After 7 years my MSI x370 Carbon did not post this week with my Vega 56 installed 
Debug was flashing at VGA and the GPU fan was running fast.
Started thinking I’d finally killed the GPU but installed an old GTX 950 and it was the same. I have swapped out the 2600x for my trusty 2200G 
and it is working fine again.
Just wanting to get your thought’s please, I’m thinking it is either a PCIE issue or a PSU issue. Don’t really want to strip my sons computer to try a 2nd PSU just yet without getting your thought’s. Any help would be greatfully appreciated.
Both gpus work with the other cpu? My x370 carbon will flash at vga for ram if things are off. Maybe your Vega is due for a dusting and thermal repaste, but that could be a waste of time. If popping out the cmos battery doesn’t do any good. The good news is the beta bios for these mobos do work for the 5800X3D, and currently 5700X. Worked no issue with a 1700X.
Secure boot issue?
I’m assuming everything went back to default anyways with cpu change, and then it worked again
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Just had a chance to put the Vega 56 in with the 2200g and it works fine, I did get an Adrenalin update when running just the 2200G.
I also took the chance to replace the cmos battery after removing the GPU as I think it had done well for 7 years. Won’t get a chance to put the 2600x back in until tomorrow, but I will let you know how that goes.
Thanks for the feedback .