CPU Fan runs at 4000 rpm after HD replacement

I have previously posted another question about what to replace when i had a faulty HD on my imac. That was answered really so this is a new post to cover the current problem.

I replaced the (failed) HD in my 2011 iMac with a normal HD and an SSD under the optical drive. I used OWC's sensor for the install of the replacement HD.

When i put everything back together, the CPU fan and only the CPU fan was running at 4000rpm. I have checked and rechecked the connections visually and even replaced the CPU fan yesterday but still no luck. I also put back the imac to it's original configuration to then try to add one piece at a time to single out the problem but the fan was going fast then too. That last point i do wonder whether that is the cause or the original HD fail and i just didnt notice at the time wheras when i replaced the HD, i was looking / hearing for any problems.

Anyway, I contacted OWC yesterday to see if they had any ideas and told them all of this. They got me to try using the OWC cable with the original disk - that way, the built in sensor on the original HD connection would be ignored. The CPU fan still went 4000 rpm and the conclusion was that it is a faulty OWC cable.

I am in the process of trying to get a replacement however i'm worried that the problem wont go away and something else is actually wrong (i dont believe the cable AND my original HD had the same problem). I finally got to run the Apple Hardware test today (as i had a clean install 'd' on its own does not work, option-d before power button does) and found the following:

new SSD, HD installed and NO OWC cable (i had taken out the sensor to return to the supplier) produced the single error:

4SNS/1/40000000:TH00-9.000

which i believe is a temp sensor problem for the HD - makes sense as i was missing the OWC cable. Then i plugged in the OWC sensor and ran the test again and got the following error:

4SNS/1/C0000008: TS2p- - 124

which is a temp sensor again but i cant find a definitive answer as to what the 'S' is n TS2p. All sorts of questions - why dont i get that error when the OWC cable is not plugged in.

I attach the output from TG Pro.

One interesting post i found stated the following:

''When U install the SSD under the optical drive(OD), the air from the OD fan that was supposed to swing under the OD to exit on the uppper vent has no where to go and hence swings across and exits mostly through the vent on the other side near the camera whereon lies a Skin sensor, which blows away/damages tricking the iMac to believe that the CPU is hot and the CPU fan starts to run at full speed of 4000rpm and the Apple hardware shows error 4SNS/1/C0000008:TS2P--124.

This skin sensor will then need to be replaced.''