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I recently left for vacation, and when I came back yesterday, my computer was behaving quite strangely.

Now it will boot up fine, all is good, but the fan is loud, and constantly moving! This has never happened to me before, and I have no idea how to fix it.

Is it hardware or software? Either way, can I fix it on my own, or do I need to get a genius in on this? I'm running OS X Snow Leopard.

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How old is the computer and when was the fan cleaned last time? What are the system temperatures? Did you physically move the computer? – AndrejaKo Feb 4 at 21:09
its less than a year old, fan never cleaned, computer never moved, and how do i check system temp? – That Guy Feb 4 at 21:19
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I'm not sure. Maybe this program may help? Since it's less than a year old, it shouldn't be overheating that much, but that would depend on the amount of dust in the area. Anyway, try that program and post temperatures. – AndrejaKo Feb 4 at 21:41
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Several things come to mind:

  1. There may be a problem with its cooling, and it's running the fans full speed to try to compensate for it. Install Marcel Bresink's free Temperature Monitor utility, and see what the temps look like.
  2. The System Management Controller -- which manages the fans, among other things -- may have gotten into a weird state. Try resetting it (instructions in Apple's KB #HT3964).
  3. It's possible your iMac's clock battery has died, meaning that it looses state (& possibly messes up the SMC) when the iMac is turned off for an extended period. You can test this by turning off network time synchronization (in System Preferences -> Date & Time pane -> Date & Time tab -> deselect "Set date and time automatically"), turning the computer off for a while (not sure how long is needed; overnight should be long enough), and then seeing if the date is still correct after you restart it. If it's reset back to 1969 or 1970, your clock battery is dead. (In either case, you should turn network time back on after the test.) Apple doesn't generally consider clock batteries user-replaceable parts, and since I don't know which specific model of iMac you have and your experience level with repairs I'll just recommend taking it to an Apple-authorized repair shop for replacement.
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Shouldn't those batteries last years? Clock batteries are usually lithium cells and it's not rare to see them hold charge for 5-8 years with no problems, sometimes even more. If it's dead, it could be a sign of a bigger problem hiding somewhere. – AndrejaKo Feb 4 at 22:25
@AndrejaKo: You're correct that the battery should last several years (I didn't notice the comment about the computer's age before posting). I'd still recommend checking, as it could be a defective battery, bad connection, etc. – Gordon Davisson Feb 4 at 22:41
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