iMac CPU Clock dropping to 800mhz

Hi JeanSawada

Is this a new problem or something that has always occurred?

What maOS version are you running on the iMac?

Try the following steps:

Reset SMC and PRAM/NVRAM as described in these two linked documents:

https://support.apple.com/en-us/HT201295

https://support.apple.com/en-gb/HT204063

Finally, start the Mac one time in Safe-Mode to clear system caches and check for disk errors:

https://support.apple.com/en-gb/HT201262

There is no need to interact with the Mac while in Safe-Mode, just allow the Mac to boot in Safe-Mode to your user desktop, this may take a long time on your system, please be patient while disk tests are being performed. After your user desktop is reached allow the Mac to remain idle for ten minutes while all hidden background processes complete and then restart.

Do the steps above make any difference?

On a general note, if this is a problem you have always had with your Mac then it may be entirely normal for a couple of reasons.

Macs of this era were not designed to be gaming machines.

Your i5-6500 CPU is a four-core four-thread design and without hyper-threading on this CPU model core efficiency is only moderate, further, many games are written single-threaded, they are only able to use a single CPU core to execute and consequently when run at high frame rates and with high levels of visual effects the single core of the CPU will quickly reach maximum temperature, even with the fan running at maximum speed, and the CPU must be throttled back quickly to prevent damage to the CPU.

Some CPU models are able to reduce their clock speed in discrete steps (Intel SpeedStep) but this is not available in all the CPU core designs which means when a single CPU core reaches maximum temperature the clock speed of the entire CPU must be cut in a single, large, reduction, and this is what you are reporting, this is to be expected when running highly demanding single-threaded games on a CPU / machine which is not optimised for gaming.

Until the CPU core(s) has/have returned to a safe temperature then the Mac will be unable to handle multiple tasks simultaneously, this is to be expected.

Exporting/rendering a video in FinalCut uses all available cores of the CPU, it is a multi-threaded application, and therefore the processing load is spread over all the available cores and no single-core will reach dangerous temperatures and no reduction in clock speed will occur.

iMove I’m not sure about as I don’t use it, but from your description it sounds as though when exporting/rendering it is running in single-threaded mode and single-core maximum temperature may be quickly reached.

A final possibility, if your iMac is used in a dusty environment, or in a household that includes cats or dogs then over time the air-intake vents may become clogged internally with dust and shed animal hair, when this happens CPU cooling will be impaired and maximum CPU core temperature reached more easily.

If this applies in your environment then cleaning of the internal air-ducting for CPU cooling may be required by a trained Mac technician.

HTH.

Will.

Hi JeanSawada.

A re-paste of the CPU heatsink sounds like a good step to take, we know that the thermal paste does dry out over time and become less efficient at moving heat quickly from the CPU die to the cooling system.

Also, you might like to run the built-in Apple Diagnostics test to see if that finds anything out-of-specification, or failing.

The linked Apple Document below describes how to use the Diagnostics Test:

https://support.apple.com/en-us/HT202731

HTH.

Will.

Newer Macs may slow the processor to reduce heat production. 94C is far too hot so you may be seeing a normal reaction to that high reading. At 60F it is still in the normal range and that would explain the normal speeds there.

As you could destroy an iMac made after 2012 by attempting to open it to follow this advice:

A re-paste of the CPU heatsink sounds like a good step to take,

it behooves you to find other ways to keep the iMac cooler. As already mentioned, checking the air intakes for dust clogs is a very good first step Remove dust/lint by vacuuming, not blowing. Blowing will drive all that goop deeper into the computer.

The air intake is on the bottom edge of the case:

NOTHING must be placed between those vents and the desk. It takes very little under the vents to start the temps rising.

The exhaust vent is on the back below the stand attachment point:

This vent is only a problem if you set the computer high above you and tip the screen fully forward on its hinge. In that position the top of the stand comes too close to the vent and temps will rise. I can duplicate that on demand with my 2017 iMac 5K.

The are software causes of heating. Useless anti-virus and so-called "cleaning" apps are a waste and will run up temps. They have no place on the modern Mac, which takes very good care of its self including automated cleaning and defragging. Chrome is a known resource hog on both Windows and Macs. Close it—don't send it to the background—when gaming.

Some web pages, regardless of browser, can stress the computer if left open in a background tab. I can make the Amazon home page run up the temps on my Macbook Pro by leaving its tab open while I worke other tabs. When I first noticed it, the effect is clearly evident in Activity Monitor, which showed amazon.com using twice as much energy as the other 12 tabs combined.