worcester 28i junior?

my boiler works now and again, (usually not), i turn on water and the fan wont run i turn off water and the fan runs! (for a short time). if i leave the water on the red light flashes rapidly and boiler shuts down. same for heating. had plumber out hes stumped apart from pcb powered fan it runs independently,isolated water pressure switch ditto overheat. no joy if i knew for certain it was the pcb i would be happy