BoilerFaultCodesThe 5 most common faults
01
B Illuminated continuously High DIY-safe
Your boiler has stopped working because it failed to light the internal flame, resulting in no heating or hot water until it is successfully restarted.
What to try first
Turn the selector dial on the front of the unit to the 'Reset' position.Wait for 30 seconds for the boiler to click and attempt the ignition sequence again.If the boiler fails to ignite repeatedly, check that your gas supply is turned on and your gas meter has credit.
02
A Flashing High DIY-safe
Your boiler's internal thermometer is sending incorrect or missing information, which has caused the system to stop working for safety.
What to try first
Turn the boiler off at the main electrical switch.Wait for 30 seconds.Turn the power back on and press the 'Reset' button.
03
B Flashing High DIY-safe
Your boiler has shut down because it has reached a temperature that is too high, often caused by a blockage or a pump issue.
What to try first
Check that all radiator valves are fully openEnsure the system pressure is between 1.0 and 1.5 barTurn the selector switch to 'Reset' for a few seconds and then back to 'On'
04
Neon A flashing High Engineer
Your boiler has lost the ability to read the temperature of the water inside it, so it has shut down to prevent overheating.
Full guide for Neon A flashing05
Neon B flashing High Engineer
Your boiler has automatically shut itself down because it detected that it is running too hot, possibly due to trapped air or a blockage in the system.
Full guide for Neon B flashingon the Alpha Cd25
Ignition electrodeGas valvePrinted Circuit Board (PCB)NTC Temperature SensorWiring harnessCirculating Pump
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Alpha shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue