I have a backup computer that I have been working on lately. After placing the components back together I plugged the power cable into the PSU. It promptly started spinning the fans but no display or posting tones. The hard disks spin as well. I attempt to hold down the power button but it does not shut it off. So is it the psu or the mobo that needs to be replaced?

Are you sure that you have the on/off header pins properly wired?

There is an interaction that occurs between the PSU and the motherboard when it is first turned on. The motherboard shorts the green wire in the 20/24 pin connector to ground to turn on the PSU. If the PSU is turning on without the power button on the front of the case then you may have a motherboard issue.

I agree with DCC about it being wired properly. Check the cable that runs from your cases power switch to your motherboard.

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