Q I just started up my iMac 27” and the fan has quickly run up to being really loud. Is there any way to fix this?

A Cooling fans in Macs work in an unusual way, to safeguard effective cooling of your precious hardware. Their default setting is maximum speed, full blast, and it is only OS X telling the hardware controller that things are OK that slows them down to their usual near-silent state.

Thus if there are software or hardware problems, such as a sensor failure or kernel panic, the fans revert to running at full pelt. If there is no obvious cause of this, shut your iMac down, leave it a minute or so, then start up again.

Observe now when the fans start running up: if from the first part of startup, it is more likely a hardware problem, needing the diagnostics available at a Genius Bar, and most probably a sensor failure.

If later, it may be software, so check entries following startup in Console’s view of the logs. It may then also be worth starting up in Safe mode (Shift key held), to prevent non-Apple items from loading. If that stops the fan from blowing hard, you will know where to look.

Updated from the original, which was first published in MacUser volume 28 issue 23, 2012.