I have been driving a well kept 95 Olds Cutlass Ciera for about 4 years now. All of a sudden one day the AC stopped working and the engine fan stopped running. I took it to my mechanic and when he plugged in to see the error codes there were more than 20 that showed up. He cleared the codes and instantly they came back when he turned the car back on. He then proceeded to attempt to fix the issue by replacing, thermostats, sensors, and he even replaced the entire computer on the car thinking that was the problem. None of it worked. Everything else on the car runs great, but this is a killer because the engine overheats when it idles and the check engine light we forever be on now. I could find a way to rig the fan or even get a new car, but I love this car and would rather just fix it. Any ideas or resources?
I would check for intermittent or bad grounding from battery to engine and battery to body and frame and the engine to the firewall, etc. and to the ECU. And check out the temperature switch’s for the cooling fans and their motors. The A/C not working is also a checklist of compressor running, cooling fans on radiator running and charge level of the refrigerant. If you know how to check all that go or take it to a competent shop.
Clearly you don’t work on your own car. Just as clearly, you need a better mechanic. Likely a MUCH better mechanic. One well versed in electrical diagnosis.
As @Ralph-66 posted, this is very likely bad grounds, a lot of them. This will take a lot of diagnostic labor to find and repair. If you really want to keep this old car running, be prepared to spend a lot of money on diagnostic time trying to fix this. Likely more than this car is worth.
Get a better mechanic!
Yup; so much going off reveals a lot of deferred preventative maintenance! 
No, deferred maintenance is not the issue. Age and the affects are.
@Dadmobile what region do you live in?
This could be multiple corroded electrical (ground) connections due to time and a salty environment.
yes; that too