Common Boiler Fault Codes

When your boiler goes wrong then it's display screen often shows either a fault or error code which helps determine the issue with your type of boiler. From our list of common boiler fault codes you should be able to identify the fault your boiler is facing.

Once you have the information it will better help your boiler engineer to resolve your boiler's fault for you.

F.49VaillantFault: eBUS: Short circuit on eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52VaillantMass flow sensor connection fault – Mass flow sensor not connected/disconnected, plug not connected or incorrectly connected
F.53VaillantMass flow sensor fault – Mass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54VaillantGas pressure fault (in combination with F.28/F.29)- No or too little gas supply pressure, gas valve closed
F.56VaillantFault: Mass flow sensor regulation – Gas valve defective, cable harness to gas valve defective
F.57VaillantFault during comfort safety mode – Ignition electrode highly corroded
F.61VaillantFault: Gas valve regulation – Short circuit/short to earth in cable harness for the gas valve Gas valve defective (coils shorted to earth). Electronic defective
F.62VaillantFault: Gas valve switch-off delay – Delayed shutdown of gas valve. Delayed extinguishing of flame signal. Gas valve leaking Electronics defective
F.63VaillantEEPROM error – Electronics defective
F.64VaillantElectronics/NTC fault – Supply or return NTC short circuited, electronics defective
F.65VaillantElectronic temperature fault – Electronics overheating due to external influences, electronics defective
F.67VaillantElectronics/flame fault – Implausible flame signal, electronics defective
F.68VaillantUnstable flame signal fault – Air in gas, gas flow pressure too low, wrong air ratio, condensate route blocked, wrong gas restrictor, ionisation flow interruption (cables, electrodes), flue gas recirculation, condensate route
F.70VaillantInvalid device specific number (DSN)- Spare part case: Display and PCB replaced at same time and DSN not set, wrong or missing output range coding resistor
F.71VaillantFlow NTC fault – Flow temperature sensor signalling constant value: Flow temperature sensor incorrectly positioned at supply pipe. Flow temperature sensor defective
F.72VaillantFlow/return NTC fault – Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73VaillantWater pressure sensor signal outside correct range (too low) – Cable to water pressure sensor has short-circuited at 5V/24 V or internal fault in water pressure sensor
F.75VaillantFault, no pressure change detection when starting pump – Water pressure sensor and/or pump defective, air in heating installation, too low water pressure in boiler; check adjustable bypass, connect external expansion vessel to return
F.76VaillantOverheating protection on primary heat exchanger has responded – Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
F.77VaillantFlue non-return flap/condensate pump fault No response from flue non-return flap or condensate pump defective
F.78VaillantInterruption to DHW outlet sensor at external controller -UK link box is connected, but hot water NTC not bridged
F.83VaillantFlow and/or return temperature sensor temperature change fault – When the burner starts, the temperature change registered at flow and/or return temperature sensor is non-existent or too small. Not enough water in the boiler. Flow and/or return temperature sensor not in correct position at pipe
F.84VaillantFault: Flow/return temperature sensor difference implausible – Flow and return temperature sensors return implausible values. Flow and return temperature sensors have been inverted. Flow and return temperature sensors have not been correctly fitted
F.85VaillantFault: Flow and return temperature sensors incorrectly fitted- Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92VaillantCoding resistor fault- The coding resistor on the PCB does not match the entered gas family. Check the resistor, repeat the gas family check and enter the correct gas family.
F.93VaillantGas group fault- Combustion quality outside the permitted range: Wrong gas restrictor, recirculation, wrong gas family, internal pressure measuring point in venturi blocked (do not use lubricant on O-ring in venturi.). Gas family check prematurely terminated.
9AWorcester BoschIncorrect HCM fitted
9UWorcester BoschProblem with the code plug (faulty or not connected)
B7Worcester BoschReplace control board
C6Worcester BoschRunning too fast / Running too slow
C7Worcester BoschFan not running / Fan not running or airflow blocked
D1Worcester BoschSensor wet or damaged (the boiler will block for 300 seconds before this condition)
D1Worcester BoschBoiler overheat
E2Worcester BoschFlow sensor shorted or damaged
E2Worcester BoschFlow sensor disconnected or damaged
E5Worcester BoschPrimary flow sensor overheated
E9Worcester BoschThe main heat exchanger has overheated. Maximum temperature 105°C
E9Worcester BoschMaximum temperature sensor on the main heat exchanger has failed
E9Worcester BoschMaximum temperature sensor on the main heat exchanger has not been recognised
E9Worcester BoschFlue gas thermostat overheat
EAWorcester BoschFailure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
EAWorcester BoschAn established flame has been extinguished. This could be caused by: moisture in the sump, due to a blocked sump/condensate trap, or a strong wind into the flue
EAWorcester BoschGas valve coil disconnected
EAWorcester BoschFlame not established during the first 4 seconds ignition period
EAWorcester BoschThere is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.
EAWorcester BoschNo flame is detected. The boiler will block during 4 attempts before going to Locking fault
A1Worcester BoschLittle or no water in the system – check pressure gauge and repressurise to 1 bar.
C1Worcester BoschFan has stopped
EFWorcester BoschThe boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
No CodeWorcester BoschAir lock or reduced water content – check pressure gauge and repressurise to 1 bar
260Worcester BoschSensor disconnected or no water content – check pressure gauge and repressurise to 1 bar
C0IdealBCC Activation Fault.
C2IdealBCC Fault.
F1IdealLow Water Pressure.
F2IdealFlame Loss.
F3IdealFan Fault.
F4IdealFlow Thermistor.
F5IdealReturn Thermistor.
F6IdealOutside Sensor Failure.
F7IdealLow Mains Voltage.
F9IdealUnconfigured PCB.
L1IdealFlow Temperature Overheat or No Water Flow.
L2IdealFlame Loss.
L5Ideal5 Boiler Resets in 15 Minutes.
L6IdealFalse Flame Lockout.

Need immediate help with a boiler fault?

Or can't find your code, get help from one of our engineers.

More Help and Advice

Engineers in home

Latest News

Catch up with what is happening with Heat Care UK now.
Woman on phone to engineer about broken boiler


For further help and advise check out our FAQs.