My opinion is worthless - I know nothing about boilers...
It may well be a PCB fault but I doubt it - the fan should cause a pressure switch to operate before the pilot flame gets lighted - your boiler may have a pressure switch fault or an issue with the fan or the seal of the case.
But like others (and many boiler engineers) - I can only guess.
BTW - you do not say the make and model of your boiler and it's in the plumbing category rather than Home Heating Systems - but no matter...
Good luck..., November 2014
