BoilerFaultCodesYour boiler has paused its heating process because an external control or smart accessory is preventing it from firing up.
This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.
Technical description: Accesory Module is blocking burner operation
What causes this fault?
This code occurs when an external control device, such as a smart thermostat or a zone valve, is sending a signal to the boiler that prevents it from firing. The boiler is technically ready to work, but it is being 'blocked' by a command from an accessory that may be malfunctioning or incorrectly wired. In many cases, this is caused by a communication failure between the wireless receiver and the boiler itself.
DIY fix possible
Check your external room thermostat or smart home app to ensure it is calling for heatReplace the batteries in your wireless thermostat or programmerEnsure any underfloor heating zones or external pumps are switched on and functional
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like S.42 — no call-out fees, no surprises.
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