BoilerFaultCodesYour boiler has temporarily paused because a safety flap in the flue system is not providing the correct signal to allow the burner to start.
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: Flue non return flap return signal blocks burner operation
What causes this fault?
This fault occurs when a safety flap inside the flue system, designed to prevent exhaust gases from flowing back into the boiler, fails to open or close correctly. The boiler’s control board pauses operation because it cannot confirm the flap is in the safe position required to start the burner. This is often caused by a mechanical blockage, a stiff hinge on the flap, or a faulty sensor failing to send the correct signal.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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.
Affiliate links — we may earn a commission at no extra cost to you.