Pretty simple I would say, check the gas burner pressure when firing if it goes to 17mbar it means GV is opening, if you have spark at electrodes and gas valve is definitely opening it should almost definitely light, if not try lighting manually with blow lamp just in case there is a problem with spark catching. If your not getting gas (burner pressure) increase when lighting, check pcb voltage to gas valve if no voltage then it's pcb if there is voltage out from pcb check at gas valve, if you have voltage ac at gas valve but no burner pressure it's the GV (it may be just selenoid tho) just a few other pointers I have had it in the past the reset doesn't work properly sometimes I have had to turn electric on and off and press the reset, and also double check the low pressure switch hasn't connected you can either check continuity through it or even link it out. Hope this helps.