Short answer
A fan fault means the control did not see the fan reach or hold the speed it commanded, or an air pressure switch did not prove. Because the fan sets the gas/air mix on premix boilers, any fan fault stops the boiler before ignition.
Common causes
Worn bearings or a seized fan.
Failed tacho feedback or wiring fault.
PCB not driving the fan.
Blocked flue or air intake causing air proving failure.
Condensate in the fan housing.
What the engineer checks
Compare commanded and actual fan speed.
Check supply and PWM signals at the fan connector.
Inspect flue and air inlet.
Repair and prevention
Replace the fan with the correct part, repair wiring, clear the flue or condensate, then re-check combustion as fan changes alter the gas/air ratio.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
Need a commercial gas engineer in London?
Gas Safe registered commercial engineers, 24 hours a day. Call now or book online and we'll confirm a time.