The System Management Controller is an integrated circuit (computer chip) that is on the logic board. As the name implies, it is responsible for power management of the computer. It controls backlighting, hard drive spin down, sleep and wake, some charging aspects, trackpad control, and some input/output as it relates to the computer sleeping.
Over time, the settings in the System Management Controller may become unusable, which can result in operational anomalies with the computer. Examples include not turning on, not waking from sleep, not charging the battery, or not recognizing the AC Adapter, among others.
Don't reset the SMC if the computer is unresponsive. An SMC reset should only be a last resort in cases where a hardware failure of the power management system is suspected. Performing an SMC reset returns the hardware, including NVRAM (Non-Volatile Random Access Memory), to default settings and forces the computer to shut down.
For most situations, a restart is sufficient. If the computer has stopped responding, try each one of these steps, one by one. Test in between steps to see if it has worked. If one step works, don't worry about the next, as you're up and running! Only go on to reset the SMC if you've tried all of the steps listed here and the computer still isn't working.
Caution: Reset the SMC only when the MacBook Air has been properly shut down. This prevents file system corruption.
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 the MacBook Air has been without power for an extended period of time.
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