When your Mac starts acting up, you'll probably run through some common troubleshooting procedures, such as restarting it, running Disk Utility, and perhaps performing a Safe Boot. Your repair repertoire should also include a couple of additional procedures that can occasionally eliminate otherwise inscrutable problems — zapping the NVRAM and resetting the SMC.
Zap the NVRAM (or PRAM)
Back in the day, the standard list of quick fixes for random Mac ailments always included "zap the PRAM." The P in PRAM stood for parameter (the RAM was just RAM — random access memory), and it referred to a small amount of special, battery-backed memory in every Mac that stored information the computer needed before it loaded the operating system. If the values in this memory got out of whack for one reason or another, your Mac might not start up correctly, or might exhibit any of numerous odd behaviors afterward. So you could press a key sequence at startup to reset (or "zap") the PRAM, returning it to default, factory values.
Modern Macs no longer use PRAM; they instead use something called NVRAM (NV for non-volatile). NVRAM serves approximately the same purpose as PRAM, but instead of storing a dozen or more pieces of information, it now contains just a few: your selected startup disk, speaker volume, screen resolution, time zone, and — if your Mac has crashed recently — details of the last kernel panic.
NVRAM corruption is fairly uncommon, but if your Mac seems to take forever to figure out which disk to boot from, if it starts up with the wrong screen resolution, or if you have weird audio problems (like there's no sound whatsoever, or the menu bar's volume control is grayed out), it doesn't hurt to reset the NVRAM — it's quick and harmless.
To reset your NVRAM, you use exactly the same procedure you once used to reset PRAM: Shut down your Mac, press the power button, and as soon as you hear the startup chime, hold down Command-Option-P-R. Keep holding down those keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally. Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure they're set the way you want them.
If you hold down Command-Option-P-R at startup and you see nothing but a gray screen that doesn't change for several minutes — no Apple logo, no progress bar, no second startup chime — don't panic. (This happened to me just last week.) The most likely cause is that your Mac isn't registering the key presses due to wonkiness with a USB device. Disconnect all USB devices (except your keyboard, if it's a wired keyboard), hold the power button down until the Mac shuts off completely, and then press it again and immediately hold down Command-Option-P-R. If that doesn't work and you're using an external Bluetooth keyboard, try plugging in a USB keyboard instead. If you're able to reset the NVRAM successfully with the wired keyboard, you can disconnect it and go back to your normal Bluetooth keyboard.
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