halstead aces high almost new everything but just stopped firing up?

have replaced switches. diaphram. pcb. i have an identical boiler in my office and i changed pcb's recently. dhw at home now quite good.
as home stopped firing (pmp is turning) and when i changed the switches around at least a flame appeared for 1 second. had shower in office. did not know the halstead boiler was so good.
help. all downstairs items seem to be working-it's the flame that is not happening.
i think the problem has been there from the beginning but accepted the dhw supply to shower was weak. now i know better.
graeme