How can i fix my combi boiler vokera excell 80sp?

I now know that either the pressure differential switch or the fan has ceased to function correctly on my combi boiler. the pressure switch was replaced (but not specific to the make of the boiler) and we were left with the same problem we had to start with. This would lead me to believe the fan was the culprit. can anyone tell me their opinion.

If i were to replace the fan i would have to remove the main heat exchanger. When i come to refilling the system presumably i would use the filling loop and just bleed the radiators. Advice on this would also be helpful.

Many thanks