MacBook Air and MacBook Pro (Late 2008 and later)
If the computer is on, turn it off by choosing Shutdown from the Apple () menu.
Connect the power adapter to a working power source.
On the built-in keyboard, press (left) Shift-Control-Option along with the power button once.
Wait 5 seconds and press the power button to start the computer.
Note: You must use the keys on the left side of the built-in keyboard.
You should only perform an SMC reset when the MacBook Air or MacBook Pro has been properly shutdown. This prevents issues from occurring to the file system.
While the power adapter does not need to be connected to reset the SMC, it may be necessary in situations where the battery needs charging or if the computer has been without power for an extended period of time. You can also visually check the SMC reset by observing the LED on the adapter. It will momentarily turn off as the SMC resets.