High
Neon 3 NOT lit on Glow-worm Ultimate 3 25R

Your boiler is failing to clear combustion gases properly, which means the safety system has turned the unit off to prevent it from operating until the airflow problem is resolved.

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.

What the manual says

Manufacturer manual

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

From this model's manual
Verified against the Ultimate 100ff 120ff installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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