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
  • How long has this throttling been going on? Spotlight could be indexing your new drive which takes a while and could be causing the issue.
    – Natsfan
    Sep 21, 2019 at 20:02
  • The fan noise and the throttling starts even before the login screen appears (when the progress bar shows about 50%). When I login I see no other processes that consume a lot of CPU (neither spotlightd or similar). Sep 21, 2019 at 20:37
  • Have you tried booting in Safe Mode or performed a hardware check?
    – Natsfan
    Sep 21, 2019 at 20:55
  • To do a hardware diagnostics press and hold the D key while rebooting your Mac. Select your language and your Mac will automatically test its hardware and inform you if anything is wrong. To boot in safe mode, restart your computer and press and hold down the shift key.
    – Natsfan
    Sep 21, 2019 at 21:03
  • @jmh: thank's for the hints! It turned out that a temperature sensor was missing, see below. Sep 22, 2019 at 20:37

1 Answer 1


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!

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