Your boiler is failing to shut off the gas properly at the end of a heating cycle, which is a safety issue requiring professional inspection.

Fault: Fuel valve switch-off delay - Delayed switch-off sequence for the gas valve after the flame is extinguished

What causes this fault?

This fault occurs when the boiler's control system detects that gas is still flowing or a flame is present even after the unit has been told to shut down. The most common reason is a physical stickiness within the gas valve or a communication glitch on the main circuit board that prevents the fuel supply from being cut off instantly.