Your boiler has detected a flame signal when it is supposed to be turned off, which suggests a potential fault in the burner control system.

Flame detected even though the appliance is switched off.

What causes this fault?

This fault occurs when the boiler's internal sensor continues to detect a flame signal after the heating cycle has finished. It is usually caused by a carbon-coated ionisation electrode providing a false reading, or a gas valve that isn't sealing completely, allowing a tiny amount of gas to keep burning. Because the boiler believes a flame is present when it shouldn't be, it enters a safety lockout to prevent overheating.