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I have a 2017 27" iMac with a 4.2 GHz Intel Core i7. It's been having some issues lately where every now and then the entire system will gradually freeze, forcing me to hard reboot the computer. So to see if there's a hardware issue, I started running prime95 in stress test mode to see if any errors were detected.

After running for about 20 minutes, I checked the core temperature readings for my CPU and they were all hovering around 95 C. Ordinarily I'd think that's way too hot but with the design of the iMac I'm not sure. I stopped prime95 and after my system was idle for a little while the core temperatures are now hovering around 60 C.

Is that temperature too hot, or is this normal for an iMac?

edit: After the system was idling all night the temperature went down to 45 C, which is of course a good idle temperature. So no worries there.

edit 2: After doing more research I've discovered this is a widespread issue with the Intel Core i7-7700k, and affects all iMacs that have it. Apparently Intel claims its temperature spiking to 90 C during normal use and hitting 100 C during heavy use is normal. Seems wrong to me, but who am I to argue with Intel?

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    I don't know if it helps you, but I have had a lot of issues with my iMacs (2009, 2011, 2013, 2017) - and ALL were heat related. When you use them to do heavy computation, especially if you use the GPU, too, they run VERY hot - then, GPUs fail (2009, 2011), SSDs fail (2009, 2013), Backlight fails (2009), etc.pp. My solution is to run heavy computation ONLY on a Linux Machine - but if you have the spare cash, you can also buy a Mac Pro. I'd buy an iMac Pro right away if it weren't for the heat issues all the iMacs have - they are just unusable for heavy computation imho. So - normal but deadly.
    – user2707001
    Nov 8, 2017 at 9:19
  • iMacs are essentially running on laptop hardware, permanently plugged in. >I stopped prime95 and after my system was idle for a little while the core temperatures are now hovering around 60 C. 60C idle is really high. I'd expect that to be in the room-temperature +5º range. What temperature is it when it's not been running a stress test for a while?
    – wrossmck
    Nov 8, 2017 at 11:36
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    All Mac are fine at basically 100C for die temperature for prolonged periods. apple.stackexchange.com/questions/54422/…
    – bmike ♦
    Nov 8, 2017 at 15:59
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    Then it's normal behavior. Abnormal behavior is when it shuts down because it got too hot. It's also important to remember that the CPU (in addition to the Mac itself) can shut itself down to prevent damage. Too much over analysis is put on CPU temps in the mistaken presumption that somehow there's no self preservation mechanism built in at multiple layers.
    – Allan
    Nov 8, 2017 at 16:00
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    @Allan You are correct, but of course my worry is that even coming close to the point where the system shuts itself down is both bad on its own (as you might be moments away from a shutdown) and still possibly damaging to the system's lifespan. And usually in my experience 100 C is the shutdown point, and 90 C is "dangerously hot", and 80 C is "too hot" and already a sign your heatsink isn't doing a good enough job. But those are the numbers I used working with older non-mac PCs. I don't know if that applies to 2017 iMacs, hence my question.
    – Bri Bri
    Nov 8, 2017 at 16:09

4 Answers 4

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Reaching a temp of 95°C is not surprising if the CPU is under heavy load for a prolonged period.

Prime95 can be useful for determining stability, but I'm not particularly a fan of it as a troubleshooting/diagnostic tool. The reason it's popular for stability testing is because users will overclock their CPUs and then need a way to test how stable they are afterwards. So, for me, prime95 is the wrong tool for the job you're trying to perform (but that's a personal opinion). However, if you're wanting to use it, then you need to run it for at least 3hrs. If your iMac continues to run it without error, then it's passed the test.

If you're particularly concerned about your hardware, I'd start with using Apple Diagnostics to test your machine.

Run Apple Diagnostics

Follow the steps below to run Apple Diagnostics:

  1. Fully shut down your iMac
  2. Restart your iMac
  3. Immediately press the D key and keep it down until you see the Diagnostics screen appear
  4. Wait for Diagnostics to finish (this typically only takes a few minutes)
  5. Once complete, one of two things will appear on the screen:
    • a No issues found message
    • a brief description of any errors found plus further instructions
  6. If the diagnostics test does find errors, take a note of what they are

Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.

Assuming the iMac passes this without error, I'd be more inclined to test your hardware in Safe Mode to see if you can recreate the issues you're having. If you can't, then you know it's most likely a software issue relating to a login item, corrupted font, or kernel extension.

For more info on Safe Mode, refer to: Use safe mode to isolate issues with your Mac.

