As part of my XP-compatible collection I have the GA-990FXA-UD5. The board is rev 1.x flashed with the latest F12 BIOS. Installing a PII 1090T works perfectly fine. Installing any of the FX CPUs I have (8320 and 8350, both tested and confirmed working in another board) leads to the system not posting and no beep codes, everything else being the same (literally just a CPU swap with a CMOS reset). The board basically behaves the same way it does when I turn it on w/o a CPU installed. Despite the FXs not posting, they do heat up.

All other components (PSU, RAM, GPU, SSD) have been tested w/ another board (not to mention they work in that one with the 1090T).

I tried blowing into the socket, maybe that single extra pin on AM3+ is not making contact, no luck (I wouldn't say I was very thorough, what are the chances of that being it...)

Also tried multiple different CMOS batteries as well as booting up w/o one.

I've been scratching my head on that one for a few hours and am out of ideas at this point. Thanks for any suggestions you might have!