Chosen Solution
So I had a mid 2007 iMac with 2.0Ghz core 2 Duo CPU, and it was really slow, so I upgraded the RAM, CPU, Wlan card, and also to an SSD around 70 bucks. It performed really well in MacOS Catalina, and I realized, that I did not plug in the mic’s cable into the motherboard. Went back, took apart the whole machine again, and assembled just as a new. Now it won’t boot. I can’t hear the startup chime, and the lcd is also off. The first 3 diagnostic leds always turn on when the machine starts. I’ve already tried SMC reset, PRAM reset (bluetooth, usb keyboards not working), switching ram modules (it won’t beep if it doesn’t have any), also tried the old HDD, issue still remains. I had even tought about baking the logic board and GPU, but I don’t think any of them are faulty just bc of a reassembly. Do you know how can I fix that?
Technically, you system can only go to OS-X El Capitan (10.11.6) While there is software which will allow to run macOS Catalina (10.15.x) While this is possible its not really something I would run on a system of this age as the systems RAM is quite limited! - 6.0 GB (Actual) 4.0 GB (Apple). The bottomline here is you likely did your system in as it has to work much harder with its limited resources. Your logic board support chips likely died. You also have a second issue in this series as the electrolytic capacitors may have failed due to Capacitor Plague. I have seen both issues, dammed and double dammed!
