Your boiler is still detecting a flame even after it has tried to shut off the gas, which is a safety conflict that requires a professional inspection.

Flame signal after closing the fuel supply – the air/gas ratio control valve might need replacing

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame even after the gas supply should have been cut off. The most common reason is a gas valve that has become stuck open or a faulty electrode that is incorrectly reporting a flame to the control board. Because the boiler cannot guarantee it can stop the flow of fuel, it shuts down immediately as a critical safety precaution.