Good PSU's Showing Fault

So, I'm trying to figure out what's wrong with my brother's computer. Installed a new motherboard, hooked it all up, and no power. Tested the PSU (while it was hooked up), and it was showing "fault."

Alright, we had another (different brand, same specs) PSU from when we tested to see if it was the PSU causing the original problem, and it tested fine. Hooked that up and the same thing happened, no power (except one lazy spin of the fans). Now, free from being hooked up, the first PSU was showing fine and the new one (which had tested good the day before) was showing fault.

I've never actually built a computer and haven't done much besides the simple stuff like replacing HDs, graphics cards, RAM, etc. so I'm sure it's possible I'm making some rookie mistake. Would this be a hookup problem? Motherboard problem? CPU problem?

Thank you ahead of time for any help.

You say you checked the power supply and it showed fault?
How did you check the power supply and how did indicate a fault?

Knowing exactly the hardware you are using in your brothers computer would be helpful.

Are you sure you hooked up all the necessary harness to the motherboard. Double check and make sure you have the cpu power hooked up.

Did you make sure the motherboard stand offs were in the proper place before installing the motherboard?

I went ahead and unplugged / unscrewed everything to see if there was a screw underneath (there wasn't) and to make sure everything was screwed into the right spots (they were). Re-screwed and plugged everything, brought someone in for another point-of-view and . . . it magically worked. Nothing had changed, so no idea why it suddenly works now.

Now I feel a little silly, but thanks Layback. Good to know there are people willing to help.

Normally on a new build it's something simple like a connector that didn't go all the way in the socket. Some of them are very tight fits.

Glad it's working for you.
Helping and learning is what we do on this forum.

Happy computing for you and your brother.