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After turning on the Mac, the fan runs at full speed  continuously (audible fan noise immediately after turning on). In this event, shutting the Mac off and turning it on again may not necessarily resolve the issue.

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Typically, this problem occurs if you press the Mac power button while connecting the power cord, the Mac will enter a mode (known as SMC) in which the fans run at full speed. Resetting the System Management Controller (SMC) may resolve the issue.

 

Resetting the SMC

Intel-base iMacs (or equivalent)

  1. Shut down the Mac
  2. Unplug the computer’s power cord
  3. Leave it unplugged for at least 15 seconds
  4. Plug the power cord back in
  5. Wait 5 – 10 seconds before turning the iMac back on

 

Macs with a Removable Battery

  1. Shut down the computer
  2. Disconnect the MagSafe power adapter from the computer, if it’s connected
  3. Remove the battery
  4. Press and hold the power button for 5 seconds
  5. Release the power button
  6. Reconnect the battery and MagSafe power adapter
  7. Press the power button to turn on the computer

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