See my reply dated December 1, 2012 in this thread:
I have a G4 not booting. When I press the power button, it lights for the duration of the press and then goes off. No fan turns, no red led lights. Nothing, just the panel light in the power switch.
Now I have tried reseating the processor, removed the ram and video card. Nothing still dead.
Also checked some voltages on the power supply and it seems ok since there are some voltages present.
Tried to reset the switch on the motherboard (grey) near the battery. Also removed the battery and waited for 15mins. Nothing.
Can anybody please help me resolve my issue since I have run out of ideas. Any idea whatsoever will help. Any questions please do not hesitate to ask.
Many thanks to all.