How to resetting the SMC on iMac

Section: How to reset the SMC on Mac desktop computers

Follow these steps for iMac, Mac mini, Mac Pro, and Xserve. If you have an iMac Pro, skip to the next section.

1. Choose Apple menu > Shut Down.
2. After your Mac shuts down, unplug the power cord.
3. Wait 15 seconds.
4. Plug the power cord back in.
5. Wait 5 seconds, then press the power button again to turn on your Mac.

For Intel-based Xserve computers that aren't responding, you can shut down locally or by using remote commands. You can also press and hold the power button for 5 seconds.

Section:How to reset the SMC on iMac Pro

How to reset the SMC on iMac Pro

Follow these steps for iMac Pro:

1. Choose Apple menu > Shut Down.
2. After your iMac Pro shuts down, press and hold its power button for 8 seconds.
3. Release the power button, then wait a few seconds.
4. Press the power button again to turn on your iMac Pro.

https://support.apple.com/en-us/HT201295

Please click iMac SMC and PRAM reset for instructions. Do the SMC reset 2-3 times back to back, sometimes it does not work the first time. Also disconnect any peripherals such as External Hard Disks when doing the reset.

I had to do this recently too because my fan started blowing really loud and fast all the time. In addition to the instructions given above, I found a variant procedure (below, from 2010 27" iMac Technician's Guide) that added pushing and holding the power button for 5 sec while the computer was unplugged.

Not sure why this would help, but it did help with the fan control some (not completely). Resetting the SMC without that added step did nothing to help with the fan control at all.