All About Programming: How to reset the System Management Controller (SMC) on your Mac

How to know if you need to reset the SMC

These symptoms might mean that an SMC reset is necessary:

Your computer's fans run at high speed, even though it isn't under heavy usage and is properly ventilated.
The keyboard backlight behaves incorrectly.
The status indicator light (SIL), if present, behaves incorrectly.
Battery indicator lights, if present, behave incorrectly on Mac notebooks with a non-removable battery.
The display backlight doesn't respond correctly to ambient light changes.
Your Mac doesn't respond when you press the power button.
Your Mac notebook doesn't respond properly when you close or open the lid.
Your Mac sleeps or shuts down unexpectedly.
The battery doesn't charge properly.
The MagSafe power adapter LED, if present, doesn't indicate the correct activity.
Your Mac performs unusually slowly, even though its CPU isn't under an abnormally heavy load.
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 turn on when you move the computer.