Dangerous CPU / GPU temperatures ?

[MortyMars]

Hello everyone,

My iMac restarted last night and the reports suggest an excessive CPU temperature.
So this morning I did some tests with a resource-hungry app, namely X-Plane 12.
The temperature rises rapidly and stabilizes at high values.
That's what it gives, should I be worried?:

sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 77.83 C
GPU die temperature: 99.00 C

sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 85.14 C
GPU die temperature: 98.00 C


sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 92.39 C
GPU die temperature: 99.00 C

sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 91.81 C
GPU die temperature: 98.00 C

sudo powermetrics --samplers smc -n 1 | grep -i temp

CPU die temperature: 87.66 C
GPU die temperature: 98.00 C

sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 84.05 C
GPU die temperature: 97.00 C

sudo powermetrics --samplers smc -n 1 | grep -i temp
CPU die temperature: 80.17 C
GPU die temperature: 97.00 C

Thank you for your opinions

 

[Apple_Robert]

1) What are the specs of your iMac?
2) What apps do you have installed on the Mac?
3) Which apps were running in the background last night?
4) What report are you looking over, to see that the iMac restarted due to temperature? Can you post a screenshot of the report?

 

[MortyMars]

Thank you @Apple_Robert for your help 👍

1) Imac 27" 2019 core i9 8 cores Radeon Pro Vega 48, 64 Go RAM, 1To SSD
2) Nothing special: Qt Creator, X-Plane 12, office automation...
3) Nothing at all but maintenance auto macOS ?
4) Attached file

 

[maflynn]

Any temps near 99c is not a good thing, regardless of the specs of the computer. If you didn't have these temps a few months, or even years ago, then, something changed internally with your computer.

You're computer is 6 years old, there's number of possible contributing factors leading to increased temps. The thermal compound has gotten old and less effective. Re-pasting may benefit the computer both the CPU and possibly the GPU. Dust infiltration, dust is an insulator and could be impacting how well your fans are able to cool the internals. Fan malfunctions, maybe the bearings or fans are not running optimally anymore.

 

[Apple_Robert]

Thank you @Apple_Robert for your help 👍

1) Imac 27" 2019 core i9 8 cores Radeon Pro Vega 48, 64 Go RAM, 1To SSD
2) Nothing special: Qt Creator, X-Plane 12, office automation...
3) Nothing at all but maintenance auto macOS ?
4) Attached file

With the apps listed in conjunction with your other posts in a different thread, it looks like you may be having app conflicts that are sending your Mac in a loop.
Do you have an app related to KnowledgeGraph kit ?

What Mike said in his reply is also very apropos and should not be overlooked.

 

[MortyMars]

Any temps near 99c is not a good thing, regardless of the specs of the computer. If you didn't have these temps a few months, or even years ago, then, something changed internally with your computer.

You're computer is 6 years old, there's number of possible contributing factors leading to increased temps. The thermal compound has gotten old and less effective. Re-pasting may benefit the computer both the CPU and possibly the GPU. Dust infiltration, dust is an insulator and could be impacting how well your fans are able to cool the internals. Fan malfunctions, maybe the bearings or fans are not running optimally anymore.

Thank you @maflynn

I was indeed considering an internal cleaning but given the fact that I have to take off the screen, I wanted to be sure of the necessity of the operation.
So I will think about it more precisely

 

[MortyMars]

With the apps listed in conjunction with your other posts in a different thread, it looks like you may be having app conflicts that are sending your Mac in a loop.
Do you have an app related to KnowledgeGraph kit ?

I admit I don't understand, I didn't post information about my installed applications, except today...?

And I don't know what is the 'KnowledgeGraph Kit'??

 

[maflynn]

I was indeed considering an internal cleaning but given the fact that I have to take off the screen, I wanted to be sure of the necessity of the operation.

Check out ifixit tear downs and/or other youtubes to see what it will take to open up your mac - then you can determine if its something that you're comfortable with.

 

[PaulD-UK]

Have a look with a torch through the vent slit in the bottom edge of the iMac.
There is a filter mesh about 2cm in, which can get totally clogged with debris if the iMac fan has run constantly in a dusty environment.

Cleaning the mesh is difficult, and really requires access to the interior.

 

[MortyMars]

Untimely restart again, I launched the following request:
log show --last 2h --predicate 'eventMessage contains "shutdown cause"' --style compact

The return is as follows:
Filtering the log data using "composedMessage CONTAINS "shutdown cause""
Skipping info and debug messages, pass --info and/or --debug to include.
Wall Clock adjustment detected - results might be strange while using --end
Timestamp Ty Process[PID:TID]
2025-09-03 13:58:25.646 Df kernel[0:e9] (AppleSMC) Previous shutdown cause: -108
2025-09-03 13:58:29.343 Df DumpPanic[113:2f1] [com.apple.DumpPanicanicprocessing] found previous shutdown cause -108

 

[MortyMars]

Check out ifixit tear downs and/or other youtubes to see what it will take to open up your mac - then you can determine if its something that you're comfortable with.

I have already had the opportunity to do this to replace the HDD of a 21.5" late 2015 iMac with an SSD.
It went well, but I still had some sweats...

