iMac 27" (late 2012) is switch off but fan run at full speed

Hi, I've never heard of this either... is it really OFF/Shutdown, or maybe didn't complete Shut Down completely?

Does it do it if you manually shut it off with the Power Button?

I think you'll need to take it in, for sure sounds like a serious Hardware issue.

Pull the AC cord out of the wall outlet. That way you know it is shut down. Give it a few seconds and then plug it back in. If you hear the fans spin up then you have a hardware problem that only Apple can fix.

Good Glad to hear that. Yes there is something terribly wrong with that one.

Good Luck and Best Wishes.

Today I experienced the same problem with my iMac 21,5". Whenever I shut it down, the fans start running on full speed. I have already made an SMC and PRAM reset, no results. I also tried to change the wall plug, but still the same issue...

Is calling Apple Care the only solution? :/

Wow, sorry to hear, but yes, that's about the only option. 😟

I know this is an older post (couple of months ago) but did anyone come up with an answer or explanation for this?

I had one of the new 21.5" iMacs and its was doing the same thing. Apple tried several things but no go, back to the Apple Store, they looked, couldn't work it out and swapped the machine. Two days after getting the new mac, which shut down perfect with no noise, I put it into sleep mode. Came back a few hours later and it wouldn't wake up, had to hold the power button in to get it back. All came back fine, but as soon as it was shut down the fans take off again.

Will be ringing Apple tomorrow, but just wondered how common this was or if anyone had come up with a reason or (better yet!) a solution other than another swap? I've reset the SMC and NVRam, still no good.

Anyone any thoughts?

Thanks for that. Interesting comments. Neither of my iMacs had teamviewer on, but both have had a USB hub, however, I didn't have the hub when I first got the first iMac, went it and bought that separately, so although it appears there is a link there, I'm not sure there is?

my first machine exhibited the fun going on full blast upon connecting to power, off when the machine was booted, and on again when the machine was off.

I am now on my third replacement iMac (27" late 2012), the last two are exhibiting identical issue with sleep: the machines will go to sleep and will not wake up, and in addition the power button is disabled, i.e. pressing the power button will not restart the machine.

I am guessing that the SMC crashed, as the only course of recovery is to reset the SMC by removing power, pressing the power button for a while, and attempting to restart (does not work all the time, so I sometimes have to repeat the SMC reset procedure).

after doing some research on the net, I found how to view the hibernate mode, and saw that my machine was set to hibernate mode 5, which is NOT an approved setting, see below.

I used the system pref -> energy saver and clicked restore defaults which restored the hibernatemode to 0.

HOWVER, upon plugging in a powered USB hub, the hibernate mode was automagically reset to 5.

once at hibernate mode 5, putting the machine to sleep manually exhibited the same symptoms, i.e. not wake from sleep and disabled power button.

this was done with an apple care tech on the phone, and he recommended replacing the iMac once again, and sending me a fourth replacement!

overnight I played some more with this

I reset the hibernate mode but this time to 3 and I tried a different USB hub.

hibernate mode stayed at 3 after plugging in the USB hub.

I then created a small shell script to monitor and display the hibernate mode:

function showhibernate() {

while true

do echo -n `date` " ";pmset -g|grep hibernatemode;sleep 10

done

}

and executed it overnight

this evening after I came home from work, I noticed that the hibernate mode somehow was switched from 3 to 7!?

unfortunately, I did not have the echo `date` on the first version, so I do not know when it was switched from 3 to 7.

however, the machine did wake up from sleep... (so it seems that 7 is not as bad as 5?)

I am guessing that some powered USB hubs cause unexpected changes to the hibernatemode, which in turn cause the SMC to crash, disabling the power button.

why plugging in some powered USB hubs but not others will cause the hibernatemode to switch to 5?

why hibernate mode would automagically switch from 3 to 7?

neither 5 nor 7 are documented in the man page, and the only recommended modes are 0 3 and 25:

We do not recommend modifying hibernation settings. Any changes you make are not supported. If you choose to do so anyway, we recommend using one of these three settings. For your sake and mine, please don't use anything other 0, 3, or 25.

For now I cancled the shipment of the fourth replacement, as this seems to be an issue affecting all iMacs (and a specific USB powered hub), so that a new machine will not resolve the issue...

bottom line, remove the USB hub, and/or try another one.

hope this helps.

I really appreciate finding this strand. I had a similar 'fan runs when I power down the iMac' issue when I purchased a new 27' iMac in January. Took it to the Apple Store. No one had heard of my issue, and no one could figure it out. They ordered me a new one.

The new one seemed to be doing fine, but now after a couple of weeks it's doing the same thing. Fan comes on every time I shut it down.

Like some who have posted here, I did have a USB hub plugged in, and I wonder if that screwed me.

I tried reseeting the hibernate mode as suggested by aviram. But this has not helped me.

Has anyone else found any new info about how to fix this issue? Seems like I shouldn't have to start all over again with another iMac. There must be a fix out there, right?

Thanks for any help anyone may have ...