Kernel_task 300% and fans on full

Hi, So I recently started reusing my old iMac after a couple years of not using it, I’ve been using it only in target display mode for my 2017 MacBook Pro via a thunderbolt cable and adapter and when I started the iMac up today the fans were on full and Activity monitor showed kernel_task reaching very high CPU levels, highest I’ve seen the kernel_task was at 495% today. As a result of course everything is extremely slow at the moment and it won’t enter target display mode either. I installed iStats menu to look at the temperature sensors, pictured below, I can’t seem to find anything abnormal and the CPU doesn’t seem hot from what I can tell, the fans are still going crazy though. I’ve already rest SMC, booted in safe mode, resetted PRAM but all to no avail. Apple Hardware testing won’t work - I just get an error message saying “Error: 0x800000etc, Cannot Load "EFI/Drivers/TestSupport.efi". So I’m kind of stumped as to what to do next, I’d rather not have to take it in to an Apple Store, especially at the moment.

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You'll need to run the onboard diagnostics to see if you get an error code. let us know what you get!

Restart your system and press the D key

Reference: Mac startup key combinations


@danj Can't seem to get the onboard diagnostics to work, have tried multiple times but all I get is a black screen with:

"Error: 0x8000000000000003, Cannot Load 'EFI/Drivers/TestSupport.efi'

Status: 0x00000003"


Sadly you hit an Apple bug in the newer firmware ;-{

Let's see if the paid version of TG Pro gets a better data point.

Without knowing where to focus its a crapshoot on which sensor is firing off.


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