PCIe slots not working

[Tom Rovak]

I have a Mac Pro 2012 5,1. When I upgraded to Sierra a couple years ago it seems PCIe slots 3 & 4 stopped working. I later went to High Sierra and still those slots are not working. How can I fix this?

 

[Macschrauber]

I have a Board with the same fault, seems the pci bridge for 3 & 4 is dead.

 

[MIKX]

No guarantees but worth trying.

Canned air + a brush like this ; clean the PCi slots..

Also,

shut down, remove power cord for a few minutes and then all PCie cards including current GPU . .. then . . install the current GPU in a different pci slot.

Now restart using the "Three Consecutive Happy mac NVRAM reset.


IMPORTANT : It is vital to HOLD DOWN the keys illustrated below until one hears the THIRD CHIME.

Worked for me in re-setting my PCIe slots when my GPU(s) were not displaying the correct Link Speed nor Link Width.