Your boiler is unable to light the flame because the gas supply is blocked or a safety sensor is cutting out.

The first and/or second gas valve does not open, check for a 24 V supply during the ignition phase. Check the resistance of the burner thermostat (unit electrically disconnected) maximum resistance of 3Ω.

What causes this fault?

This fault occurs when the boiler is unable to establishes a flame because the internal gas valves are failing to open or a safety sensor has detected overheating. The most common reason is an electrical failure in the gas valve motor or a tripped burner thermostat which acts as a safety cut-out to prevent damage. Essentially, the boiler is trying to start, but the 'gates' that allow gas into the burner are staying shut.