Your boiler is performing a routine safety check by starting the fan before the burner ignites.

fan pre-run

What causes this fault?

This is a normal part of the boiler's startup sequence where the fan clears any leftover gases from the combustion chamber to ensure a safe ignition. It is not a fault, but rather a status update showing that the system is preparing the burner to fire up. If this code stays on the screen without the boiler eventually firing, it may indicate that the fan is struggling to reach the required speed.