Out of nowhere, the screen turned a crazed green and yellow, and then the computer shut down. Here are a few things you can do to troubleshoot and figure out exactly why your video card or display is freaking out.
Step One: Reset the PRAM/VRAM and SMC
Turn on your Mac and hold down Command+Option+P+R until the computer reboots. This resets the PRAM / VRAM, which is where things like startup disk selection, screen resolution, and speaker volume are stored. Sometimes this can correct display issues, and if so, continue using your Mac as you did.
The other option is to reset the SMC (System Management Controller). This controls everything on your computer ranging from the power to the fans. Every Mac has a slightly different process for doing this which usually involves unplugging the power cord on a desktop, or removing the battery on a laptop. Once you reset the SMC a lot of your setting are restored to factory defaults and your display problems may get solved.
Step Two: Boot Into Safe Mode
The next step to figure out what's going on with your graphics card or display is to boot into safe mode to see if the problems persist. Power on your Mac and hold down the Shift key until you get past the Apple logo. This boots into a stripped down version of OS X.
Here, you can see if the display problems are persisting. This might be screen glitches, pixelated graphics, or large black squares everywhere. If they are, it's probably a hardware issue and you should move on to the next step. If not, it's likely a software problem, and you have a few different options for troubleshooting:
- Restart the computer again in normal boot mode to see if the problem resolves itself (this does actually happen).
- If not, go back into Safe Mode and check for software updates (Apple logo > Software Updates). If you get an update for your graphics card or logic board, install it.
- Double-check your display options by going into System Preferences (Applications > System Preferences). Choose "Displays" and make sure the resolution and refresh rate are correct.
- If you've recently installed a software update that possibly caused the issue, it's also worth checking out Apple's recently released updates and downloading and installing the most recent combo update again. Sometimes a simple re-install can fix strange issues that might have cropped up with display drivers.
If none of those work, it's time to run the Apple Hardware test to see if it's a hardware issue.
Step Three: Run Apple Hardware Test
A lesser known feature of Macs is the Apple Hardware Test. Like the name suggests, this is a way to test for hardware failures on your computer. It's not foolproof, but it might help you troubleshoot your issue moving forward.
- Reboot your Mac and hold down the "D" key until the Apple Hardware Test starts (if you're on Snow Leopard or earlier you need to put in the install disc first).
- Select your language, and then select the "Basic test" option. Let it do its thing. If an error occurs, the Apple Hardware Test should tell you which piece of hardware is failing and you've found your problem. If not, select the "Perform Extended Testing" option. This might take an hour or two to complete.
While a hardware failure is never fun, hopefully the Apple Hardware test will actually show it so you can get the faulty hardware replaced.