Chosen Solution
I disassembled my parents’ old iMac with the intention of cleaning it and repasting the CPU (ten year old thermal paste is certainly more like thermal dust). After reassembly, however, the CPU fan was running at a constant full speed. After running Apple Hardware Test, the error code returned was 4SNS/1/C0000008 TCOH– 124. Per the popular CNET article, this is the CPU temperature sensor. According to Macs Fan Control, the sensor was still reading accurately (it seems), with idle temperatures around 33 C and loaded temperatures not getting above 40 C. After a quick terminal command (I can’t remember or find the source of where I found this) showed the CPU speed to be 39 out of 100. I would just use Macs Fan Control to keep the noise down except the CPU is throttled so the machine is much slower now. Additionally, I later disassembled the iMac again to swap an SSD in, and I also removed the logic board again to ensure I had plugged in the CPU temperature sensor. I had, and after reassembly again the problem persisted. SMC and PRAM resets were fruitless, so I’m wondering if a replacement logic board is the next step? I’m okay with that since you can readily find a refurbished one for around $100, I just want to be sure that’s the problem. Thanks.
Well, the good news is you don’t need a new logic board! TCOH is a sensor error for the heat sink per the Apple service manual ‘Excessive processor heatsink temperature.’ Review: Check CPU heat sink sensor cable connection to logic board. It is located on back side and requires removal of the logic board. Symptom: If the sensor is disconnected, the CPU fan will run at full speed. Here’s the connection point
Apple used a propritory heat sensor on the hard drive for this machine. You will need to install an OWC Thermal sensor or the CPU fan will run at full speed. Here’s what you need: OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010 Hard Drive Upgrade https://eshop.macsales.com/item/OWC/DIDI…
