Fans at full speed after SSD replacement (4sns/1/c0000008:TCOH-124)
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?
Is this a good question?
35 
329 
971 
The Spare SATA port does not use or offer an interface for a thermal sensor. Please disconnect it and only use the HD drives thermal sensor (or if you switched out the drive then use the sensor on it only.
by Dan
I will try to exchange the SSD with the HD to check whether it works and keep you updated. However, since I can see a temperature for both the HD and the SSD, the OWC cable seems to do its job, and since it worked well for 2.5 year with the same configuration I don't understand why it doesn't work now
by Daniel Plai
@danielplai - swishing the drives won't candle things here. All you're doing is going in circles. Trust me! I've messed up the inductors and ended up wasting days trying to figure our what I did that messed up a simple drive upgrade.
Before you do anything take a nice tight picture of the area I posted and post that image so we can see it. Please remove the OWC sensor on the Spare SATA port drive (under the optical drive per the gouge you posted). It is not needed and does not have even a place to hook up. If you disconnected the optical drive sensor connection thinking the OWC would work it won't!
by Dan