High Lockout
Neon 3 NOT lit on Glow-worm Glow-worm Micron 100FF 120FF

Your boiler has stopped working because it cannot confirm that the internal fan is safely clearing waste gases out of the flue.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Air flow proving fault - that is fan or air pressure switch

What causes this fault?

This fault occurs when the boiler's safety system detects that the fan isn't spinning fast enough or the air pressure is incorrect to safely push exhaust fumes outside. The most common reasons are a seized fan motor, a perished rubber tube leading to the sensor, or a blockage within the flue pipe itself.

This issue is more common during the first cold snap of autumn, as fan bearings can seize up after being idle throughout the summer months.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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