Chosen Solution
Fan runs high on my late 2009 imac all the time (4000 rpm), and there isn’t anything taxing the CPU. Monity shows that my CPU Heatsink and CPU Diode are both at a temp of 0 degrees. The Northbridge Diode and Northbridge Proximity 1 are also at 0 degrees. After running a hardware test I get the following error: 4SNS/1/C0000008:TC0H–124 Searches seem to indicate that means a CPU temperature sensor error. I’d be happy to replace it, but I can’t seem to find one online. Does it go by a different name? Any help is appreciated!
Directly from the service manual:
To be sure I would install a good thermal sensor app like TG Pro you’ll want the full version. Review this IFIXIT guide: iMac Intel 21.5" EMC 2308 Logic Board Replacement. You’ll need to pull the logic board out to inspect the plug and if needed replace the heat sink as the sensor is epoxied to it.
The Copper plate chip is the GPU. Both have a sensor glued to the heat sink you’ll need to replace the heat sink if the sensor is bad.
