The Power Management Unit controls many of the low-level functions of your MacBook. When you reset the PMU, the date and time settings are reset and any RAM disks you installed are deleted. Only use the PMU reset option when you have problems with the monitor or keyboard backlight, hard disk cycling, sleep functions, battery or other hardware-related issues. Intel-based MacBooks use a System Management Controller, or SMC, that performs the same function as the PMU.

Step 1

Click the Apple menu, and then select the "Restart" option to check if the problem resolves before attempting to reset the SMC (see Tips).

Step 2

Shut down your computer and plug in the power adapter.

Step 3

Press the "Shift-Control-Option" keys and the power button at the same time, and then release the keys after one second. The power light indicator may blink.

Step 4

Press the power button to start your MacBook.