 

[MortyMars]

Have a look with a torch through the vent slit in the bottom edge of the iMac.
There is a filter mesh about 2cm in, which can get totally clogged with debris if the iMac fan has run constantly in a dusty environment.

Cleaning the mesh is difficult, and really requires access to the interior.

I see that everyone agrees that internal cleaning is necessary.
I think I'm really going to have to resolve it...

 

[Happy_John]

First thing I'd do: Crack open the case ( not a lovely job on a slim iMac), repaste the CPU and GPU, clean out fans and check fan is working properly and is not obstructed.

 

[MortyMars]

Third untimely stop.
Strangely, it seems to intervene during a standby because I have not yet been interrupted during a task.
All my messages regarding the malfunction are sent without problems, overheating, or slowdown, from the problematic iMac.

I think this time I managed to isolate the system report corresponding to the problem.
It is attached...

 

[PaulD-UK]

"shutdown cause: -108" seems to be a memory error.

First thing to try is resetting the SMC (and the PRAM).

One way to reduce high CPU temps is to take the RAM door off, as the CPU is directly behind the SODIMMS and this helps with increased airflow to cool that area.

This won't help reduce the GPU temp, as that can only get its cooling airflow from the base slots.

EDIT: IONVMeController.cpp suggests an SSD problem.
Does your iMac have an original Apple blade, or a replacement - OEM or NVMe + adapter?

 

[MortyMars]

First thing I'd do: Crack open the case ( not a lovely job on a slim iMac), repaste the CPU and GPU, clean out fans and check fan is working properly and is not obstructed.

Welcome to the discussion @Happy_John
You confirm what I feared and that everyone recommends: open the iMac...

 

[MortyMars]

"shutdown cause: -108" seems to be a memory error.

First thing to try is resetting the SMC (and the PRAM).

One way to reduce high CPU temps is to take the RAM door off, as the CPU is directly behind the SODIMMS and this helps with increased airflow to cool that area.

This won't help reduce the GPU temp, as that can only get its cooling airflow from the base slots.

Thank you for these good ideas @PaulD-UK
I'll try that right away.

 

[Happy_John]

Welcome to the discussion @Happy_John
You confirm what I feared and that everyone recommends: open the iMac...

Yeah :-( It's not a very complicated or difficult thing to do, but it's very stressful :-( Just be very careful removing the screen.

 

[Hajj.david]

While doing an internal cleaning also replace the thermal paste. They tend to go bad after 5 years or so.

 

[Allen_Wentz]

IMO prior to doing anything else with your Mac you should try it with the existing apps disabled and running a cocktail of different challenging applications. My point being that the first thing to do is verify that one of your existing applications is not causing the problem.

Doing hardware maintenance (cleaning/repasting) may not be necessary if the problem is software.

 

[ignatius345]

Yeah :-( It's not a very complicated or difficult thing to do, but it's very stressful :-( Just be very careful removing the screen.

I had a go of it a few years ago on my old 5K iMac. I used the parts and tool kit from iFixit and it was very good. Along with the other tools and the replacement adhesive strips, there was a special piece of foam that held the screen firmly at just the right angle. IIRC the thing I spent the most time on was getting all the old gunky adhesive off the display before putting on the new adhesive strips.

 

[canadianreader]

Could be that air vents are clogged meanwhile you could use this app to increase cpu fan speed to lower the temp https://crystalidea.com/macs-fan-control/download

 

[Happy_John]

I had a go of it a few years ago on my old 5K iMac. I used the parts and tool kit from iFixit and it was very good. Along with the other tools and the replacement adhesive strips, there was a special piece of foam that held the screen firmly at just the right angle. IIRC the thing I spent the most time on was getting all the old gunky adhesive off the display before putting on the new adhesive strips.

The foam piece, or anything similar, is essential if you are doing it on your own. Otherwise it's a "more than two hands" job to detatch and reattach the screen without damaging, chipping or cracking it.

Manhandling the screen is definitely the least pleasurable part of the process. The rest is fine.

 

[PaulD-UK]

The foam wedge is part number 944-4365, you can google where to find one.

The most important thing is not to insert a sharp metal cutting tool to get the screen off.
Only use the pizza wheel cutter, or a blunt plastic guitar pick.

If you poke anything in too deeply along the top you risk ruining the screen by slicing through the unprotected ribbon cable that runs close to the glass the full width of the top of the screen, to take the signal to the pixel matrix. 😱

 

[MortyMars]

"shutdown cause: -108" seems to be a memory error.

First thing to try is resetting the SMC (and the PRAM).

One way to reduce high CPU temps is to take the RAM door off, as the CPU is directly behind the SODIMMS and this helps with increased airflow to cool that area.

This won't help reduce the GPU temp, as that can only get its cooling airflow from the base slots.

EDIT: IONVMeController.cpp suggests an SSD problem.
Does your iMac have an original Apple blade, or a replacement - OEM or NVMe + adapter?

I did an SMC + PRAM reset but I have not yet been able to judge an improvement made knowing that I am forced to move and that I am no longer in front of the iMac concerned.

Regarding the history of the SSD, I do not have it since it is a computer bought second-hand. So I don't know if it's the original SSD or if it has been replaced