Guide To Boiler Fault Codes

Modern boilers are, on the whole, very reliable but like any complex machinery, they can suffer problems over time. Before you call a heating engineer and say, “my boiler is broken” it’s worth doing some diagnostic work yourself. Of course, this only applies if you can’t smell gas. If you can smell gas then you should contact your local gas board immediately (they should have an emergency number).

What are Boiler Fault Codes?

There’s often a fault code shown on the boiler’s front panel. If you can identify this code and give the information it indicates to your plumber or engineer, they can have an idea as to what’s wrong before they arrive. This means they can come prepared with the right parts and you can get your boiler repaired and working again faster.

The instruction manual for your boiler will usually have a list of codes and what boiler faults they identify, probably in a ‘troubleshooting’ or similar section. Codes vary between manufacturers, so it’s important to have the correct information for identifying boiler issues with your system. If you don’t have the instructions to hand, here are some of the common codes for the most popular makes of boiler.

Worcester Bosch Boiler Fault Codes

There are two types of error on these boilers. Locking errors which require you to press the Reset button in order to clear them, and Blocking errors which will clear themselves automatically. Note that some codes have multiple meanings.

Locking Errors

9A Wrong HCM fitted
9U Code plug faulty or not connected)
B7 Control board needs replacing
C6 Running too fast or too slow
C7 Fan not running or airflow blocked
D1 Sensor is wet or damaged (boiler will block for 300 seconds before D1 shows)
D1 Sensor disconnected or damaged
D1 Boiler overheating
E2 Flow sensor is shorted or damaged or disconnected
E5 Primary flow sensor has overheated
E9 Main heat exchanger has overheated – maximum temperature is 105°C
E9 Maximum temperature sensor on main heat exchanger has failed
E9 Maximum temperature sensor on the main heat exchanger not recognised
E9 Flue gas thermostat overheating
EA Four ignition attempts failed, the boiler will wait 30 seconds before another attempt is made
EA Flame has been extinguished – this could be caused by: moisture in the sump, a blocked sump or condensate trap, or a strong wind into the flue
EA Gas valve coil not connected
EA Flame not established during the first four seconds of ignition
EA Pump is not detecting water – check the pressure gauge

Blocking Errors

A1 Lack of water in the system – check pressure gauge
C1 The fan has stopped
EA No flame detected. The boiler will block 4 attempts then go to Locking fault
EF Operating at minimum burner load and there is a greater than 18°C Flow and Return temperature difference
No Code Air lock or low water content – check pressure gauge
260 Sensor disconnected or no water – check pressure gauge

Vaillant Boiler Fault Codes

F.49 eBUS fault
F.52 Mass flow sensor connection fault
F.53 Mass flow sensor fault
F.54 Gas pressure fault
F.56 Mass flow sensor regulation fault
F.57 Fault during comfort safety mode
F.61 Gas valve regulation fault
F.62 Gas valve switch-off delay fault
F.63 EEPROM error
F.64 Electronics/NTC fault
F.65 Electronics overheating
F.67 Electronics/flame fault
F.68 Unstable flame signal fault
F.70 Invalid device specific number (DSN)
F.71 Flow NTC fault
F.72 Flow/return fault or return temperature sensor fault
F.73 Water pressure sensor signal in the wrong range
F.74 Water pressure sensor signal outside correct range
F.75 Pump/water pressure problem
F.76 Overheating on primary heat exchanger
F.77 Flue non-return flap/condensate pump fault
F.78 Interruption to DHW outlet sensor at external controller
F.83 Flow and/or return temperature sensor temperature change fault, could mean not enough water in the boiler
F.84 Flow and return temperature sensors return implausible values
F.85 Flow and return temperature sensors wrongly fitted
F.92 Coding resistor fault
F.93 Gas group fault

Ideal Boiler Fault 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