If you experience issues related to volume, display resolution, fan or disk start up, resetting NVRAM might help. 

1. Shut down your Mac.

2. Turn on your Mac and immediately press and hold these four keys together: Option, Command, P, and R. 


Option keyplusCommand keyplusP keyplusR key

You can release the keys after about 20 seconds, during which your Mac might appear to restart.


NOTES: