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I'm on Mountain Lion and for two days my fan RPM was 6000 and the temperature was 92C. I decided to restart and now it's 3300 RPM at 68C.

How come? Is this some kind of bug? I remember having problems with high RPM a month or so ago but it was also very hot outside... the problem somehow went away but now it reappeared... since it's also hot outside now I thought that's how it is - but no - there is something wrong and it looks like it's not the dust or hardware problem.. something in the OS is strange.. any ideas?

UPDATE: it's 5500 RPM again - I hate it! :( It was great during the entire day - around 3500 and now for no apparent reason it's 4500-5500... Again no activity on the CPU.. the temperature shows 65C so no idea why the RPM has to be that high.

UPDATE2: back to 3500 RPM all by itself - no restart... Are these fluctuations normal? That's just how it is? I know that being at 6000 RPM constantly was not normal, but what about these spikes to 5000 and back for no apparent reason?

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Sounds like the symptoms of a runaway process using a lot of CPU. Sometimes there are also valid reasons. If it reoccurs your best course of action is to open the Activity Monitor, and look for something using a lot of CPU time and try to determine if there is a valid reason for this (an example might be you have added lots of little files that are being indexed by mds (the indexer daemon for Spotlight)

  • This would have been my answer, yesterday Dropbox went crazy and thrashed the fans of my Mother's MacBook for hours; she didn't know it was Dropbox, until I got her to open Activity Monitor and look for something using the CPU up. Of course, we'll never know what it was, but if it happens again, you might be able to draw a conclusion. – stuffe Aug 5 '13 at 15:01
  • I wrote in the title that there was "no activity" :) so no really active processes in Activity Monitor – davidhq Aug 5 '13 at 15:44
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You should try to reset your SMC. The SMC is a micro-controller on the logic board that controls the power functions for your computer.

Follow this guide to do that. No data on your HDD/SSD will be lost.

  • One cause for rampant fans can be a corrupt SMC, so if next time it happens you are unable to locate a process using up all your CPU and the fans are still going nuts, then try this, but personally I wouldn;t feel the need to do it if and until it happens again. – stuffe Aug 5 '13 at 15:03
  • I tried SMC reset the last time and it didn't help... this time I just restarted and the problem went away - this also suggests that the problem was not in the SMC but in the OS – davidhq Aug 5 '13 at 15:45
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It was dust in the fans after all! :-)

I just didn't want to believe it can cause such erratic behaviour... I thought dust means constantly higher RPM, not that strange jumping behaviour.

Anyway: I recommend you clean up your fans first if you haven't done so in the last 6 months and then troubleshoot further if the issue doesn't go away...

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