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.

Code	Manufacturer	Meaning
F.49	Vaillant	Fault: eBUS: Short circuit on eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52	Vaillant	Mass flow sensor connection fault – Mass flow sensor not connected/disconnected, plug not connected or incorrectly connected
F.53	Vaillant	Mass flow sensor fault – Mass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54	Vaillant	Gas pressure fault (in combination with F.28/F.29)- No or too little gas supply pressure, gas valve closed
F.56	Vaillant	Fault: Mass flow sensor regulation – Gas valve defective, cable harness to gas valve defective
F.57	Vaillant	Fault during comfort safety mode – Ignition electrode highly corroded
F.61	Vaillant	Fault: 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.62	Vaillant	Fault: Gas valve switch-off delay – Delayed shutdown of gas valve. Delayed extinguishing of flame signal. Gas valve leaking Electronics defective
F.63	Vaillant	EEPROM error – Electronics defective
F.64	Vaillant	Electronics/NTC fault – Supply or return NTC short circuited, electronics defective
F.65	Vaillant	Electronic temperature fault – Electronics overheating due to external influences, electronics defective
F.67	Vaillant	Electronics/flame fault – Implausible flame signal, electronics defective
F.68	Vaillant	Unstable 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.70	Vaillant	Invalid 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.71	Vaillant	Flow NTC fault – Flow temperature sensor signalling constant value: Flow temperature sensor incorrectly positioned at supply pipe. Flow temperature sensor defective
F.72	Vaillant	Flow/return NTC fault – Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73	Vaillant	Water 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.75	Vaillant	Fault, 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.76	Vaillant	Overheating protection on primary heat exchanger has responded – Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
F.77	Vaillant	Flue non-return flap/condensate pump fault No response from flue non-return flap or condensate pump defective
F.78	Vaillant	Interruption to DHW outlet sensor at external controller -UK link box is connected, but hot water NTC not bridged
F.83	Vaillant	Flow 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.84	Vaillant	Fault: 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.85	Vaillant	Fault: Flow and return temperature sensors incorrectly fitted- Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92	Vaillant	Coding 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.93	Vaillant	Gas 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.
9A	Worcester Bosch	Incorrect HCM fitted
9U	Worcester Bosch	Problem with the code plug (faulty or not connected)
B7	Worcester Bosch	Replace control board
C6	Worcester Bosch	Running too fast / Running too slow
C7	Worcester Bosch	Fan not running / Fan not running or airflow blocked
D1	Worcester Bosch	Sensor wet or damaged (the boiler will block for 300 seconds before this condition)
D1	Worcester Bosch	Boiler overheat
E2	Worcester Bosch	Flow sensor shorted or damaged
E2	Worcester Bosch	Flow sensor disconnected or damaged
E5	Worcester Bosch	Primary flow sensor overheated
E9	Worcester Bosch	The main heat exchanger has overheated. Maximum temperature 105°C
E9	Worcester Bosch	Maximum temperature sensor on the main heat exchanger has failed
E9	Worcester Bosch	Maximum temperature sensor on the main heat exchanger has not been recognised
E9	Worcester Bosch	Flue gas thermostat overheat
EA	Worcester Bosch	Failure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
EA	Worcester Bosch	An 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
EA	Worcester Bosch	Gas valve coil disconnected
EA	Worcester Bosch	Flame not established during the first 4 seconds ignition period
EA	Worcester Bosch	There is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.
EA	Worcester Bosch	No flame is detected. The boiler will block during 4 attempts before going to Locking fault
A1	Worcester Bosch	Little or no water in the system – check pressure gauge and repressurise to 1 bar.
C1	Worcester Bosch	Fan has stopped
EF	Worcester Bosch	The boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
No Code	Worcester Bosch	Air lock or reduced water content – check pressure gauge and repressurise to 1 bar
260	Worcester Bosch	Sensor disconnected or no water content – check pressure gauge and repressurise to 1 bar
C0	Ideal	BCC Activation Fault.
C2	Ideal	BCC Fault.
F1	Ideal	Low Water Pressure.
F2	Ideal	Flame Loss.
F3	Ideal	Fan Fault.
F4	Ideal	Flow Thermistor.
F5	Ideal	Return Thermistor.
F6	Ideal	Outside Sensor Failure.
F7	Ideal	Low Mains Voltage.
F9	Ideal	Unconfigured PCB.
L1	Ideal	Flow Temperature Overheat or No Water Flow.
L2	Ideal	Flame Loss.
L5	Ideal	5 Boiler Resets in 15 Minutes.
L6	Ideal	False Flame Lockout.