Me and my brother purchased and built a custom budget computer for our little brother for Christmas. It has been running fine for about a week, but last night it would no longer start. When the power button is pressed, the PC's fans (including the CPU fan) start up for about half a second before the PC shuts down again. Unless the power cable is removed, the PC will keep trying to start and die every ~3 seconds.
I have tried removing and reattaching the power cords (both the 24 pin for the motherboard and the 8 pin for the CPU), removing and reinserting the RAM chips and disconnecting the SSD. The PC still won't start, so I've got to assume that one of the components have died, I'm guessing the PSU.
But how do I figure out for sure which component is faulty? As far as I know, the only candidates are the PSU, the mother board, the CPU and the RAM.
Update: I have left every PSU cable connected to the PC, but removed the 24 pin from the motherboard and used it to jump start the PSU. That works! All the fans are spinning and the PSU is showing no sign of the behavior I'm experiencing when the 24 pin is connected to the motherboard. When I reconnect the 24 pin to the motherboard, the behavior I described resumes. Does that rule out the PSU being a faulty component, or could it be that the PSU is suffering from severely reduced efficiency and chokes when I connect it to the motherboard? (It's a 650W power supply connected to nothing but a mini-ATX motherboard with an i5 CPU and 4 fans)
Update: Also, I forgot to mention, the green lamp on the motherboard is shining happily when the power is connected. I don't know if that's relevant to the problem in any way.