vokera aquanova green light flashing?

i have a vokera aquanova water heater which has a flashing green light ,it goes through the ignition sequence several times before going to a permanent red light. can hear fast clicking noises from the pcb area also cannot see any sparks from electrode . Burner etc looks very clean , fan is also working ,that leaves the gas valve ,pcb and differential pressure switch to be checked. Anyone got any idea as to what could be wrong with it?.Thanks