Periodic CPU usage results in brief temperature spikes, why?

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Level 1

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Periodic CPU usage results in brief temperature spikes, why?

I use TG Pro to monitor the temperature on my 2020 27" iMac (iMac20,1). I have TG Pro set to boost the fans when any sensor gets above 185*F and to alert me if the temp goes above 196*F. Recently the fans have started coming on for brief periods (2-3 seconds), even when the computer is idle. The high temps are at the 8 CPU cores. The fans may come on several times in a 10-15 minute period, then no activity for several hours.

I have been trying figure out what process is causing this periodic CPU use by watching Activity Monitor, but so far I have not caught the process "in the act."

The TG Pro software keeps a log, so I can see precisely when the fans/temperatures are elevated.

Q. What MacOS log would I look at to determine what process was running at a given time?

Or is there a better way to pinpoint the source of this issue, rather than comparing logs?

MacOS Sequoia 15.7.1

iMac 27″, macOS 15.7

Posted on Oct 30, 2025 7:47 AM

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Answer 1

den.thed

Level 10

90,718 points

Oct 30, 2025 8:51 AM in response to Gary Wright4

I do not have or use TG Pro. However, I have used MacFanControl for almost 20 years and occasionally get the same short spikes as you describe.

The most noticeable spike for me, is when I connect an external SSD to update a CarbonCopyClone. The CCC clone update spikes the CPU for about 20-30 seconds until it is complete. During that 20-30 seconds, MacFanControl ramps up fan speed about a 1000rpm, then returns back to the normal default speed.

To catch a spike using Activity Monitor. You have you have Activity Monitor open and set on CPU% during the time of the actually CPU spike.

Another thing that you might want to try. Is set the TG Pro Fan Control on a more stable temp. sensor, like the CPU Diode or the Platform Controller Die instead of on All CPU's.

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