Fan issue on iMac Retina 5K, 27-inch, 2020

[SpringKid]

First, story time:

Yesterday all of a sudden my iMac started becoming extremely slow, as if some process was going nuts. I checked Activity Monitor just to find that it was the kernel_task process (pid 0) that was seemingly consuming all CPU. I tried this and that, killing different running things, and all of a sudden everything is back to normal. I reboot the computer, still fine. However, after a few minutes, I have the same issue again.

After a lot of random troubleshooting, I realize that my computer is completely silent — and the fan is showing 0 RMP! And the CPU/GPU temps are getting close to 100 C. Ouch.

So, the fan is dead. I started watching a "how to get into your iMac" video, and got some things to pry the screen off. But first I come to my senses and remember "oh, maybe I should do a PRAM reset". I do that (while having the computer laying down), and all of a sudden the fan works again!

I move the computer back to my desk, plug everything back in, boot it up and... Same issue, and 0 RPM.

I shut it down, unplug my network cable, two Thunderbolt devices, one USB cable and two USB keys. I boot it up and the fan is working!

So now I reconnect everything again, one device at a time and reboot for each device. The fan is spinning. So now everything is connected again, the fan works and everything is back to normal. I can hear the fan, two different fan apps shows the RPM, so yeah, working fine for real.

But if I run "sudo powermetrics --samplers smc" in the terminal, for some reason "Fan speed" doesn't show up at all as if that sensor isn't available, only thermal levels and temps are showing. Fanny and Macs Fan Control still show the fan speed, so they can apparently read the needed sensor.

So, tl; dr:

Anyone with this iMac, does "sudo powermetrics --samplers smc" show your fan or not, or do I still have some weird fan issue?

 

[Sippincider]

2019 27" iMac19,1.

Both TG Pro and "sudo powermetrics --samplers smc" are showing fan speed here.

Discussions.apple.com suggests resetting the SMC 2-3 times (???) to correct fan issues, and to have "any wired external peripherals" unplugged when doing this.

 

[SpringKid]

Thanks for checking.

In total, I have reset it three times now, although I didn’t do the last two with nothing connected. I will try that anyway, just to make sure.

 

[SpringKid]

Nope, still no fan showing up in powermetrics. Oh well, everything else says there’s a fan and it’s spinning as it should, so I guess this is fine. And I have no idea if the fan showed up there earlier either.

 

[TwoH]

Got Macs Fan Control? Can you at least bump the fans up to max rpm on that to see if you can hear the fans spinning when at max rpm

 

[Nermal]

Anyone with this iMac, does "sudo powermetrics --samplers smc" show your fan or not, or do I still have some weird fan issue?

It does not show fan speed on my 2020.

 

[SpringKid]

It does not show fan speed on my 2020.

Oh! So maybe that is how it is on this model after all.

 

[gilby101]

Anyone with this iMac, does "sudo powermetrics --samplers smc" show your fan or not, or do I still have some weird fan issue?

2019 27" iMac19,1.

Both TG Pro and "sudo powermetrics --samplers smc" are showing fan speed here.

2019 iMac: Yes, powermetrics shows fan speed in 2nd and subsequent bursts of output.

But 2020 is different in that the 2020 has a T2 chip, but the 2019 does not - this changes the whole SMC configuration. So I doubt that a 2019 result is definitive.

What does TGPro https://www.tunabellysoftware.com/tgpro/ (the most reliable gui app) say about your fan?

 

[specialstyle]

2019 iMac: Yes, powermetrics shows fan speed in 2nd and subsequent bursts of output.

But 2020 is different in that the 2020 has a T2 chip, but the 2019 does not - this changes the whole SMC configuration. So I doubt that a 2019 result is definitive.

What does TGPro https://www.tunabellysoftware.com/tgpro/ (the most reliable gui app) say about your fan?

That's really interesting regarding the T2 chip -- I guess that does change SMC stuff n things

 

[Titus]

I bought my iMac in July 2020 and I have a fan issue. It runs very loud, then I reboot and it's normal. I can hear the regular fan. When it is running loud, the computer back heats up. The only thing I can do is reboot it. I wonder if I should take it in?

 

[specialstyle]

They're not gonna do much for you. One test you can do is boot the machine and leave it awake, no apps open and see if the fan spins up loud. It shouldn't. But a lot of times mine will do the same thing depending on the websites I'm browsing -- some use a ton of energy and will do that. I've found that there are some people that leave 24,000 tabs open on their browser and it turns out they're basically melting the machine ;-)

 

[Titus]

I only open one tab at a time. Sort of an OCD thing.

 

[specialstyle]

Ah good lol. You can open Activity Monitor and jump to the energy tab to see what's using the most juice. It could be because of an app, or if the OS is doing something in the background like Spotlight indexing... which can take considerable time and resources. That might be a good place to start. But if the fan is spinning up, that's a good thing because it's doing what it needs to do. The other option is to wipe it clean and start fresh, but that is generally something you don't need to do on Mac.

 

[gilby101]

I bought my iMac in July 2020 and I have a fan issue.....

As well as following the suggestions of @specialstyle, get TGPro https://www.tunabellysoftware.com/tgpro/ (free trial) as that will allow you to watch the CPU and other temperatures alongside the fan speed.

When lightly loaded, the fan should be running at ~1200 rpm and only increase when the temperatures go up. And Activity Monitor can help you discover what is causing increased fan speed (assuming it is software issue).

It is possible you have a hardware issue with perhaps a build up of dust and fluff as the likely cause.