Your boiler is currently preparing to fire up by starting its internal fan, which is a normal part of the ignition process.

DHW mode: Fan start-up

What causes this fault?

This is a normal status message rather than a mechanical failure, occurring every time the boiler prepares to heat your water or radiators. It indicates that the control board has sent a signal to the internal fan to begin spinning, which clears out the combustion chamber before the burner ignites. If the boiler stays on this code for a long time without firing, it may suggest the fan is struggling to reach the required speed.