Let’s face it: even boilers have bad days!
Especially because those bad days most commonly occur when the boiler is turned back on after a nice long break from not having to work hard to heat your home.
The good news is that, more often than not, our boiler can tell us what’s wrong with it and it doesn’t always require the expertise of a boiler engineer to help get it working again.
When a boiler stops working it generally shows a fault code, or error code.
However, different models have different fault codes and here at Gregor Heating, we do see three common codes crop up which are:
EA (Worcester) or F27 (Vaillant) – blocked condensate pipe. This usually appears in extreme cold weather when the outside pipe is frozen. You may have to wait for the condensate to defrost. If the pipe runs outside, pour hot water over the pipe to defrost it, or use your hairdryer.
U9, A1 or No code (Worcester), F75 (Vaillant), or F1 (Ideal) – low water pressure. This usually requires you to top up your boiler. Follow your manufacturer’s manual to do this
F75 (Vaillant) – sensor change. This is very common with these boilers and usually means the fault is most likely to do with the boiler’s sensor not being aware that the pump has started running

All your boiler’s fault codes will be listed in your installation and servicing manual which is in place for guidance when an engineer is carrying out diagnostics but can be useful to the user to know what’s wrong to pass this info on when booking a visit. 

Once you’ve established which fault error is relevant, you will need to refer to your boiler’s manual or visit the manufacturer’s website. Both will be useful resources for helping you to work out what the code is as well as provide helpful tips on how to fix it.
As accredited installers of the three of the top tried and trusted boiler brands in the UK, Worcester Bosch, Vaillant and Ideal, we have provided a list of the most common fault codes below by brand. So read on to find out which one might be relevant for you.
Good luck, if in doubt and the boiler has stopped working, then don’t delay and call Gregor Heating on 0117 935 2400 today!


Worcester Bosch 

With most Worcester Bosch boilers, the first step is to consider the fault errors:
Locking error – this will show a flashing fault code as well as a warning triangle and you will need to press the reset button to stop this.
Block error – if there is a blocking error, no data will be shown and your boiler will stop working. You will be able to find the cause code by pressing the spanner / return button.
Cause code – this is a 3 digit number shown when pressing the spanner / return button.
Fault error – this is an alpha numeric code that will be shown and will indicate the error by showing the fault group (they vary by boiler make and model).
Maintenance code – this begins with an H and will tell you that there is an issue that will not stop the boiler working but will need the attention of an engineer.

Common Locking Errors

  • 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 – 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, or 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.

Common Blocking Errors

  • 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 Fault 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-circuited 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 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 – 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.


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.


Good luck! If you are in any doubt and the boiler has stopped working, then don’t delay, call Gregor Heating on 0117 935 2400 today!