Chosen Solution
I installed a OWC Mercury Electra 6G SSD in this late 2015 21.5 iMac. Previously it had a 1TB Fusion drive. It boots fine but the CPU is maxed out with kernel_task consuming all of the processor, and the fan is running at max speed. I should also mention that during the installation, the screen fell open and the LCD cable was damaged. I have ordered a new LCD cable, but in the meantime, the built-in display does not work at all. I am running it on an external display. Could this be the cause of the system freaking out?
The display cable carries other signals in this case the thermal sensors. So when you damaged the cable you disconnected the sensors as well. SMC is designed to go into save mode when it looses access to needed sensors. So until you replace the cable this is the correct reaction from the system. Hopefully no further damage is present.
Did this fix resolve the problem. I had someone do a RAM and SSD upgrade, it ran fine afterwards but the screen fell out that night. Since the screen has been replaced, the fan doesn’t stop running at full speed and it takes 15 minutes to open Adobe Premiere etc….
More info (including causes and solutions) about the SSD upgrades on iMacs causing high fan speeds and maxed out CPU can be found on Crucial’s website here:https://uk.crucial.com/support/articles-…
