Your boiler is unable to confirm that the burner has successfully lit, so it has safely shut itself down to prevent gas buildup.

Flame sensing fault.

What causes this fault?

This fault occurs when the boiler's control system cannot confirm that a flame has ignited after several attempts. The most common reason is a worn-out or dirty sensing probe that can no longer detect the heat, or a fault within the internal wiring that prevents the signal from reaching the main control board.