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

fan pre-run

What causes this fault?

This is a normal operational stage and not an actual fault with your boiler. It occurs because the boiler's internal computer is running the fan to clear the combustion chamber of any lingering gases before the burner safely ignites. You will likely see this code for a few seconds every time you turn on your heating or hot water tap.