RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Update 3
Called Apple and they had me boot into Safe Mode and then back into regular boot. Since then (October 9th) it hasn't happened that I've noticed. We'll see how it goes.

Update 2
No process that is driving the CPU or GPU high. The temperature is consistently average, no spikes. The only weirdness is the 0 RPMs while it happens, so likely a fan motor sensor issue? Finally got around to resetting SMC just now. Going to see how it behaves over the next few days. If it still happens I guess it's time to call Apple. This is a BTO machine and I'm hoping I don't need to send it somewhere.

Update 1
Happened again. Macs fan control listed the fan at 0RPMs as it was happening. The RPMs then came back as soon as the fan went back to normal.

Original Post
I've had this new iMac for about a month now. More recently I've noticed that about once a day (not always every day, at least that I've noticed) the fan would suddenly go max speed, stay that way for a few minutes, and then suddenly go back to normal. There's no gradual speed-up/down. It's very abrupt. When I feel for the heat, the air coming out doesn't seem much hotter than normal.

I'm not doing anything out of the ordinary and Activity Monitor for CPU and GPU don't show heavy usage. Though, I always open Activity Monitor about 20 seconds after I notice the fan going. I'm going to leave AM open for the time being.

Regular usage consists of having Photoshop, Nova (a code editor), Chrome, Safari, Plex, Reason, iTunes, Slack, Google Hangouts Chat, Skype, and Outlook open. I noticed it was indexing Spotlight once, but that that was only one time. When I say "regular usage" I mean I never notice the fans spinning up at all during usage.

Plex doesn't have anything scheduled during the day. iTunes isn't always playing. Chrome does have several tabs open, but no videos or other sites that require heavy lifting. Same with Safari.

I have the 3.8 GHz i7 model with the 5700 GPU 8 GB GPU.

Anyone else having the random high speed fan problem?

If there's intensive CPU/GPU usage, how long does that usage have to be going on for before the fans kick in? Could it be that there's a big spike in activity for 10-20 seconds, and then stops, but the fans keep going for a few minutes anyway?
 
Last edited:

wilberforce

macrumors 6502a
Aug 15, 2020
681
523
Install Macs Fan Control (free), so you can monitor temperatures of CPU, GPU, etc, and what is causing the fan to spin up.

I don't have any unexpected high speed fan. (Only when doing intensive CPU/GPU for longer than about 20 seconds). It also tends to gradually ramp up and down, not abrupt.

Also suggest reset SMC, as this is a common fix for fan misbehavior. It fixed fan issues on my MBP.

 
Last edited:

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Install Macs Fan Control (free), so you can monitor temperatures of CPU, GPU, etc, and what is causing the fan to spin up.

I don't have any unexpected high speed fan. (Only when doing intensive CPU/GPU for longer than about 20 seconds). It also tends to gradually ramp up and down, not abrupt.

Also suggest reset SMC, as this is a common fix for fan misbehavior. It fixed fan issues on my MBP.


Thanks for the suggestion! I've installed Macs Fan Control so I'll keep an eye on that and compare it to Activity Monitor readouts to see what's happening.

I'll reset the SMC later tonight too.

Yeah normally when I'm doing something intensive, like a game or rendering a video or 3D model, the fan gradually speeds up to maximum, and then gradually goes back down. So this abrupt behaviour is weird.
 
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mj_

macrumors 65816
May 18, 2017
1,396
1,010
Austin, TX
Make sure to set Activity Monitor to show all processes, not just yours. It might be a rogue background process running under a different user account (View - All Processes).
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Make sure to set Activity Monitor to show all processes, not just yours. It might be a rogue background process running under a different user account (View - All Processes).

I do have it set to All Processes. Good reminder though. Maybe it's one of those system-owned users.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Okay so it just did it again. This time it wasn't as long. Maybe about a minute? I was in the other room when I heard it go up. Same abrupt jump to max speed.

Nothing out of the ordinary in Activity Monitor that I caught, either for CPU or GPU. Again, I wasn't looking at it right at the moment it happened. But the history graphs seem normal.

I did notice that in the Macs Fan Control in the menu bar, the RPMs was 0 as it was happening. The RPMs returned as soon as the fan went back down to normal speed. It went from 3000 RPMs down to the normal 1200, abruptly again. Could it be some kind of sensor glitch? And is that a hardware problem or potentially software?
 

