If you're seeing random weirdness, you'll likely need to reset your "NVRAM."
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
- Turn on your Mac.
- Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
- Hold these keys until the computer restarts and you hear the startup sound for a second time.
- Release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.
If issues related to these features continue on a desktop Mac (like an iMac, Mac mini or Mac Pro), its logic board battery may need to be replaced. The logic board battery on desktop computers helps retain NVRAM settings when your Mac is unplugged. You can take your Mac to an Apple Store or Apple Authorized Service Provider to replace the battery on the logic board.