has overheated.
E9Worcester BoschMain heat exchanger has overheated.
E9Worcester BoschMax temperature sensor has failed or can't be recognised.
EAWorcester BoschFlue gas thermostat has overheated.
EAWorcester BoschBoiler has failed to ignite 4 times. Boiler will wait 30 seconds before another attempt is made.
EAWorcester BoschFlame has been extinguished due to moisture in sump, blocked sump or wind in flue.
EAWorcester BoschGas valve coil has disconnected.
EAWorcester BoschFlame has not extinguished within 4 seconds of ignition.
EAWorcester BoschPump is not detecting water.
F.00VaillantInterruption: Flow sensor: NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC defective
F.01VaillantInterruption: Return sensor: NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC defective
F.02VaillantInterruption: DHW outlet sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics
F.03VaillantInterruption: Cylinder sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics
F.10VaillantShort circuit: Flow sensor: NTC defective, short circuit in cable harness, cable/casing
F.11VaillantShort circuit: Return sensor: NTC defective, short circuit in cable harness, cable/casing
F.12VaillantShort circuit: DHW outlet sensor: Only in conjunction with F.91NTC defective, short circuit in cable harness, cable/casing
F.13VaillantShort circuit: Cylinder sensor: Combination product: Warm start sensor/cylinder sensor short circuitCombination product with actoSTOR: Short circuit cylinder sensor (NTC) only in combination with F.91NTC defective, short circuit in cable harness, cable/casing
F.20VaillantSafety switch-off: Temperature limiter: Incorrect earth connection between cable harness and product, flow or return NTC defective (loose connection), black discharge via ignition cable, ignition plug or ignition electrode
F.22VaillantSafety switch-off: Low water pressure: No or insufficient water in the product, water pressure sensor defective,
F.23VaillantSafety switch-off: Temp.spread too large: Pump blocked, insufficient pump output, air in product, flow and return NTC connected the wrong way round
F.24VaillantSafety switch-off: Temp. incr. too fast: Pump blocked, insufficient pump output, air in product, system pressure too low, non-return valve blocked/incorrectly installed
F.25VaillantSafety switch-off: Flue temp. too high: Break in plug connection for optional flue gas safety temperature limiter (STB), break in cable harness
F.26VaillantFault: Fuel valve not working: Gas valve stepper motor not connected, multiple plug on the PCB not plugged in correctly, interruption in cable harness, gas valve stepper motor defective, electronics defective
F.27VaillantSafety switch-off: Flame simulation: Moisture on the electronics, electronics (flame monitor) defective, gas solenoid valve leaking
F.28VaillantStart-up failure: Ignit. unsuccessful: Gas meter defective or gas pressure monitor has triggered, air in gas, gas flow pressure too low, thermal isolator device (TAE) has triggered, condensate duct blocked, incorrect gas restrictor, incorrect spare part gas valve, value in D.052 does not correspond to the printed value on the current gas valve, fault on the gas valve, multiple plug on PCB incor- rectly plugged in, break in cable harness, ignition system (ignition trans- former, ignition cable, ignition plug, ignition electrode) defective, ionisa- tion current interrupted (cable, electrode), incorrect earthing of product, electronics defective
F.29VaillantStart-up failure: Ignit. unsuccessful: Gas supply temporarily stopped, flue gas recirculation, condensate duct blocked, defective earthing of product, ignition transformer has spark failure
F.32VaillantFault: Fan: Plug on fan not correctly plugged in, multiple plug on PCB not correctly plugged in, break in cable harness, fan blocked, Hall sensor defective, electronics defective
F.49VaillantFault: eBUS: Short circuit on eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52VaillantContact fault mass flow sensor/Venturi: The mass flow sensor/Venturi is not connected electrically – The plug is not plugged in correctly – The plug is not plugged in – The plug is defective – The slot is defective (loose connection) – Mass flow sensor/Venturi defective
F.53VaillantCombustion regulation fault: The combustion regulation has detected a fault – Gas flow pressure too low – Liquid gas coding resistance used when operating with natural gas – If the fault occurs again after being cleared: – Gas valve defective – Mass flow sensor/Venturi defective, wet or blocked (if the fault occurs again after being cleared): Do not wet the sensor, do not use any lubricants on the O-ring on the Venturi.
F.54VaillantFault in the gas supply (in combination with F.28/F.29): There is insufficient gas supply to operate the unit – Gas isolator cock(s) closed – Gas flow pressure is too low – Gas valve defective
F.56VaillantSafety switch-off: CO limit exceeded: Safety switch-off: CO limit exceededA component in the combustion regulation is defective– Contact fault at the gas valve (plug not plugged in correctly or not plugged in, plug defective, slot is defective (loose connection))– If the fault occurs again after being cleared: The gas valve is defective
F.57VaillantFault: Measuring program: Active comfort protection mode has detected a regulation fault– Ignition electrode highly corroded
