Your boiler has detected a potential internal gas leak or an electrical failure that stops the gas supply from closing correctly, so it has shut down for safety.

Fault:Fuelvalveswitch-offdelay Delayedswitch-offsequenceofgasvalveassemblydetected – Flameindicatorlight(ignitionandmonitoringelectrodeindicates delayedextinguishingoftheflamesignal) – Gasvalveassemblydefective – PCBdefective

What causes this fault?

This fault occurs when the boiler's internal computer detects that the gas flame is taking too long to go out after the heating is turned off. The most common reason is a mechanical failure in the gas valve that prevents it from closing fully, or a communication error on the circuit board that keeps the valve open when it shouldn't be.