Macs are great, stylish, well-designed and built personal computers. The iMac in particular is a wonderful all-in-one PC that works well out of the box and rarely goes wrong. But that very reliability means that on the rare occasion when your Mac does go wrong it can be unnerving. Especially when it isn't clear what the problem is.
A classic example of this is an overheating iMac. It may not even feel particularly hot, but if your iMac's fans are whirring away you will be concerned. And if a sustained blast of unusually loud fan activity is followed by your iMac shutting itself down you have a problem.
If this is a one-off occurance after a long spell of intense use you shouldn't worry too much, especially if you are using an iMac that is more than a couple of years old. But we do recommend you at least carry out the first of our tips on how to stop your iMac overheating. See also: It's easy to upgrade RAM on the new 27in iMac
Apple announced the new Haswell iMac on 24 September, read about the New Apple iMac price and availability.
Fix overheating iMac the easy way
The first thing you should do is download and install fan control software. We recommend smcFanControl 2.2.2. This is a free software utility that lets you set a minimum speed for your iMac's built-in fans. It works only with Intel Macs, and won't go beneath a minimum speed so you can't overheat your iMac, and it won't stop the fans whirring it up when the iMac gets too hot. It's just a means of increasing the work the fans do all of the time to keep down the general temperature of your iMac's components.
In the event described above in which your iMac has overheated and shut down we recommend setting a minimum fan speeds of 2800rpm for the CPU and 2200rpm for the hard drive. See if that helps - you can always increase the fan's work further.
There are a couple of caveats, however. For one thing increased fan use leads to increased noise. More importantly increasing the work of your existing fans will wear them out more quickly and may mask an underlying hardware fault that is causing the problem.
Fix overheating iMac: what is the problem?
If this was a Windows PC we'd immediately suggest that the problem is likely to be a virus. That is much less likely with an OS X system, of course, but it is a possibility. So install and run a free Mac security program such as Avast Free Antivirus for Mac, scan your iMac and rule out the possibility of an infection. You can uninstall the program once you are happy.
That means your problem is likely a hardware issue. It's possible that a component has failed or is failing and the fans are working harder to try - unsuccessfully - to cover this up.
To find out where the problem is you want to run the Apple Hardware Test. Simply start up from your system installation disk or a secondary hard disk while holding down the "D" key until the AHT starts up. Run it and note down any codes it gives you. A Google Search of those codes should tell you of any hardware problems your iMac has. Failing that a Mac repair professional definitely will. And depending on your DIY chops you may need to have that guy on speed dial to fix the underlying problem.
It's possible, for instance, that you need to replace a component such as a hard drive or GPU. We've heard of cases of both - the latter in particular when you are using your iMac to watch a lot of HD movies.
Fix overheating iMac the crazy way
Okay, this is a little out there but was suggested by a Macworld reader in the US. He recommends attaching an enclosed suction-blower fan hooked up to any common 9vDC/AC adapter taped to the hottest point of the back of your iMac. This will suck out the hot air.
It might just work, although it will ugly up your iMac plenty. Personally I'd be concerned that it may also mask an underlying problem. But if you just need to get things moving it's worth considering.