Your boiler is currently preparing its internal fan to safely clear any leftover gases before it begins the ignition process.

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 the combustion chamber. It occurs every time there is a demand for heating or hot water to ensure the system is safe to ignite. If the code stays on the screen for a long time without the boiler firing up, it may indicate that the fan is struggling to reach the required speed.