Your boiler has temporarily paused because a safety flap in the flue system is not providing the correct signal to allow the burner to start.

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.