Your boiler’s air fan is not spinning at the correct speed to safely push exhaust fumes out of the flue, causing the system to shut down for safety.

Fan speed variation (too high on start); Fans blocked, plug not inserted correctly on fan, hall sensor defective, fault in cable harness, electronics defective

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is spinning too slowly or not at all, meaning it cannot safely clear exhaust gases. The most common reasons are a build-up of dirt within the fan casing, a mechanical failure where the motor has burnt out, or a communication issue between the fan and the main circuit board.