Question PC doesn't work with GPU installed, works fine without

[Mooey23]

So I built my new build yesterday in preparation for my 3080 which should be arriving soon. My card hasn't arrived yet so I figured I'd use my old GPU (1060) from my last build until it comes but when I connect it the PC won't start and the red light on the motherboard tells me it failed the post test at the GPU.

The GPU works fine in the older system I've even put it back in to make sure it's still working and it is. The new system runs fine without the GPU I have windows installed and it's been functional for hours but as soon as I connect the GPU I have nothing but a red light on the motherboard and a black screen.

The issue isn't with the PSU being too small as it's 850W. The GPU is getting power as the fans start for a second when the PC is turned on.

Does anyone have any advice in how I can fix my issue?

 

[anotherdrew]

@Mooey23

It's just a wild ass guess, but it could be an issue with the PCIe slot. Does your new motherboard have 2 x16? Have you tried the 2nd? If it works in the 2nd then you could have a dud slot (probably inserting a card causes a short or something) and you should RMA the motherboard.