One of the scariest things a Mac user can experience is a kernel panic, which is when a Mac stops in its tracks, darkens the display, and puts up the message, "You need to restart your computer. Hold down the power button until it turns off."
If you see the kernel panic message, first off, relax; there is nothing you can do at this point to make it go away except to restart your Mac.
Shut Down Your Mac After a Kernel Panic
- When you see the restart message, press and hold the power button until your Mac begins to turn off.
With that out of the way, it's time to try to figure out what went wrong, or at least how to get your Mac back to working condition. The good news is that getting your Mac working again may be as simple as powering it back on. In all my years of working with Macs and providing technical support, I have only once seen the kernel panic screen associated with a permanently failing Mac. Even then, the Mac could have been repaired, but it was a good excuse to replace it instead.
What Causes a Kernel Panic?
There are quite a few reasons why a Mac may have a kernel panic, but most of them are temporary and may not be seen again. These include poorly written applications, plug-ins, add-ons, drivers, and other software components.
Many times you only see a kernel panic when unusual conditions occur, such as two or more specific apps running while most of your memory is in use. Simply restarting your Mac will correct the problem. Other times, the kernel panic comes back to visit from time to time, not quite on a regular basis, but often enough that you get really tired of seeing it.
In those cases, the problem is once again usually software related, but it can also be failing hardware, or a combination of software and hardware problems, such as wrong versions of drivers for a specific piece of hardware, such as a printer.
The most hair-pulling kernel panic is the one that happens every time you try to start up your Mac. In this case, the problem is usually hardware related, but it can also still be something as simple as a corrupt system file or driver.
Resolving a Kernel Panic
Since the majority of the time a kernel panic is transitory, it is tempting to just restart your Mac and get back to work. I won't fault you if you go that route. I do that quite often when I have a good deal of work to get done, but if you have the time, I suggest doing the following.
Restart Using Safe Boot
- Start your Mac by holding down the shift key and pressing the power on button. Keep pressing the shift key until your Mac boots up. This process is called a Safe Boot. During a Safe Boot, your Mac does a basic check of the startup drive's directory structure. If everything is okay, the OS loads the bare minimum number of kernel extensions it needs to run. This means no startup or login items are run, all fonts except those used by the system are disabled, and the dynamic loader cache is dumped.
- If your Mac starts up fine in Safe Boot mode, then the basic underlying hardware of the Mac is functioning, as are most system files. You should now try starting your Mac normally (simply restart your Mac). If your Mac restarts without any problems, then some wayward app or driver, or some type of interaction between apps and hardware, probably caused the kernel panic. If the kernel panic doesn't reoccur in a short time, say a day or two of use, you can consider it just a minor inconvenience and get on with using your Mac.
- If your Mac won't start up after restarting from Safe Boot mode, then the likely problem is a startup or login item, a corrupt font or font conflict, a hardware issue, a corrupt system file, or a driver/hardware issue.
Isolate your hardware by disconnecting everything but your keyboard and mouse from your Mac. If you're using a third-party keyboard that requires a driver in order to work, try temporarily replacing the keyboard with the original Apple-supplied keyboard. Once everything but the keyboard and mouse are disconnected, try restarting your Mac. If your Mac starts up, then you will need to repeat the startup process, reconnecting one piece of external hardware at a time and restarting after each, until you figure out which device is causing the problem. Remember that devices such as wired routers, switches, and printers can all be the source of problems.
If you still can't start up your Mac without a kernel panic, then it's time to check some basics. Restart your Mac using the OS X installation DVD or the Recovery HD partition. Once your Mac boots to the installation or recovery screen, use Disk Utility to run a Repair Disk on all drives connected to your Mac, starting with the startup drive. If you run into problems with your hard drive that Repair Disk can't fix, it may be time to replace the drive.
Disable all startup and login items, and then start up again in Safe Boot mode (hold down the shift key and press the power on button). Once your Mac boots, you will need to disable startup and login items from the Accounts or Users & Groups preference pane.
There are also system-wide startup items that some applications install. You can find these items at: /Library/StartupItems. Each startup item in this folder is usually located in a subfolder identified by the application's name, or some semblance of the application's name. You can move all of the subfolders to the desktop (you may need to provide an administrator password to move them).
Once the startup and login items are disabled, restart your Mac normally. If your Mac starts without any problems, reinstall startup and login items, one at a time, rebooting after each, until you find the one that's causing the problem.
You can use FontBook to check any fonts you installed with FontBook. Once again, start in Safe Boot mode, and then launch FontBook, which is located at /Applications. You can select multiple fonts and then use the Font Validation option to check for errors and corrupt font files. If you find any problems, you can use FontBook to disable the relevant fonts.
Reinstall OS X using the OS X Update Combo. Restart your Mac in Safe Boot mode, if you haven't already, go to the Apple web site, and download the most recent OS X Update Combo for the system you're using. Installing the Update Combo, even if your Mac is already at the same version level as the updater, will replace any corrupt or outdated system files with current working versions. Installing the Update Combo should not affect any user data on your Mac. I say "should not" because we're dealing with a Mac with problems, and anything can happen. Make sure you have a current backup of your data.
If the Update Combo doesn't get things working, you may have to consider reinstalling OS X using the installation media (OS X through 10.6.x) or the Recovery HD (OS X 10.7 and later). If you're using OS X 10.5 or earlier, you can use the Archive and Install option to preserve user data that is already present. OS X 10.6 and later does not have an Archive and Install option. Ideally, reinstalling the OS will only erase and install the system files, leaving user files intact. Once again, it's safer to have a current backup of your data before updating or reinstalling the OS.
Once you reinstall the OS, you will also need to run Software Update (Apple menu, Software Update) to bring your Mac up to the current OS level. Be sure to also reinstall any drivers, plug-ins, and add-ons. It's best to reinstall them one at a time, and reboot after each, just to make sure that none of them were the original cause of the kernel panic.
If You Can't Resolve the Kernel Panic
If reinstalling the OS and updating any third-party apps and drivers doesn't resolve the kernel panic, then it's a good bet the issue is with hardware. Be sure to check out the hardware troubleshooting section above. If you still have problems, chances are the issue is the internal hardware of your Mac. It can still be something basic, such as bad RAM or a hard drive that isn't working correctly. I have loads of memory and multiple drives from other Macs that make it fast and easy to swap hardware around for troubleshooting purposes, but most people don't have the luxury of an in-house parts department. For this reason, consider taking your Mac to an Apple or authorized third-party service center. I've had good luck with Apple's Genius Bar. Making an appointment is easy, and the diagnosis is free.