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I have a MacBook Air with OS X Lion and the fan started running loudly. I presume it is because the system is overheating and would like to determine the cause.

What tool can I use to see what is driving the system to become overheated?

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up vote 10 down vote accepted

The fan is turning on because the CPU is being taxed. You can look at what processes are using the CPU with Activity Monitor:

screenshot of Activity Monitor

image credit

Activity Monitor is a built-in application. Type "Activity Monitor" into Spotlight and it should be the first result.

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thanks. I just checked my Activity monitor for the same issue. Firefox was using 85% of my Macbook OSX Lion's CPU. After closing it, it took a minute or two but then the fan stopped. – tim peterson Aug 2 '12 at 1:14

Activity Monitor defaulted to show only "My Processes". None of those seemed to be the source of the problem.

However, when I switched the Activity Monitor view to "All Processes" Carbonite was the clear culprit. About 60 seconds after I set Carbonite to "Pause for 24 hours" its CPU utilization dropped to < 1% and my annoyingly loud fan slowed down then stopped.

I'm only using Carbonite, because I had a few months left on my old license, but I'm not sure its worth listening to a noisy fan for hours on end while it trickles my bits to the cloud.

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