My Mac 3,1 isn't recognizing one CPU - Dual Xeon I have a mac 3,1 with dual Xeon's. It definitely has 2 CPUs because when I bought it I checked system information and is said 2 CPUs and I put better thermal paste on both of them just just to improve thermals which weren't bad but I figured it wouldn't hurt.
When I put everything back together it booted and I think I remember checking to make sure both cpu's where working correctly which, if I did ally check, they both where.
I recently downloaded cinabench and saw that is only recognized 1 cpu but I thought nothing of it and assumed it was just cinabench being weird.
Today I purchased a used hd 5570 and once I had installed the card and booted the computer I saw that in the system info it too only recognized one cpu. (I am fairly certain it had nothing to do with the card because in hindsight it seemed slower than when normal.
If you need any more information just ask, I'm happy to give it to you. I'd really appreciate some help, thanks.

Model Name: Mac Pro
Model Identifier: MacPro3,1
Processor Name: Quad-Core Intel Xeon
Processor Speed: 2.8 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache: 12 MB
Memory: 16 GB
Bus Speed: 1.6 GHz
macOS
This is almost certainly a hardware problem. As you're not sure that your second CPU was recognized after you unplugged and replugged it, it is likely that this is the event that caused the problem.
I would suggest shutting down the Mac, unplug power - and then remove the second CPU. Boot the system up and check that it works, and that System Information says 1 processor.
Then shut the Mac down again, unplug power - and carefully replug the second CPU. Boot up again and check System Information for the Processor Count.
If it still says 1 CPU, your problem is most likely either a defective CPU or a defective socket. The easiest way to check if to try with a different CPU. Perhaps you can get one cheap off eBay or perhaps you have access to one.
If it is a defective socket, then your options are very limited. With reasonable cost the only you can really do is to exchange the whole board.