Your boiler has stopped working because the internal fan isn't spinning fast enough to safely clear away exhaust fumes.

Fan speed too low.

What causes this fault?

This fault occurs when the boiler's control system senses that the fan isn't rotating at the required speed to safely vent exhaust gases out of the flue. The most common reasons are a build-up of dust and debris inside the fan motor, a mechanical seafing of the bearings, or a communication failure between the fan and the main control board.