Computer won't boot

[TheBlackOut]

I am helping a friend build his computer, he has plugged his 24 pin connector in his motherboard and when he does that, the power supply turns on and the CPU heatsink fan spins. When we plugged in the 2x4 power connector the power connector that powers the CPU and try to turn on the power supply, it simply doesn't even turn on anymore.

We've tried 3 different PSUs, swapped out RAM modules, and swapped the motherboard with another one (the same one, we returned the one and got another thinking it was a mobo issue), and now we're stuck. This has happened with both mobos.

Is it the CPU? I am thinking the CPU is the culprit but I think maybe we're overlooking something. Any advice would help.

mobo is a Gigabyte GA-880GA-UD3H

 

[marsbound2024]

What wattage size are the PSUs and what type of system are we talking about spec-wise?

 

[MagnusTheBrewer]

Do a minimal install outside the box. Check your case connectors for the proper orientation to pin one. Look for the embossed triangles and, do not depend on the cable labels.