Resetting the PMU
The Power Management Unit (PMU) is an integrated circuit (computer chip) typically found on the logic board of Macintosh computers. As its name implies, the PMU is responsible for managing all aspects of the computer's power. It controls hard disk spin down, sleep and wake, some charging aspects, and how any devices attached to the computer affect sleep.
Corrupted settings in the PMU can result in operational anomalies such as your computer not turning on, not displaying video, or not waking from sleep, among other things. In these situations, you may need to reset the computer's PMU.
Resetting the PMU will not resolve issues in which the computer stalls or is unresponsive. A PMU reset should be used only as a last resort, as it is an intrusive quick fix. Reset the PMU in the case of a hardware failure or when the power management system is suspected. Resetting the PMU returns the computer hardware, including NVRAM, to default settings and forces the computer to shut down.
Before you reset the PMU, try restarting the computer instead, which may resolve the issue. If the computer has stopped responding, try these steps, in order, until the computer responds:
1. | Force quit (Command-Option-Escape). |
2. | Restart (Command-Control-Power). |
3. | Force shut down (press the power button for 10 seconds). |
If none of the above steps resolves the situation, reset the PMU. End users can reset the PMU with the top housing on without voiding the warranty, while Apple Authorized Service Providers (AASPs) also have the option of resetting the PMU with the top housing off.
MORE INFO
Refer to Knowledge Base document 300574, “Mac mini: How to Reset the PMU.”
Resetting the PMU With the Top Housing On
1. | Unplug all cables from the computer, including the power cord. |
2. | Wait 10 seconds. |
3. | Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer. |
4. | Let go of the power button. The computer should start up. |
NOTE
The above procedure resets the computer's PRAM. After resetting the PMU, be sure to reset the time, date, and other system parameter settings.
If none of the above steps resolves the situation, reset the PMU with the top housing off as instructed in the following section.
Resetting the PMU With the Top Housing Off
AASPs can also reset the PMU by directly pressing the PMU reset switch located on the logic board. Follow these steps to access the PMU reset switch on the logic board:
1. | Unplug all cables from the computer, including the power cable. |
2. | Remove the top housing. |
3. | If necessary, remove the internal frame to access the battery. |
4. | Measure the voltage of the battery in the battery holder. The battery should measure +2.6 V to +3.0 V. If the battery voltage is good, proceed to step 5. If the battery does not have minimum voltage, replace the battery, wait 10 seconds, and then proceed to step 5. |
5. |
NOTE
The above procedure resets the computer's PRAM. After resetting the PMU, be sure to reset the time, date, and other system parameter settings.
