The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac: 1. From Apple menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off). 2. Unplug all cables from computer, including power cord. 3. Press and hold the power button for 5 seconds. 4. Release the power button. 5. Attach the computer’s power cable, making sure power button is not being pressed. Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed. 6. Press the power button to turn on the computer.