Your boiler is attempting to start and creating a spark, but it is failing to detect a flame and is shutting down for safety.

Ignition sequence: the spark starts and the gas valve opens.

What causes this fault?

This fault occurs when the boiler's control system fails to confirm that a flame has successfully lit after opening the gas valve. The most common reasons are a worn-out sensor that can no longer 'see' the flame, or a fault within the gas valve preventing fuel from reaching the burner. Over time, soot buildup or moisture can also interfere with the electrical signal required to keep the boiler running.