Your boiler is currently trying to light the gas burner with a spark, but it is failing to sense a flame and stay on.

Flashes to indicate spark operation (stops after detection).

What causes this fault?

This fault occurs when the boiler's internal computer tries to light the gas but cannot confirm that a steady flame has been established. The most common reasons are that the spark is too weak to ignite the gas, or the sensor that detects the flame has become covered in soot and can no longer 'see' the fire.