Fix fan noise and CPU throttling after imac SSD upgrade

I've replaced the original 3,5" 3TB HDD in my imac (27", late-2013, 3,4 GHz Intel Core i5, A1419, iMac14,2) with a new 2,5" Samsung 860 QVO 4TB SSD-drive. The system was equipped initially with a fusion drive and the old SSD (PCIe) is still installed.

First, I connected the new SSD via USB and copied over everything. Then I replaced the drive and checked that everything was working. After that, I upgraded to OSX Mojave (from High Sierra).

Now everything seems to be working fine, except that the system fan is running at full speed and the CPU is throttled (a process called "kernel_task" is eating up the available performance). After all, the system is much slower than before the upgrade (so I'm considering of going back to my old HDD/fusion drive).

I've already tried a lot of things: - installing OWC temperature sensor -> doesn't change anything - SMC reset - PRAM reset - installing different tools to take over fan control -> fixes the fan noise, but CPU throttling remains - checking the filesystem with fsck when booting in single mode

Note, when I installed the different fan control tools, all reported temperatures of the system, even from all hard drives/disks, are perfectly OK. I guess it's just some compatibility issue (i.e., the SMC is complaining about the non-certified SSD).

Is there a way to get more information about why the SMC is turning the fan on and throttling the CPU ?

What else can I try to fix this?

Thanks for your help!

[UPDATE:] Running Mac Diagnostics at startup showed error PFM006, which is, according to https://support.apple.com/de-de/HT203747, some error in the SMC.

After doing some further research on the CPU throttling, I found a command line command to display the limit (see below). Note, both limits are below 100 which means that the CPU is throttled:

 $ pmset -g thermlog
 Note: No thermal warning level has been recorded
 Note: No performance warning level has been recorded
 2019-09-22 12:15:14 +0200 CPU Power notify
    CPU_Scheduler_Limit     = 40
    CPU_Available_CPUs  = 4
    CPU_Speed_Limit     = 21

I did some further testing today. It turned out, that the display temperature sensor was missing. After I installed it correctly, the problems (fan noise and cpu throttling) were gone!