BoilerFaultCodesYour boiler has stopped working because it has detected a blockage in the exhaust system or a failure in the pump that removes waste water.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flue non-return flap closed. Flue non-return flap return signal blocks burner operation (only in conjunction with the multi-functional module) or condensate pump defective, heat demand is blocked.
What causes this fault?
This fault occurs when the boiler's safety systems detect that waste gases cannot exit the flue or waste water cannot be pumped away. The most common cause is a failure in the external condensate pump or a physical blockage in the non-return flap, which triggers a shutdown to prevent water damage or a buildup of exhaust fumes inside the unit.
This fault is more common during the winter, as external condensate discharge pipes can freeze and cause water to back up into the pump or boiler.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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