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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)
9A Incorrect HCM fitted
9U Problem with the code plug (faulty or not connected)
B7 Replace control board
C6 Running too fast / Running too slow
C7 Fan not running / Fan not running or airflow blocked
D1 Sensor wet or damaged (the boiler will block for 300 seconds before this condition)
D1 Sensor disconnected or damaged (the boiler will block for 300 seconds before this condition)
D1 Boiler overheat
E2 Flow sensor shorted or damaged
E2 Flow sensor disconnected or damaged
E5 Primary flow sensor overheated
E9 The main heat exchanger has overheated.
Maximum temperature 105°C
E9 Maximum temperature sensor on the main heat exchanger has failed
E9 Maximum temperature sensor on the main heat exchanger has not been recognised
E9 Flue gas thermostat overheat
EA Failure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
EA An 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
EA Gas valve coil disconnected
EA Flame not established during the first 4 seconds ignition period
EA There is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.
Blocking Errors (Will clear automatically)
A1 Little or no water in the system – check pressure gauge and repressurise to 1 bar.
C1 Fan has stopped
EA No flame is detected. The boiler will block during 4 attempts before going to Locking fault
EF The boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
No Code Air lock or reduced water content – check pressure gauge and repressurise to 1 bar
260 Sensor 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.49 eBUS fault Short circuit on the eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52 Mass flow sensor connection fault Mass flow sensor not connected/disconnected, plug not connected or incorrectly connected
F.53 Mass flow sensor fault Mass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54 Gas pressure fault (in combination with F.28/F.29) No or too little gas supply pressure, gas valve closed
F.56 Fault: Mass flow sensor regulation Gas valve defective, cable harness to gas valve defective
F.57 Fault during comfort safety mode Ignition electrode highly corroded
F.61 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 Fault: Gas valve switch-off delay Delayed shutdown of gas valve

Delayed extinguishing of flame signal

Gas valve leaking Electronics defective

F.63 EEPROM error Electronics defective
F.64 Electronics/NTC fault Supply or return NTC short circuited, electronics defective
F.65 Electronic temperature fault Electronics overheating due to external influences, electronics defective
F.67 Electronics/flame fault Implausible flame signal, electronics defective
F.68 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 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 Flow NTC fault Flow temperature sensor signalling constant value:

Flow temperature sensor incorrectly positioned at supply pipe

Flow temperature sensor defective

F.72 Flow/return NTC fault Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73 Water 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.74 Water 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.75 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 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 Flue non-return flap/condensate pump fault No response from flue non-return flap or condensate pump defective
F.78 Interruption to DHW outlet sensor at external controller UK link box is connected, but hot water NTC not bridged
F.83 Flow and/or return temperature sesnsor temperature change fault When 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.84 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 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 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 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.
Communication fault No 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
C0 BCC Activation Fault.
C2 BCC Fault.
F1 Low Water Pressure.
F2 Flame Loss.
F3 Fan Fault.
F4 Flow Thermistor.
F5 Return Thermistor.
F6 Outside Sensor Failure.
F7 Low Mains Voltage.
F9 Unconfigured PCB.
L1 Flow Temperature Overheat or No Water Flow.
L2 Flame Loss.
L5 5 Boiler Resets in 15 Minutes.
L6 False Flame Lockout.
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