Your boiler is preparing to start the heating process by running the internal fan to clear out the combustion chamber.

fan pre-run

What causes this fault?

This is not actually a fault, but rather a normal part of the boiler's starting routine where the fan spins up to clear any residual gases from the burner area. It occurs every time the system receives a request for heat or hot water to ensures the appliance is safe to ignite. If the boiler stays stuck on this code without firing up, it usually indicates the fan cannot reach the required speed or the air pressure switch isn't detecting enough airflow.