Resolved: Flashing back to older 780 Ti BIOS resolved issue
 
I bought some old hardware locally with one GPU being a GTX 780 Ti SC w/ ACX cooling. It was testing it on a test bench which had an Intel 9100F, Gigabyte Z390 Aorus Pro Wifi, 32GB G.Skill Ripjaws V, and Samsung 970 Evo SSD, which it did fine with. After testing I then moved it over to its home system, which is an Intel 10100, MSI B460M Pro-VDH, 16GB G.Skill Ripjaws V, and a WD Blue M.2 SSD, and it would not boot. Plugged in a different GPU and booted fine. Moved the 780 Ti to an older system with an Intel 3570k in it and it boots, but only displays. No gaming. Going into GPU-Z the device ID has now changed and no driver recognizes it because it's not a valid device ID. Looking online it looks like the correct device ID for the 780 Ti is 10DE 100A, but this one is now reading 10DE 102A in GPU-Z. The BIOS version still shows up as 80.80.34.00.80, which seems to be correct for the card. It almost seems like a bit somehow got flipped on the device ID. To be clear, other than testing it with Unigine Superposition, 3D Mark, Fortnite, and Apex Legends I have not done anything to alter the software of this card. Problem is now I'm assuming it need a BIOS flash back to the correct device ID. Can someone please confirm that this is the correct course of action? Can the Device ID be changed back? Or is the card completely bricked? Thanks for any help.

See the Video BIOS Collection over at TechPowerUp to get the VBIOS.rom..Seems you need the 80.80.30.00.80 for a GTX 780 Ti SC w/ ACX (Device ID: 10DE 100A) and that's the one I would roll with.
 
You can put your working GPU in the primary PCIe slot and the 780 in the secondary slot..Then use NVIDIA NVFlash for Windows with the -i=1 command to flash the secondary card (your 780) while leaving the primary card untouched.
 
I put the NVFlash folder on the Desktop and put my VBIOS.rom in it..Then in Command Prompt I type in cd desktop\nvflash_windows_5.165

 
Then nvflash --protectoff

Then I use nvflash -4 -5 -6 GTX760.rom to flash my primary card..I use nvflash -i=1 -4 -5 -6 GTX760.rom to flash it if it's in the secondary slot.

That's just an example on how I do it with my GTX 760 with my VBIOS.rom and NVFlash 5.165 for Windows.

You can rename the VBIOS.rom file to simplify it or change it to name it what you want.

Hope all that gives you a good direction to go in.

I will try that as a last ditch. Otherwise I will probably try to bake it to reflow the solder and hope that it may fix a problem with an ID resistor.

cbayer8
Otherwise I will probably try to bake it to reflow the solder and hope that it may fix a problem with an ID resistor.

Been there and done that successfully more than a handful of times..But flash the VBIOS first, as stated above..Good luck.

UPDATE: I got it back. I was using the most recent BIOS 80.80.34.00.80, which was not working. Switched over to an older BIOS 80.80.30.00.80 and that fixed it. Thank you everyone that pitched in the help. I truly appreciated it. I've been working on PCs for a while, but mostly its hardware level stuff. I have never had to go this far down the rabbit hole for GPUs before.