zapping the P-RAM
clears out some types of problems
sometimes worth doing
How to reset NVRAM on your Mac
If you experience issues related to settings stored in NVRAM, resetting NVRAM can help.
NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. Settings that can be stored in NVRAM include sound volume, display resolution, startup-disk selection, time zone, and recent kernel panic information. The settings stored in NVRAM depend on your Mac and the devices you're using with your Mac.
If you experience issues related to these settings or others, resetting NVRAM can help. For example, if your Mac starts up from a disk other than the one selected in Startup Disk preferences, or a question mark icon briefly appears before your Mac starts up, you might need to reset NVRAM.
Older Mac computers store similar information in PRAM (Parameter RAM) instead of NVRAM. The steps for resetting PRAM are the same as the steps for resetting NVRAM.
How to reset NVRAM
Shut down your Mac, then turn it on and immediately hold down these four keys together: Option, Command, P, and R. Keep holding the keys for about 20 seconds, during which your Mac might appear to restart. (If you have a Mac that plays a startup sound when you turn it on, you can release the keys after the second startup sound.)
When your Mac finishes starting up, you might want to open System Preferences and adjust settings that have been reset, such as sound volume, display resolution, startup disk selection, or time zone.
- If you're using a desktop Mac instead of a notebook, and settings such as sound volume or time zone are reset every time you shut down and unplug your Mac, you might need to replace the battery inside your Mac. This small battery is on your computer's logic board, and it helps NVRAM retain settings when your Mac is unplugged. You can take your Mac to an Apple service provider to replace the battery.
- If you experience issues with sleep, wake, power, charging your Mac notebook battery, or other power-related symptoms, you might need to reset the SMC (System Management Controller).