Question Is my PSU or Motherboard Bad? (GPU PCIe Slots)

Kikiyai

Reputable
Jul 2, 2016
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I have an ASUS Prime B250-PLUS LGA1151, and sometimes it does not POST.
PSU: Corsair RM1000x - 1000W
When I put a GPU (RX580) in the first/top PCIe slot, it most likely will post, but rarely it does not POST.
And when I try to put the GPU in the lower slot, it does not POST at all.
And two GPUs installed does not POST at all.
Also, both my case fans have a 3 pin header and molex connector and when I plug in both the molex and 3 pin connector, it does not post.
When it doesn't POST, the computer makes a clicking sound, like an engine trying to turn over, and the GPU lights flicker in sync to the clicking sound.

I am confused if the motherboard is bad or the PSU?
I can answer any clarifying questions you might have for me.