Today I ran into an interesting issue. I booted up my Mac this AM as I had needed to shut it down last night since it was going to be quite warm in the room where I keep the computer with the air conditioner off. After letting the AC cool the room down, I turned it back on as normal. On boot, it was ok, but within about 5 minutes the fans went into ballistic overdrive mode and began to sound like a small airplane was about to take off.
I immediately checked the internal temperature sensors (I have the iStat Pro widget installed, but there are many ways to do this) and saw that everything was within normal ranges for my late 2010 iMac. The fans, however, were going at full tilt still. Quickly checking my system processes and open apps, nothing was driving the CPU or graphics card to extremes, so I was at a total loss as to what could be causing the iMac to think I was sitting in a sauna all of a sudden.
Next up, web searches to find out if this was any kind of known issue, and what do you know, it is. Apple even has a KB article that details excessive fan speed as a symptom of an unusual, but known, situation.
NOTE: You’ll be resetting some pretty critical components of your Mac, so the author takes no responsibility if you don’t read that KB article and/or don’t follow the full instructions on that page.
The System Management Controller (SMC) is a set of hardware monitoring and control operations that are in charge of very low-level components like fans, hard drive speeds, etc. They exist to handle things that OS X may not be able to deal with, such as making sure the system doesn’t overheat when the processor goes off the charts. Something inside the OS itself would be just as frozen, but the SMC can shut down the Mac to save the hardware until you can figure out what’s wrong.
In my case, the SMC got confused and though the system was overheating even when it clearly was not. Other symptoms of the SMC needing to be reset can be odd lights on the keyboard (e.g. the CapsLock light won’t ever shut off) or just general hardware flakiness that seems unrelated to system settings or operations within OS X. Luckily, resetting the SMC doesn’t cause any data or preferences loss, and is pretty painless.
First, go read the KB article. There are a few steps you should really take to rule out OS X or application issues that could just as easily be causing the problem you’re seeing. In my case, I shut down all apps and rebooted to rule out that something might be running the CPU or GPU at full tilt by accident.
After that, gracefully shut down your Mac. If you cannot gracefully shut it down (Apple menu, Shut Down…) then the problem is most likely not SMC related. If you shut down cleanly, leave the Mac alone for about 30 seconds.
Then, for Intel-based Mac desktops:
Unplug the power cable from the desktop. You must then wait at least 15 seconds before you plug it back in. This step basically forces the SMC to reset when you plug the power back in – the desktop recognizes that there was a total power loss at some point, but now it’s back, triggering the event.
After plugging the power back in, wait at least 5 seconds to allow the hardware to figure out that it’s come back from a power loss, then press the power button. The machine should now boot up just like normal.
If all went well, the hardware hiccup you were experiencing should have disappeared. In my case, the fans sped up on boot, but returned to their default behavior within about 3 minutes. Since then, they’ve been operating as expected, only revving up into “airplane mode” if I do something that taxes the video card or CPU.
The KB article also has instructions for how to perform this operation on non-desktop Macs, such as MacBook Pro and Air devices, so it appears to be a universal control for all Intel-based Mac computers. Be sure to follow the instructions exactly, as not waiting for the correct length of time, or not hitting the right keyboard triggers at the right time can cause the SMC reset to be skipped.
Of course, if this doesn’t fix the problem, you may need to call into Apple Tech Support or stop by a Genius Bar to get more help. Gremlins like this do turn up from time to time – based on the descriptions in the KB article – so knowing how to do this can be handy.