Common Boiler Fault Codes

Although most new boilers are very reliable, all machines have the potential to encounter a fault at some point in their life. To make things easy to diagnose, boilers often display an Error Code when they are not working. This can be used to identify the issue, cutting down on investigation time.

Ableforce mainly install Worcester Bosch, Vailant, and Ideal boilers. These units all tend to be reliable, but just in case, we have detailed the descriptions of common fault codes for each manufacturer below.

Despite this useful information, we would still strongly advise that any further diagnostics or repair is performed by a fully qualified Gas Safe registered company such as Ableforce Services.


Worcester Bosch Boilers

As the leading manufacturer of gas boilers in the UK, there are a significant number of these units installed in thousands of homes. Whilst these tend to be some of the most reliable boilers available, it’s still good to know what the issue is if something went wrong.

Locking Errors (Reset button needs to be pressed)

9AIncorrect HCM fitted
9UProblem with the code plug (faulty or not connected)
B7Replace control board
C6Running too fast / Running too slow
C7Fan not running / Fan not running or airflow blocked
D1Sensor wet or damaged (the boiler will block for 300 seconds before this condition)
D1Sensor disconnected or damaged (the boiler will block for 300 seconds before this condition)
D1Boiler overheat
E2Flow sensor shorted or damaged
E2Flow sensor disconnected or damaged
E5Primary flow sensor overheated
E9The main heat exchanger has overheated.
Maximum temperature 105°C
E9Maximum temperature sensor on the main heat exchanger has failed
E9Maximum temperature sensor on the main heat exchanger has not been recognised
E9Flue gas thermostat overheat
EAFailure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
EAAn established flame has been extinguished.
This could be caused by:
moisture in the sump, due to a blocked sump/condensate trap.
a strong wind into the flue
EAGas valve coil disconnected
EAFlame not established during the first 4 seconds ignition period
EAThere is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.

Blocking Errors (Will clear automatically)

A1Little or no water in the system – check pressure gauge and repressurise to 1 bar.
C1Fan has stopped
EANo flame is detected. The boiler will block during 4 attempts before going to Locking fault
EFThe boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
No CodeAir lock or reduced water content – check pressure gauge and repressurise to 1 bar
260Sensor disconnected or no water content – check pressure gauge and repressurise to 1 bar



Vaillant Boilers

As a pioneer of the Combi Boiler and continuing to deliver excellent quality products, many homes have chosen a Vaillant Boiler for their heat and hot water requirements.

All error codes

F.49eBUS faultShort circuit on the eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52Mass flow sensor connection faultMass flow sensor not connected/disconnected, plug not connected or incorrectly connected
F.53Mass flow sensor faultMass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54Gas pressure fault (in combination with F.28/F.29)No or too little gas supply pressure, gas valve closed
F.56Fault: Mass flow sensor regulationGas valve defective, cable harness to gas valve defective
F.57Fault during comfort safety modeIgnition electrode highly corroded
F.61Fault: Gas valve regulationShort circuit/short to earth in cable harness for the gas valve

Gas valve defective (coils shorted to earth)

Electronic defective

F.62Fault: Gas valve switch-off delayDelayed shutdown of gas valve

Delayed extinguishing of flame signal

Gas valve leaking Electronics defective

F.63EEPROM errorElectronics defective
F.64Electronics/NTC faultSupply or return NTC short circuited, electronics defective
F.65Electronic temperature faultElectronics overheating due to external influences, electronics defective
F.67Electronics/flame faultImplausible flame signal, electronics defective
F.68Unstable flame signal faultAir 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.70Invalid 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.71Flow NTC faultFlow temperature sensor signalling constant value:

Flow temperature sensor incorrectly positioned at supply pipe

Flow temperature sensor defective

F.72Flow/return NTC faultFlow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73Water pressure sensor signal in the wrong range (too low)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.74Water pressure sensor signal outside correct range (too low)Cable to water pressure sensor has short-circuitedd at 5V/24 V or internal fault in water pressure sensor
F.75Fault, no pressure change detection when starting pumpWater 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.76Overheating protection on primary heat exchanger has respondedCable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
F.77Flue non-return flap/condensate pump faultNo response from flue non-return flap or condensate pump defective
F.78Interruption to DHW outlet sensor at external controllerUK link box is connected, but hot water NTC not bridged
F.83Flow and/or return temperature sesnsor temperature change faultWhen the burner starts, the temperature change registered at flow and/or return temperature sensor is non-existant or too small.

Not enough water in the boiler

Flow and/or return temperature sensor not in correct position at pipe

F.84Fault: Flow/return temperature sensor difference implausibleFlow 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.85Fault: Flow and return temperature sensors incorrectly fittedFlow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92Coding resistor faultThe 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.93Gas group faultCombustion 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.
Communication faultNo communication with PCB



Ideal Boilers

UK Manufacturer Ideal has been producing boilers for over 100 years and offer a great value and dependable reliability.

All error codes

C0BCC Activation Fault.
C2BCC Fault.
F1Low Water Pressure.
F2Flame Loss.
F3Fan Fault.
F4Flow Thermistor.
F5Return Thermistor.
F6Outside Sensor Failure.
F7Low Mains Voltage.
F9Unconfigured PCB.
L1Flow Temperature Overheat or No Water Flow.
L2Flame Loss.
L55 Boiler Resets in 15 Minutes.
L6False Flame Lockout.