F.61VaillantFault: Fuel valve actuation: The gas valve cannot be actuated– Cable harness supply line to the gas valve is defective (short to earth, short circuit)– Gas valve defective– PCB defective
F.62VaillantFault: Fuel valve switch-off delay: Delayed shutdown of gas valve detected– Secondary light (ignition and monitoring electrode indicates delayed extinguishing of the flame signal)– Gas valve defective– PCB defective
F.63VaillantFault: EEPROM: Electronics defective
F.64VaillantFault: Electronics/sensor: Flow or return NTC short circuited, electronics defective
F.65VaillantFault: Electronics temp.: Electronics overheating due to external influences, electronics defective
F.67VaillantFault: Electronics/flame: Implausible flame signal, electronics defective
F.68VaillantFault: Flame signal unstable: Air in gas, gas flow pressure too low, incorrect air ratio, condensate duct blocked, ionisation flow interruption (cable, electrode), flue gas recirculation, condensate duct
F.70VaillantFault: Invalid Device Specific Number: If spare parts fitted: Display and PCB replaced at same time and DSN not reset, incorrect or missing output range coding resistance
F.71VaillantFault: Flow sensor: Flow temperature sensor signalling constant value:– Flow temperature sensor incorrectly positioned at supply pipe– Flow temperature sensor defective
F.72VaillantFault: Flow/return sensor: Flow/return NTC temperature difference too great → flow and/or returntemperature sensor defective
F.73VaillantFault: Water press. sensor: Interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line to water pressure sensor or water pressure sensor defective
F.74VaillantFault: Water press. sensor: The line to the water pressure sensor has a short circuit to 5 V/24 V or internal fault in the water pressure sensor
F.75VaillantFault: Pump/ water shortage: Water pressure sensor and/or pump defective, air in the heating installa- tion, insufficient water in the product; connect external expansion vessel to the return
F.77VaillantFault: Flue non-ret. valve/condens. pump: No response from flue non-return flap or condensate pump defective
F.78VaillantInterrupt.: DHW outlet sensor on ext. contr: UK link box is connected but the hot water NTC is not bridged
F.80VaillantFault: actoSTOR inlet sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronicsSensor plug has short to earth to the casing, short circuit in cable har- ness, sensor defective
F.81VaillantFault: cylinder charging pump: Only in conjunction with F.91Cylinder is not fully charged after specified time.– Check cylinder charging sensor and cylinder sensor– Air in the actoSTOR pump– Inspect cable harness for pump– Check the impeller sensor and/or limiter in the product– Diverter valve defective– Secondary heat exchanger blocked– Pump faulty
F.83VaillantFault: NTC temp. gradient: When the burner starts, the temperature change registered at the flow and/or return temperature sensor is non-existent or too small.– Insufficient water in product– Flow or return temperature sensor not in correct position at pipe
F.84VaillantFault: NTC temp. diff. implausible: Flow and return temperature sensors returning implausible values.– Flow and return temperature sensors have been inverted– Flow and return temperature sensors have not been correctly in-stalled
F.85VaillantFault: NTCs fitted incorrectly: The flow and/or return temperature sensors have been installed on the same pipe/incorrect pipe
F.90VaillantFault: Communication: Check the cable harness from the product to the actoSTOR module (PEBus).If the product is to be operated without an actoSTOR module, setD.092 = 0.
F.91VaillantSensor/actuator fault on actoSTOR module
F.92VaillantCoding resistance fault: The coding resistance on the PCB does not match the entered gas group: Check the resistance, repeat the gas family check and enter the correct gas group
F.93VaillantPoor combustion quality: Combustion regulation has detected poor combustion quality– Incorrect gas restrictor has been installed for the gas family (anothergas family is present)– Recirculation– Mass flow sensor/Venturi defective (wet, blocked): Do not wet thesensor, do not use any lubricants on the O-ring on the Venturi.
LEDactoSTOR moduleVaillantactoSTOR electronics status: LED on: Communication OKLED flashing: Communication not OK LED off: No power supply
ErrVaillantCommunication fault: Communication fault between display and PCB in the electronics box

Worcester Bosch Boiler Fault Codes

Worcester Bosch are one of the leading boiler brands in the UK. At Gas safe, we are Worcester Bosch accredited installers meaning we have the skills and training to fix all common faults with Worcester Boilers.

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Vaillant Boiler Fault Codes

Vaillant are another leading UK boiler manufacturer with global coverage. At Gas Safe, we are Vaillant accredited installers and so customers can be assured that we have the training and expertise to fix all faults with any model of Vaillant boiler.

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Need a Fast Boiler Repair?

If your boiler is showing an error code and is in need of a repair, we can help. We offer fast and efficient boiler repair services across the South West, with most problems fixed on site. Give us a call today Bristol: 0117 964 0078 Somerset: 01934 811 566 for a prompt response or fill out a form below and we’ll get back to you.

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