Sensor Errors Galore (4MOT now 4SNS/TGHO)

Been a fun few days. This desktop went white screen about 2 weeks ago and I ran diagnostics on it and got a 4MOT error indicating the HDD fan had gone out. Ordered the part from here, and yesterday got to replacing it.

I took the entire unit apart, inserted the new fan after cleaning out dust everywhere, and then put the thing carefully back together. In the process, I noticed the airport card wires had taken the connectors with them. No big deal, this thing is plugged in all the time anyway.

After carefully putting everything back, I fired it up. This time, it was white screen and all the fans going at full bore. Ran diagnostics and got a totally different error this time:

4SNS/1/C0000008: TGOH—124

Research showed me the GPU sensor had gone off. I thought maybe I disconnected the cable by accident and so took everything apart again this morning. Checked everything, all looks good, but same deal once put back together. So I’m stuck. This thing is worth maybe $350, so taking it in is not going to happen. Before I scrap it, I wanted to get some advice.