Modern boilers tend to be pretty reliable, but like any other piece of equipment, they are not immune from faults. It’s always a pain when your boiler stops working, of course, but before you call a qualified boiler engineer to come out and fix it, it’s a good idea to attempt at least some basic diagnosis of the problem.

Most modern boilers will display some kind of fault code. This might show you that the issue is something simple that you can fix yourself. But even if it doesn’t, passing the information in the code on to our engineer when you call will help them to understand what might be needed even before they arrive.

Below we look at the fault codes for some of the most popular types of boiler in the UK to give you an idea as to what your system is trying to tell you.

Worcester Bosch Boilers

Worcester Bosch is the leading manufacturer of gas boilers in the UK and its systems are installed in lots of homes. On the whole, these boilers are extremely reliable, but it’s still worth knowing what the fault codes mean, should something go wrong.

Note that there are two kinds of error code on these boilers, locking errors that need the reset button to be pressed in order to continue, and blocking errors that will clear automatically.

Locking Errors

  • 9AIncorrect HCM is fitted
  • 9UCode plug faulty or not connected
  • B7Replace control board
  • C6Running too fast or too slow
  • C7Fan not running or airflow is blocked
  • D1Sensor wet or damaged or disconnected (the boiler will block for 300 seconds before showing D1)
  • D1Boiler overheating
  • E2Flow sensor shorted, damaged or disconnected
  • E5Primary flow sensor has overheated
  • E9The main heat exchanger has overheated – maximum temperature is 105°C
  • E9Maximum temperature sensor on the main heat exchanger failed or has not been recognised
  • E9Flue gas thermostat overheating
  • EAFour ignition attempts have failed. The boiler waits 30 seconds before another attempt is made
  • EAAn established flame has been extinguished. This could be caused by either moisture in the sump, due to a blocked sump/condensate trap, or a strong wind into the flue
  • EAThe gas valve coil is disconnected
  • EAFlame was not established during the first 4 seconds of the ignition period
  • EAThere is an error because the pump is not detecting water in the system – usually this means low pressure, check the pressure gauge and repressurise the boiler to 1 bar.

Blocking Errors

  • A1Not enough in the system – check the pressure gauge and repressurise the boiler to 1 bar.
  • C1The fan has stopped
  • EANo flame is detected. The boiler will block during 4 attempts before going to a locking fault
  • EFThe boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
  • 260Sensor disconnected or no water content – check the pressure gauge and repressurise the boiler to 1 bar
  • No CodeIf no code is shown but the boiler is still not operating, this could mean an airlock or reduced water content – check the pressure gauge and repressurise the boiler to 1 bar

Ideal Boilers

UK Manufacturer Ideal has been producing boilers for over 100 years and offers good value and dependable units.

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

Vaillant Boilers

As a pioneer of the Combi Boiler and still delivering high-quality products, many homes have a Vaillant Boiler installed to serve their heating and hot water requirements.

All error codes

  • F.49eBUS fault – A short circuit on the eBUS, eBUS overload or two power supplies with different polarities on the eBUS
  • F.52Mass flow sensor connection fault. The mass flow sensor is not connected/disconnected, plug not connected or incorrectly connected
  • F.53Mass flow sensor fault. The mass flow sensor is 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 regulation. The gas valve is defective, or cable harness to gas valve is defective
  • F.57Fault during comfort safety mode. The ignition electrode is highly corroded
  • F.61Fault: Gas valve regulation. A short circuit/short to earth in the cable harness for the gas valve, or gas valve defective (coils shorted to earth), or electronics defective
  • F.62Fault: Gas valve switch-off delay. Delayed shutdown of gas valve or delayed extinguishing of flame signal, or gas valve leaking, or electronics defective
  • F.63EEPROM error. The boiler’s electronics are defective
  • F.64Electronics/NTC fault. Supply or return NTC has short-circuited, electronics defective
  • F.65Electronic temperature fault. Electronics overheating due to external influences, or electronics defective
  • F.67Electronics/flame fault. An implausible flame signal, or electronics defective
  • F.68Unstable flame signal fault. Air in the gas, or gas flow pressure too low, or wrong air ratio, or condensate route blocked, or wrong gas restrictor, or ionisation flow interruption (cables, electrodes), or flue gas recirculation, or condensate route problem
  • F.70Invalid device-specific number (DSN)
  • F.71Flow NTC fault. Flow temperature sensor signalling constant value, or flow temperature sensor incorrectly positioned at supply pipe, or flow temperature sensor defective
  • F.72Flow/return valve or temperature sensor defective
  • F.73Water pressure sensor signal in the wrong range (too low)
  • F.74Water pressure sensor signal outside the correct range, or cable to water pressure sensor has short-circuited, or internal fault in water pressure sensor
  • F.75No pressure change detection when starting pump. Water pressure sensor and/or pump defective, air in the system, too low water pressure in boiler
  • F.76Overheating protection on primary heat exchanger. Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
  • F.77Flue non-return flap/condensate pump fault. No response from flue non-return flap or condensate pump is defective
  • F.78Interruption to DHW outlet sensor at external controller. UK link box is connected, but hot water NTC not bridged
  • F.83Flow 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, or not enough water in the boiler, or flow and/or return temperature sensor not in correct position
  • F.84Flow/return temperature sensor difference is implausible
  • F.85Flow and return temperature sensors incorrectly fitted
  • F.92Coding resistor fault. The coding resistor on the PCB does not match the entered gas family
  • F.93Gas group fault. The combustion quality is outside the permitted range. This could mean, wrong gas restrictor, recirculation, wrong gas family, internal pressure measuring point in venturi is blocked (do not use lubricant on O-ring in the venturi), or the gas family check has prematurely terminated.
  • Communication fault. There is no communication with the PCB