Question CPU throttling that won't go away

[stereosapien]

I have a late 2012 iMac that recently started showing CPU throttling. I don't know what caused it, a couple years ago I upgraded the internal fusion drive and disconnected the internal HDD, removed any excess dust and it worked fine since then. I noticed that throttling starts once the temperature on the platform controller hub die reaches 80 C (which is somewhere in the motherboard, and not the HDD sensor). That's all good and expected, except that the processor remains throttled once the fans kick in and the temperature goes down. Resetting the pram or nvram, or even unplugging it from the A/C doesn't do anything.

1. Any suggestions on how to reset or even disable throttling?
2. What could have caused this to begin with?

Thanks

 

[Lutfij]

Just out of curiosity, when you gave the innards a good cleanup, while you were replacing the storage in the iMac, did you replace the thermal paste in the cooling assembly between the IHS of the CPU/heatsources and the contact of said cooling assembly? By now the thermal paste might've ended up crusty and unable to migrate heat from heatsource to the base plate of the cooling assembly.

Another question, did you restore the OS installation when your storage migration was complete or did you clone the prior OS to the new drive?

 

[stereosapien]

I never took the fan/heatsink/CPU assembly apart and the sensors are not reporting high temps for the processor, so I don't think that would be the issue. Tho it's possible that the paste is crusty, I don't know what the life expectancy is there.

I didn't clone the drive, I think I just re-installed everything.