Your boiler is performing a routine start-up check by running its internal fan before lighting the burner.

fan pre-run

What causes this fault?

This code is not actually a fault, but a normal status message showing that the boiler's fan is spinning up to clear the combustion chamber. It occurs every time the boiler prepares to heat your water or radiators to ensure the system is safe before the gas ignites. If the boiler stays on this code for a long time without firing up, it may indicate a struggle to reach the required air pressure.