Your boiler has stopped heating because the internal mechanism that pushes air through the system is not spinning fast enough to allow for safe combustion.

Fan speed too low.

What causes this fault?

This fault occurs when the boiler's control system detects that the internal fan isn't rotating fast enough to safely expel exhaust gases. It is usually caused by a build-up of dust on the fan blades, a failing motor, or a communication issue from the main control board. If the fan cannot reach the required speed, the boiler will shut down as a safety precaution to prevent fumes from staying inside the unit.