CPU stuck at 0.8Ghz on iMac late 2013 2.7Ghz because of kernel_task

I know this thread is pretty old but firstly I would like to say what an impressive informative post, I am trying not to be too hard on Apple but you are probably far more skilled at fixing iMacs than most apple techs, they are trained to replace parts rather than pin down a fault hence the 'replace the motherboard solution' Apple would come up.

I run a PC Laptop repair shop and fix many Apple products. I too have an A1418 with exactly the same issue works fine but CPU is maxed out with kernel task. I also have tried everything you mentioned, I could of written this post myself.

After much trouble shooting from what I can figure the problem relates to a fan/hd sensor but I cannot seem to locate where this sensor is. I will keep looking and keep you posted if I find any solution.