Your boiler has stopped working because the internal fan, which clears exhaust gases, isn't spinning fast enough to operate safely.

Fan speed too low.

What causes this fault?

This fault occurs when the boiler's control system detects that the fan isn't reaching the necessary speed to safely push exhaust fumes out of the flue. The most common reasons are a build-up of dirt inside the fan mechanism, a block in the air supply tubes, or the electronic controller failing to send enough power to the motor.