iMac fan running all of the time

Before taking it in reset the PRAM per the instructions above, if that does not handle it please do a SMC reset. Instructions for that are:

1. Shut down the computer.
2. Unplug the computer's power cord and all peripherals.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.

Apple recently replaced the hard drive in my wife's 27 ' iMac. Got it home and restored from time machine. Couple of file related problems with that but I digress. Fan started going crazy. Tried all the unplugging routines to no avail. Then read your suggestion. It seems to have worked, at least for the last hour or so. Related, sometimes when I would unplug and then wait and replug it would pop a circuit breaker in the house but just for that room. I conclude that the power drain with the fan running at warp speed was the culprit. Thanks for your help. Only thing that seems to have worked. Still need to take it in and solve the file related problems.

You can try an SMC reset, which can sometimes fix a goofy fan:

1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.

What you're describing, however, sounds like a temperature sensor that's either acting correctly or it's not. I'd install iStat Pro from http://islayer.com/apps/istatpro/ , and monitor the temps and fan speeds.

I'd also run the Apple hardware test to see if it finds any obvious hardware issues. This Apple note tells how to run it: http://support.apple.com/kb/HT1509 (if you have Snow leopard, look down towards the bottom of the note for directions).

When the fan starts roaring, have you looked at your Activity Monitor (on your hard drive in applications/utilities) to see if there are any programs that are driving up CPU usage to high levels? That could contribute to an overheating situation.

Reset the PRAM and NVRAM. Seems like some conjection in the PRAM and NVRAM:

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

Your computer's PRAM and the NVRAM are reset to the default values. The clock settings may be reset to a default date on some models. If this doesn't resolve your issue, let me know please.

Hi, looks like it's been a while since any replies (from what I can see). My iMac seems to be acting up. I tried the SMC method and the PRAM reset method. Never worked with the SMC reset method. Seemed to work with the pram rest method the first time but the fan came on... tried it again and so far it's staying off but it's only been a few mins.

That did the trick. Shut down, unplug everything; wait 15 seconds, plug everything back in, start up.

Thanx.

Your welcome.