- shut down your Mac
- turn it on and immediately press and hold these four keys together: Option, Command, P, and R.
- release the keys after about 20 seconds, during which your Mac might appear to restart.
- Shut down your Mac
- Hold the left Shift+Control+Option keys down, then press and hold the power button down. Keep all four buttons pressed down for ten seconds, then let go.
- Turn on your Mac.