Resetting the power-management chip in your Mac

This FAQ provides:

It was developed for the following reasons:

  • Finding specific instructions — where available — by searching the AppleCare Knowledge Base can be difficult due to the various types of power-management chips employed in different Mac models.
  • Lists of such documents in other AppleCare Knowledge Base documents are incomplete, omitting information for older Mac models.

Types and functions of power-management chips

Every Mac includes an integrated circuit (IC) or microcontroller chip responsible for all of the power-management functions of the computer. Depending on when your Mac® was built, the power-management chip is known by one of the following names:

  • SMC: Systems Management Controller.
  • SMU: Systems Management Unit.
  • PMU: Power Management Unit.
  • CUDA: Capacitive Unit Discharge ASIC (Application-Specific Integrated Circuit).

The activities performed by power-management chips have become increasingly sophisticated as Macs have evolved. Depending on your Mac model, power-management activities can include:

  • Power management in startup, run, idle, doze, sleep, wake, and shutdown (off) power modes.
  • Processor and bus slewing, which adjust processor performance — and hence power consumption — to workload.
  • Controlling power to other ICs, including:
    • Ambient Light Sensor (ALS).
    • Graphics processor power management.
    • Keyboard backlight LED.
    • Peripheral device controllers, such as for USB® or FireWire® peripherals.
    • Sleep LED (Light Emitting Diode).
    • Sudden Motion Sensor (SMS).
  • Display brightness.
  • Battery charge, including monitoring charge level and controlling charge.
  • Power adapter identification.
  • Thermal conditions, including fans and fan speeds.

Instructions for resetting power-management chips

Most of the AppleCare Knowledge Base documents linked in the table below directly address resetting the power-management chip for a specific Mac model. However, some of the documents include that information as part of other troubleshooting or repair activities covered by the article. Read the relevant document throroughly before proceeding.

Macintosh Family Power-management chip reset instructions by model
eMac® All models.
iBook® All models.
iMac® Intel®-based iMacs.
iMac G5 (iSight, Ambient Light Sensor).
iMac G5 (Earlier models).
iMac (Flat Panel).
Mac Mini Intel-based.
MacBook® All models.
MacBook Air® All models.
MacBook Pro All models.
Mac Pro All models.
Power Mac® Power Mac G5.
Power Mac G4 Cube.
Power Mac G4 (AGP Graphics/Gigabit Ethernet).

Power Mac G4 (PCI Graphics).

    PowerBook® PowerBook G3 and G4.
    XServe® XServe G5.

    If your Mac is not listed in the table above, you may be able to find instructions for resetting the power-management chip in:

    • The hardcopy manual that shipped with your Mac. If you cannot find this manual, you may be able to download it from the official Apple Manuals page.
    • The service manual for your Mac. At the time of this writing, a Web page also dubbed "Apple Manuals" offered downloads of a variety of Mac service manuals.
    Did you find this FAQ helpful? You will find a wealth of additional advice for preventing or resolving Mac OS X problems in Dr. Smoke's book, Troubleshooting Mac® OS X.
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