How can I repair a Main combi 24 he?

The boiler works for about a minute but then cuts out as if it were up to temperature. The same thing happens in both hot water, and heating modes. When I link out the air pressure switch it works fine (until it reaches temperature). The flue and trap are clear.
The air pressure switch has now been repaced, but this hasn't cured the problem.
Could this problem be caused by the fan?