When your Mac is starting up, all sorts of things are happening behind the scenes, and if you have to, you can interrupt the process to reinstall the operating system or troubleshoot problems. Below are what I think are the most useful keys you can hold down while your Mac boots, but if you need more information, jump to the end of the article. Spoiler alert: You can learn what each stage of startup means, and then you’ll be the king of troubleshooting. I hereby dub thee as such, loyal reader.
Oh, and one more note before we begin—the way to use these is to hold down the keys I list simultaneously right after you start up your Mac, and then you’ll wait for something to happen. In some cases (like resetting the PRAM/NVRAM), you have to listen or watch for a specific event, so pay close attention to the instructions, all right?
Apple Hardware Test (AHT)/Apple Diagnostics
Key to Hold Down: D
Uses: You’re worried that your Mac has a hardware problem, and you’d like to check before you go see a Genius.
Notes: It depends on the age of your machine as to whether you’ll be using AHT or Apple Diagnostics, but the shortcut is the same. I’ve found that these tests aren’t definitive—just because they say there aren’t any problems doesn’t mean that there’s not something a more thorough test would find—but they’re useful anyway if they come up with a diagnosis.
Keys to Hold Down: Command-Option-P-R
Uses: Your Mac can’t remember what disk to start up from; it won’t save volume settings.
Notes: Again, what you’re actually resetting (PRAM or NVRAM) differs depending on the age of your Mac, but the key combo is the same. You’ll want to hold the keys down until you hear the startup chime twice.
Keys to Hold Down: Command-V
Uses: Your Mac won’t start up, but you can’t figure out why.
Notes: Verbose mode is handy if your computer seems to be getting partway through the boot process and stopping. It’ll output text describing exactly what it’s doing at every step, and you can tell where it’s getting hung by where the text stops. So you could use Safe Boot (see below) to potentially remove a problematic login item, for example, if you saw that listed as where things went awry.
Related Apple Support Article: “How to start up your Mac in single-user or verbose mode.”
Safe Boot/Safe Mode
Key to Hold Down: Shift
Uses: This can be helpful if your Mac won’t start up, as it’ll disable all sorts of potentially problematic login items, fonts, extensions, and the like.
Notes: It also cleans out certain cache files, which can be a troubleshooting step on its own. Be sure to reboot afterward—your Mac won’t work properly in Safe Mode!
Related Apple Support Article: “Try safe mode if your Mac doesn’t finish starting up.”
Keys to Hold Down: Command-R
Uses: You suspect that you might need to reinstall your operating system to address flakiness or slowdowns; Disk Utility tells you that your startup disk needs repair; you want to restore from a Time Machine backup.
Notes: Booting into Recovery Mode will let you do all sorts of stuff, like reinstalling the operating system, using Disk Utility to repair your startup drive, setting a firmware password, resetting a lost user password, and restoring from backup.
Related Apple Support Article: “OS X: About OS X Recovery.”
Keys to Hold Down: Shift-Control-Option-Power (left side), but see the Apple Support page below for exceptions by model
Uses: You’re having power- or battery-related problems (such as a Mac that does nothing when the power button is pressed); your fans are running like crazy all of the time; or lights and indicators (such as your keyboard backlight) don’t work properly.
Notes: I wrote a dedicated article about resetting the SMC fairly recently.
Related Apple Support Article: “Resetting the System Management Controller (SMC) on your Mac.”
Hungry for extra startup info? Then have I got more stuff for you. First, Apple has an extensive list of keys you can use when your Mac starts up, including plenty I haven’t covered here. Then if you need even more data packed into your brain, check out this helpful page about exactly what’s happening while your Mac is booting. Once you know all that, you’ll become a master startup diagnostician. Plus, it’s just really cool to be able to say “Oh, yeah, I can tell that my computer just found the ol’ boot.efi file” when you see the Apple logo appear onscreen.
Of course, “cool” is relative, and your mileage may vary on that. But I will think you’re cool, I promise.