Your boiler is failing to detect its own flame correctly, causing it to shut down shortly after trying to start.

The correct working of the flame supervision is shown by the 'Burner On' neon going out and the ignition sequence starting up. After a short period the boiler will shut down.

What causes this fault?

This fault happens when the boiler's internal safety system fails to confirm that a flame has successfully lit. It is usually caused by a worn-out sensor that can no longer 'see' the flame, or a fault in the control board that prevents the ignition sequence from completing. When the boiler cannot verify it is burning gas safely, it shuts down immediately as a protective measure.