You can reset the NVRAM or PRAM by shutting down the Mac and then starting it up again while holding down the Option, Command, P and R keys all at once. If the Mac is not maintaining its settings for volume level, display resolution, time-zone information or preferred start-up disk, information held in the NVRAM may have become corrupt. Modern Macs also store some settings in an area of the system called the nonvolatile random access memory (NVRAM) older Macs use parameter random-access memory (PRAM) in a similar manner. might fix erratic Mac behavior, although it will not help with a damaged power supply or another major hardware problem. (You may also have to redo any preferences for your display and power-management settings.) While performing a reset does not generate an alert box or notification, you can tell if you were successful if the odd Mac behavior stops. The guide also lists a series of things you should try before resetting the S.M.C. reset process for all its Intel-based Macs, including those with removable batteries and desktop models.

Apple’s support site has a full guide to the S.M.C.

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