potterton performa 30 he flashing flu problem?

i have had a problem now since xmas where my heating works for a while and then just stops working and results to a light flashing saying it has a faulty fan/flu, i have had various parts replaced a white lead sensor lead, and a new pcb and yet it still keeps failing and not just on the heating but also on the hot water. if anyone can help with some advice please do comment.