Your boiler has detected that a flame is still burning inside the unit even though it should have turned off, which is a safety concern that requires an immediate professional check.

Flame detected after gas has been shut off.

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame even after the system has instructed the gas to stop flowing. The most common reasons are a sticking gas valve that hasn't fully closed or a faulty detection electrode that is sending a false signal back to the main control board.