Micron 50FF?

Initial fault was boiler would not start. This was a seized fan, now replaced.Boiler ran for a few hours then stopped again. Then found it would only run with cover off. Air pressure switch replaced. Boiler ran for a few days at a time between resets. Each time it stops the LEDs indicate software fault. If I remove the cover, it will start normally. With the cover on, it will runthe purge cycle, but then takes 2 or 3 attempts to fire up.( Burner light on for about 2secs, then restarts the lighting routine. ) Sometimesit will light OK, others it fails with the software error LEDs. Anyone any ideas as to what the problem might be. BTW my heating engineer has carried out the work so far, but when I phoned him today, he sounded stumped, but is coming again tomorrow. Just thought I'd see if I could get any ideas for him.