The System Management Controller, or SMC, is a controller in your Mac that is responsible for handling a number of power- and controls-related features of your computer. In general, the lights, buttons, charging, fans, and optional display modes are all examples of items that rely on the SMC for proper behavior.
While some of the problems encountered by a faulty SMC can be simply frustrating, others can sometimes be damaging. For instance, the SMC is responsible for handling the sudden motion sensor in laptops that triggers mechanical hard drives to park their reading heads so as not to damage the system. However, if this feedback is not working correctly, then the heads may continue to read and potentially damage the drive if a strong enough shock is experienced.
The specific symptoms you might encounter from a faulty SMC include the following:
- System does not show signs of starting up after the power button is pressed
- System gets very hot without fans responding
- The battery does not seem to be charging fully
- Fans blaring without notable change in system temperature or CPU usage in Activity Monitor
- Keyboard backlights do not turn on
- Automatic screen and keyboard dimming do not work, or seem to behave erratically
- Your Mac unexpectedly powers off or goes into sleep mode
- The system will not wake from sleep mode, even if the lid on a portable system is closed and re-opened
- iMacs that support Target Display mode will not enter or exit this mode
- Battery charge level lights, or other status indicator lights do not accurately reflect the status of the system
If your Mac is experiencing any of these behaviors, then an SMC reset might be the required routine to fix your system. This can be done on any Mac, but the steps needed depend on the type of Mac you have:
Newer Macs with built-in batteries
- Shut down the system and plug in the power adapter
- Hold the Shift, Control, and Option keys on the left-side of your keyboard
- Press the power button while these keys are held, and then release all keys at once
- Start up the system normally
Older portable Macs with removable batteries
- Shut down the system and disconnect the power adapter
- Remove the battery
- Press the power button and hold it down for 5 seconds
- Release the button, connect the power, and start up the system.
Desktop and Server Mac systems
- Shut down the system and unplug the power cord
- Wait 15 seconds, and then attach the power cord
- Wait another 5 seconds, and then start up the system
When you perform these steps, you may see the MagSafe power light turn off and back on, or see the status light on your Mac’s chassis blink. This is normal behavior, and indicates the SMC has been properly reset.