Computer Shutdown within seconds after pushing power button

by Arnold
(TN, USA)

My daughter said she was working on her computer when the screen started acting up so she powered it off and turned it back on. However, when she pushed on the power button on the front it only stayed on no more than 2 or 3 seconds.

So I took it on myself to check it out. Made sure the power outlet was ok - all the cable connections were ok - but it still would not stay on more than 2-3 seconds.

After opening up the cabinet I checked all loose cable connections inside. Checked the PSU and it had 12-13VDC going to the motherboard. But when I removed the four wire cable on the motherboard from the PSU the CPU fan would stay on and run.

You could see an arch inside the cable connector when I plugged it on the motherboard. Now is there a short in the PSU or the motherboard?? or what??

Remember I am not a whiz at this kind of work. I could so use pro help here.