A1312 iMac overheating after PSU replacement, shutting off screen
I have a mid-2011 27" iMac (12,2) that stopped running while in a sleep state overnight. The machine's drive had been replaced with an OWC SSD that included a temp sensor about a year ago and it had been running without issue. I rarely ever heard the fans running.
I found that the power supply unit had input voltage, but all outputs were 0. I installed a new PSU from iFixit and the machine booted normally, but the fan(s) started running (fast) and after 15-20 seconds the screen went black. Soon (4-5 sec) the fans stop and then if I press a button on the keyboard, the screen comes on, the fan(s) start, then after 10-15 sec, the screen goes dark.
I installed TG Pro. The app indicates everything is working, except the Power Supply Proximity temp sensor reads high (106 max) and the CPU fan is at max speed (~2108 rpm). All other readings are "green" and the ODD & HDD fans are at min speed.
I removed, then reinstalled the new PSU, to make sure that all the connectors were properly seated. I had already blown the dust off the fans (there wasn't much, because they were cleaned when the SDD was installed).
Can anyone suggest a next step? I don't know where the Power Supply Proximity sensor is, though it seems to be working (I see the temp rise from 85 to 106 while the screen is lit).