Fans behaving strangely (2020)

[Nermal]

Hi everyone,

I have a 2020 iMac (i9, 5700 XT) running 15.5. I've never had any fan issues in the past.

Last night I stepped away from the computer for half an hour, and when I came back I found the fans running at full speed for no apparent reason. I've never heard them run as fast as they were. I shut the computer down, turned it off at the wall, and gave the vents a blast of compressed air. A little dust came out, but not much. I turned the computer back on and the fans began quietly, but about halfway through bootup they returned to full speed.

I ran "sudo powermetrics --samplers smc" to see whether it reported anything strange, and the fan speed immediately returned to normal. I guessed there was some sort of glitch with a sensor and that explicitly querying it caused it to get reset. I then ran diagnostics as a precaution, but it didn't find any issues. Everything was fine for the rest of the evening, including a couple of hours of gaming (fans spun back up, of course, but to their usual level).

This morning, just before I was about to leave for work, the fans suddenly spun up again. I wasn't doing anything strenuous on the machine. This time the powermetrics command didn't fix the issue. I had to get to work so I didn't have a chance to scrutinise the values properly but the temperatures looked fine (less than 40 degrees for both CPU and GPU).

Does anyone know what might be going on here?

 

[Nermal]

Does anyone know whether the fan sensor is part of the main fan assembly? I'm wondering whether I should buy a replacement one sooner rather than later, even if I don't end up needing it, given that it'd probably take a little while to arrive.

 

[Nermal]

I'm back home, and the computer is currently behaving normally. I've run Macs Fan Control and taken a look, and everything seems OK to my layman's eyes. It'll be interesting to compare if/when the issue recurs.

 

[Fishrrman]

Could it have been a "runaway process" of some sort?
If it happens again, I'd open Activity Monitor and check the CPU usage, to see if anything is "over-working" ...

 

[CouponPages]

I've got the same setup, 2020 i9, 5700 XT and I definitely noticed an increase in fan noise over the last week, even when I wasn't actively running anything.

My best guess is there are background maintenance tasks set to trigger when the system isn't busy so they don't slow down day to day activities. For what it's worth, I think they should throw a notification so you know what's going on.

 

[Nermal]

Could it have been a "runaway process" of some sort?
If it happens again, I'd open Activity Monitor and check the CPU usage, to see if anything is "over-working" ...

I'd be amazed if it was. I've had the machine for almost five years and I've never heard the fan go that loud before. It doesn't ramp up, but just goes from the slowest speed right up to the fastest in about a second. It just happened again, but only for a few seconds: I couldn't even get Macs Fan Control open before it returned to normal.

My best guess is there are background maintenance tasks set to trigger when the system isn't busy so they don't slow down day to day activities. For what it's worth, I think they should throw a notification so you know what's going on.

I've had background processing in the past, and it's nowhere near as loud as this was.

 

[Nermal]

Here we go, it just had another blast and I was able to get MFC open.



So that's higher than the "max" and I don't know why. It had returned to normal by the time I got Activity Monitor open.

The first time this happened I immediately shut down the machine, so I'm not sure whether it'll ever get "stuck" on this high RPM or whether it will always recover after a short amount of time.

 

[Nermal]

Ugh, I reset SMC and NVRAM the other day and the problem seemed to go away, but it's back again. Fan is currently at ~3400 despite the CPU and GPU only being around 35°. It's a very cold day today. I see "mds" is using around 45% CPU so I'm not sure whether that's related or not.

While I've been typing this the fan has crept up to 3700.

Edit: Same issue under a different user account (with my main one logged out, not still running in the background).

 

[Nermal]

Managed to get into diagnostics while it was happening. It returned fan error PPF004. I'm guessing I do indeed need a new one.

I don't actually understand the issue: if the fan's running too fast then wouldn't it be that something is telling it to run too fast? Would replacing the fan actually fix that? But the repair guide does suggest fan replacement under these circumstances so I'll give it a shot anyway.