If your Mac is running slowly or erratically and nothing else — including resetting the NVRAM — has worked, you can reset the SMC (system management controller).
However, you should do this only after you’ve tried all other standard troubleshooting issues. Why? SMC is a chip in your Mac that’s involved in running a lot of the physical parts of the machine, like LED indicators, keyboards and other peripherals, cooling fans, and power buttons. It also plays a role in the behavior of your hard drive, how your computer behaves in sleep mode, and the power supply.
To reset SMS, shut your Mac down, unplug all cables, wait a couple of minutes and then plug the cables back in and start up the Mac. After you perform normal troubleshooting, these symptoms may indicate that an SMC reset could be necessary:
The computer fans run at high speed, though the computer is not experiencing heavy usage and is properly ventilated.
The keyboard backlight behaves incorrectly on Mac computers that have this feature.
The status indicator light (SIL) behaves incorrectly on Mac computers that have this feature.
Battery indicator lights, if present, behave incorrectly on Mac notebooks that have a non-removable battery.
The display backlight doesn't respond correctly to ambient light changes on Mac computers that have this feature.
The computer doesn't respond to the power button when pressed.
A Mac notebook computer doesn't respond properly when you close or open the lid.
The computer sleeps or shuts down unexpectedly.
The battery doesn't charge properly.
The computer is performing unusually slowly, though it isn't experiencing abnormally high CPU utilization.
Application icons may bounce in the Dock for an extended amount of time when opened.
Applications may not function correctly, or they may stop responding after being opened.
A computer that supports target display mode doesn’t switch into or out of target display mode as expected, or it switches into or out of target display mode at unexpected times.
The illumination around the I/O ports on a Mac Pro (Late 2013) doesn’t activate when you move the computer.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)
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