I have attended an ACE HIGH combi and the fault I have is that it will go through the start-up sequence, the burner strikes up then after about 2 secs it drops out and the 'Power' LED (green) flashes and needs to be manually reset.
I have gone to the manual and can find no reference to flashing LEDs in the fault finding section.
DHW seems fine so obviously, flame rec, air pres sw, thermistors etc must be OK.