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  • Thanks for the tips. I've already tried both the Apple diagnostics (which it passed) and eight passes of memtest86 (which it passed) so I figured maybe prime95 was the next thing to try. Unfortunately with this particular crash, there's no way I know of to trigger it, much less trigger it consistently, and I can't do normal work on my system in safe mode so that's not an option. Honestly I have almost no ideas about how to fix this problem!
    – Bri Bri
    Nov 8, 2017 at 15:46
  • Okay, so what you're telling me is that you've run Apple Diagnostics and the iMac passed. You then ran memtest86 (an excellent tool!) and all your RAM passed! And now you tried prime95, which you manually stopped, but up until that point it hadn't failed. It really seems you don't have a hardware issue. But if you wanted to be sure, take it into Apple - they can run different diagnostics to confirm your hardware. :) If they find nothing, we need to start investigating a software issue.
    – Monomeeth ♦
    Nov 8, 2017 at 21:04
  • Finally, as for this question, I feel you've got your answer about the 95°C. This is reasonable for a CPU under heavy load for a prolonged period.
    – Monomeeth ♦
    Nov 8, 2017 at 21:05
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I also have a 27" 5K iMac, though mine is a Late 2015 model, so the generation before yours. I have the (at the time) top end 4.0GHz i7 6700K processor (it's officially 4.0GHz with "turbo boost" to 4.2GHz, but in practice according to iStatMenus mine never exceeds 4.0GHz).

The 4 cores' DTS temperature readings very rapidly reach their design maximum of 99 degrees C under full load. I mean within a few seconds. Then the CPU frequency automatically throttles back to prevent overheating. Because of this, I don't believe I am getting the full performance that this CPU is capable of (benchmark results are way below the same CPU's typical performance in a PC or Hackintosh). I think the iMac has completely inadequate cooling for these very high performance CPUs to run at full speed.

The following image shows the iStatMenus sensor list when my CPU is fully loaded on all 8 virtual cores. Of particular relevance are the "CPU Core (1/2/3/4) - DTS", which are all varying between 98-100C, the overall "CPU Cores" figure at 99C (I believe this is the one used to decide when to throttle back the overall frequency to keep it under 100C), and the Fan which is running at full speed.

Sensor readouts from iStatMenus at full CPU load

And here is Intel's own Power Gadget software showing how rapidly the temperature rises after the onset of load, followed shortly afterwards by the frequency being reduced to about 3.8GHz and remaining there.

Sensor charts from Intel Power Gadget at full CPU load

In conclusion: Yes, it is normal for an iMac, but it's not ideal. These high-end i7s produce a lot of heat, and the iMac simply can't get rid of it fast enough. IMO it's disingenuous of Apple to sell iMacs with CPUs that they claim can run at 4+ GHz when in fact they can't for more than a few seconds because the cooling is inadequate.

On the matter of why your Mac is slowing down, I highly recommend full-time use of an on-screen sensor program such as iStatMenus. It enables you to keep an eye on what your Mac is doing at all times, and quickly identify the cause of any slowdown - often these can be due to I/O load rather than CPU.

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  • While a good approach, I don't think this actually answers the question. Especially since all that is offered is similar comparisons from 1 Mac to another (different models) and then a third party software that senses what your Mac is doing. Unbeknownst to you, Apple has a built in sensor application tool called "Activity Monitor" that does very similar to what iStatMenus does.
    – Melvin Jefferson
    Apr 11, 2018 at 17:41
  • I'm well aware of Activity Monitor, thanks. It doesn't show temperatures/voltages, nor CPU frequency, nor provide long-term charts. iStatMenus does, and it puts charts and numbers unobtrusively in the menubar, therefore is always visible without taking up screen estate, and one can immediately see at a glance what the computer is up to. The question was "is it normal for a 27" iMac to have these CPU core temperatures?" I provide another data point for my admittedly not identical but close model of iMac to say "yes it is". I answered the question.
    – Ben23
    Apr 12, 2018 at 21:16
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YES

95° under load is reasonable given a few characteristics.

  • 100% CPU load generates a huge amount of heat
  • depending on how long it's at 100%, CPU can easily reach 90°+

However

Taking a temperature reading after a stress test will lead to misleading results. The CPU is still generating heat, so it may take a long time to cool down after a stress test.

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  • You cannot compare all Mac systems like that. Many MBPs and iMacs e.g. are sloppily built and have too much thermal paste globbed on which is also not of the best quality. 60°C when idle is 'normal' for those. Without exact model given this answer is misleading (for 'idle') in many cases. With your 40° you are on the lucky side.
    – LаngLаngС
    Nov 8, 2017 at 13:57
  • @wrossmck You are correct. After my system was idling overnight its temperature went back down to 45 C.
    – Bri Bri
    Nov 8, 2017 at 15:43
  • That said, I'm not sure 95 C is actually reasonable. I've been doing some research of the maximum safe temperature for my CPU, an i7-7700K, and I'm seeing that it's 100 C. It's recommended that it be at least 15 C lower than that when under load, and mine stays around 95 C, with spikes up to 97 or 98 C. That means it's on the verge of shutting the computer down, which suggests to me that the heatsink isn't working as well as it's supposed to.
    – Bri Bri
    Nov 8, 2017 at 19:47
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This app works great. You can set two temp points to keep your processor between the two ranges and avoid system throttling. You'd be surprised how ramping the fans up can improve temps and performance! Apple sides with less noise over performance. https://www.crystalidea.com/macs-fan-control

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  • What's your relationship to/with the developer, if any?
    – Allan
    Feb 19, 2019 at 21:56
  • I have no relationship. Found it in conclusion to my research prompted by processor temps in the 90s for small tasks on my iMac. Now it keeps below 65c with small ramps up and down by fans. I dont mind the noise. As far as I can tell so far it's free? Hasnt said trial or anything in app.
    – Chris
    Feb 21, 2019 at 15:38

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