Fan turns on when iMac powered off.

I got a USB hub for Christmas, and after we connected it to the iMac, when it only had my external drive connected, I connected my iPhone and the computer shut off. I couldn't power up again, and had to unplug/replug to get it to work. Then it went to sleep and the fan started blowing loudly. I've done all sorts of troubleshooting but nothing has worked. While the iMac is on the fan and everything runs normal, when it goes to sleep or power off the fan turns on. I took it to the Apple store today and they ran a diagnostics test, and reset the PRAM, and it was running fine. At least we think it was, because they checked it right in the middle of the very crowded and noisy store. Maybe it was still doing it but we couldn't hear it. Now I get home and plug it in and the fan kicks on again, before I turn it on. I have made an appointment for Monday again, but I was just wondering if anyone else has had this problem after connecting USB hubs, and what your solution was.

Resetting the PRAM is not going to solve that one. You need to reset the System Management Controller. Do this: Shut down your iMac and remove the AC cord from the receptacle and leave it that way for about 15 minutes. Turn it back on and see if the Fan speeds return to normal. (CPU fan should be running at about 1100 to 1200 RPM)

You didn't mention which year and model iMac you have, however, older iMacs (2011 and earlier) use three fans. The newer iMac models (2012 and 2013 have only one fan).

chscag said:

Resetting the PRAM is not going to solve that one. You need to reset the System Management Controller. Do this: Shut down your iMac and remove the AC cord from the receptacle and leave it that way for about 15 minutes. Turn it back on and see if the Fan speeds return to normal. (CPU fan should be running at about 1100 to 1200 RPM)

You didn't mention which year and model iMac you have, however, older iMacs (2011 and earlier) use three fans. The newer iMac models (2012 and 2013 have only one fan).

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Sorry, my bad. 21 in iMac, purchased May 2013, so it's not a year old yet. Running Mavericks 10.9.1. The fan is in the back, right behind the stand, at the thickest part of the computer. I unplugged it over night. Removed the plug from the computer itself. I even took it off when we went to the Apple store because they told me to bring in the computer only. He did a Comand N for network diagnostic, or something like that. He couldn't find anything wrong with it, and like I said, maybe it was just so noisy in the store we couldn't hear the fan.

I have to unplug it every time I'm not using it because I don't want the fan running like that, and it kicks on even when it just goes to sleep mode.

OK, yes that makes a difference. Since yours is a 2013 model it contains only one fan which cools the entire chassis. Since you already reset the SMC and the fan remains on during power down and sleep, there's something else going on. Definitely take it back to the genius bar and have them run a diagnostic.

Also, some advice: If you haven't purchased Apple Care, you should. The new iMacs are closed machines and pretty much impossible for a do it yourself person to repair. Apple care will extend your warranty to three years. One trip to the genius bar and it will pay for itself.

Thanks for the advice chscag. My 1 year warranty runs until May, and I'll make sure to get Apple care before then.
He did mention that sometimes faulty USB hubs, or any peripheral that's faulty, can damage the logic board, and he said that's what it sounded like. Of course, once he actually plugged it in and started running diagnostics on it everything came up fine. I'm just frustrated more than anything now, so I just wanted to see if anyone else has had this problem. I have found people with the same problem, but all the replies to them were to take the computer back for repair. Sigh!
On the bright side of things, I should be happy that the USB hub fried my warrantied computer, and not my external drive with all my work.