Chosen Solution
About three years ago I added a 1TB Samsung 850EVO SSD behind my optical drive (iMac 21,5” EMC 2428), so I have a 1TB SSD, a 1TB HD and the optical drive still inside. After installation everything worked fine until, after 15 minutes, the fans started turning very fast for no reason (no overheating), thus I added an OWC cable that includes a thermal sensor. This solved my problem back than. However, I now just replaced my old SSD by a 500GB Samsung 860EVO since my previous SSD was getting old and had problems while writing (high amount of TBW). Again I used the OWC cable that includes a heat sensor for the SSD. I started my Mac in Apple Diagnostics mode (Hold letter ‘D’ while starting) an this returned no errors. Ten minutes after a normal startup, the fans start going crazy again (CPU and Optical Drive fans turn at full speed). I checked the temperature of the sensors (Mac Fan Control) but they all seem to be normal (I can see a temperature indication for both the SSD and the HD thanks to the OWC cable).
I restarted my Mac in Diagnostics mode and it returned: 4sns/1/c0000008:TCOH-124. I did a PRAM & SMC reset but the problem is still there after a couple of minutes. I opened my iMac up again and checked all the connectors (disconnect and reconnect). Again everything works fine, no errors found during diagnostics but 15mins later it starts all over again. Could somebody help me with this issue? (I don’t think the sensors are damaged since they all return a normal temperature indication) Update (11/18/2019) Hi Dan, Thank you for your help I added an SSD as shown in the following installation guide (https://www.ifixit.com/Search?query=Inst...). I then added the OCW cable to make sure that the SSD temperature didn’t get too high since it it stuck behind the optical drive. I also noticed that right before the fans go crazy, the ambient temp sensor gives a very high number, right before getting to 24°C (as shown in the picture underneath).
Since Mac fan control still yields normal temperature indications for the CPU temp sensors I think these still work fine?
TCOH-124 is a CPU temperature sensor error I’m a bit lost here on how many drives you now have in your system. This system only has three SATA ports: HD drive, Optical drive & Spare. The optical drive is only able to support SATA II (3.0 Gb/s) drives whereas the HD & Spare are full SATA III (6.0 Gb/s). Only the 3.5” HD drive needs the OWC in-line sensor as Apple used the HD’s internal sensor for SMC thermal management. So if you replaced the original HD drive to something else you’ll need to sensor. The Spare SATA port (used for a second drive placed under the optical drive) does not need any sensor and the optical drives its own sensor. So unless you swapped it out there’s no issues here. But I’m not sure if the drives and their sensors is your issue at all. I’m suspecting you have damaged the small inductors on the logic board!
One or both likely got damaged when you lifted the Ambient temp sensor.
