BoilerFaultCodesYour boiler has detected a blockage in the exhaust pipe or a problem with the fan, causing the system to shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Air flow rate safety device: Obstruction of the flue occurs; fan will continue to run but boiler turns OFF.
What causes this fault?
This fault occurs when the boiler's safety sensors detect that harmful exhaust gases are not being cleared from the system effectively. The most common reasons are a mechanical failure of the fan, a blockage in the external flue pipe, or a perished internal seal that is leaking air.
This issue can occur during periods of high winds, which may create a back-pressure in the flue that prevents the fan from extracting gases correctly.
DIY fix possible
Check the external flue pipe (the white pipe on your outside wall) for any visible obstructions like bird nests, leaves, or debris.Ensure there are no items such as garden furniture or plants blocking the airway of the flue.If the exterior is clear, do not attempt to open the boiler; you must call a Gas Safe engineer to inspect the internal fan and air pressure switch.
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 Red flashing light (Flue) — no call-out fees, no surprises.
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Glow-worm 23C is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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