Your boiler is unable to blow out the waste gases properly because the fan is not spinning fast enough to stay safe.

Not enough fan driving force

What causes this fault?

This fault occurs when the boiler's internal computer detects that the fan isn't reaching the speed required to safely clear exhaust fumes from the system. It is usually caused by the fan motor wearing out over time or a communication failure where the control board can no longer tell the fan how fast to spin.