Your boiler is performing a routine check of the internal fan system before it begins the heating process.

fan pre-run

What causes this fault?

This code appears when the boiler's control board sends a signal to the fan to spin up and clear any remaining gases from the combustion chamber. It is a standard safety procedure to ensure the system is clear and safe before the burner ignites. If the code stays on the screen for a long time, it suggests the fan is struggling to reach the required speed to allow the ignition sequence to continue.