Your boiler is still detecting a flame even though it has tried to shut off the gas supply, which is a safety conflict.

Flame signal after closing the fuel supply

What causes this fault?

This fault occurs when the boiler's internal sensor detects a flame or an electrical current that looks like a flame even after the gas has been told to turn off. The most common reason is a gas valve that hasn't fully closed or an ignition electrode that is sending a false signal back to the main control board.