Boiler attempts to ignite but fails; you hear clicking from the ignition, then the boiler locks out and displays A01.

Safety first: Switch the boiler off at the fused spur and close the gas isolation valve before removing any covers. Only a Gas Safe registered engineer should work on gas components.

• Reset the boiler: Follow the user manual to perform a reset. If A01 clears and does not return, monitor operation.
• Check gas supply: Ensure other gas appliances work. Verify the gas cock under the boiler is fully open. If no gas to any appliance, contact your gas supplier.
• Inspect flue terminal (user level): From outside, make sure the flue is not blocked by debris, snow, or vegetation, which can affect combustion.
• Engineer checks:
  • Remove burner and clean the burner head and injectors.
  • Inspect and clean the ignition and flame sensing electrodes; check ceramic for cracks and correct gap/position.
  • Test ignition lead for damage and good connection to the PCB and electrode.
  • Measure gas inlet and burner pressure; adjust or replace the gas valve if out of spec.
  • Check ionisation current and replace the flame detection electrode if weak.