The System Management Unit, or SMU, is a very small component in an iMac but it plays a very important role. In iMacs built after mid-2006, this component is called the System Management Controller, or SMC. Without it, your iMac would not be able to turn on and off or avoid overheating. If you find your iMac is not powering on efficiently or the fans seem to be running for longer than usual, resetting the iMac's SMU can often solve the problem.
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Click the Apple logo in the upper left corner of your screen and choose "Shut Down..." to turn off the iMac.
Leave the iMac turned off for at least 15 seconds.
Press and hold the power button on the back of the iMac.
Plug the power cable into the back of the iMac while continuing to press the power button.
Plug in the power cable and then release the power button.
Press the power button again to turn on the iMac. Your SMU has been reset.
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