ferroli optimax 25c problem?

hi there i have a optimax 25c ,,it wasnt lighting so i replaced the ignition pcb ,,now it lights ok ,,but when i turn hot tap on it just bangs and runs cold ,comes up with f03 ,,i have replaced the pump and the diverter valve ,,,,if i press the reset button 3 times it goes into test mode and the hot water runs fine (mega hot) ,,,im strugling what to change next ,,,help please its costing a fortune ,,lol thanks