DeltaMac

macrumors G4
Jul 30, 2003
11,192
3,168
Delaware
RPM reported at zero, but the fan still kicks to high speed.
If the temp sensor dies, I would expect the fan would fail-safe to full speed.
Might be worth watching the TEMPS, to see if the reported temps also change or reset at the same time that the fan kicks up.
Try running your built-in diagnostics to see if any trouble codes are reported.

My guess would be: bad fan (the RPM report would come from the fan motor)
Might be fun to open up the case, and watch the fan to see if it actually stops spinning.
But, kinda messy to do that on a fairly new unit.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
RPM reported at zero, but the fan still kicks to high speed.
If the temp sensor dies, I would expect the fan would fail-safe to full speed.
Might be worth watching the TEMPS, to see if the reported temps also change or reset at the same time that the fan kicks up.
Try running your built-in diagnostics to see if any trouble codes are reported.

My guess would be: bad fan (the RPM report would come from the fan motor)
Might be fun to open up the case, and watch the fan to see if it actually stops spinning.
But, kinda messy to do that on a fairly new unit.

Is there somewhere I can see a temp history graph?

I suppose if the fan loses it's self-control then I can always manually control it with Macs Fan Control? But that's not ideal.

If it is a hardware issue, I'm hoping I can take this in for a same-day repair because I can't be without the machine for work.

I'll see if I can run the diagnostics tool first and then I'm going to get in touch with Apple Support this week to see what they say. I do have AppleCare too.
 

wilberforce

macrumors 6502a
Aug 15, 2020
681
523
Okay so it just did it again. This time it wasn't as long. Maybe about a minute? I was in the other room when I heard it go up. Same abrupt jump to max speed.

Nothing out of the ordinary in Activity Monitor that I caught, either for CPU or GPU. Again, I wasn't looking at it right at the moment it happened. But the history graphs seem normal.

I did notice that in the Macs Fan Control in the menu bar, the RPMs was 0 as it was happening. The RPMs returned as soon as the fan went back down to normal speed. It went from 3000 RPMs down to the normal 1200, abruptly again. Could it be some kind of sensor glitch? And is that a hardware problem or potentially software?
I have never seen Macs Fan Control report zero rpm. Maybe a sensor issue: computer sees low (zero) rpm, so immediately tries to increase it to 1200 (and beyond), to no avail.
 
Last edited:

wilberforce

macrumors 6502a
Aug 15, 2020
681
523
Is there somewhere I can see a temp history graph?
Intel power gadget will show temp history of the CPU

 

DeltaMac

macrumors G4
Jul 30, 2003
11,192
3,168
Delaware
Is CPU temp history going to help the OP? CPU temps should correlate with fan speed - but if the fan speed is incorrectly reported (RPM sensor intermittent, for example), then CPU temp may not be relevant, just coincidental. Note that there is only the one fan in the 2020 iMac.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Is CPU temp history going to help the OP? CPU temps should correlate with fan speed - but if the fan speed is incorrectly reported (RPM sensor intermittent, for example), then CPU temp may not be relevant, just coincidental. Note that there is only the one fan in the 2020 iMac.

I think it might be useful just to have that information so I can correlate it with activity monitor graphs and see if there was increased activity when it happened. It's more for when I'm away from the machine when it starts to happen.

Trying to pin down if it's some process that's causing the CPU to run high, or if everything is otherwise normal. In which case, I guess it would be a fan sensor issue? The zero RPMs is super weird.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
I have never seen Macs Fan Control report zero rpm. Maybe a sensor issue: computer sees low (zero) rpm, so immediately tries to increase it to 1200 (and beyond), to no avail.
Intel power gadget will show temp history of the CPU



Thanks, I've installed the tool. I'll leave it open and see if it catches anything.

Your comment about the zero RPMs is interesting. Makes sense as a cause. If that's the case, hopefully it's a quick same-day repair.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Just an update.

Been paying close attention and there's no consistent thing about when it happens. No process that is driving the CPU or GPU high. The temperature is consistently average, no spikes. The only weirdness is the 0 RPMs while it happens, so likely a fan motor sensor issue?

Finally got around to resetting SMC just now. Going to see how it behaves over the next few days. If it still happens I guess it's time to call Apple. This is a BTO machine and I'm hoping I don't need to send it somewhere.
 

RoboCop001

macrumors 65816
Original poster
Oct 4, 2005
1,457
289
Toronto, Canada
Called Apple. They couldn't guess as to the problem but had me start up the iMac in Safe Mode and then back into normal mode. Apparently that does some things during startup. That was on Friday (the 9th). So far, it hasn't happened again, at least not while I'm home/awake. But we'll see! lol. At least they said that fan replacement is a same-day service if needed.
 
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