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

Flame detected after gas valve shut. Relight, check gas valve closes promptly. Check for electric mains problem. Replace gas valve

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame for several seconds after the demand for heat has stopped and the gas should have been cut off. The most common reason is a mechanical failure in the gas valve where it is failing to close fully, or a faulty control board is incorrectly interpreting signals from the flame sensor.