Every now and then you may experience problems with odd video output on a Mac system. Recently we reported on an ongoing issue in which some Mac systems have been displaying snow output when screen adapters are used. At other times people have reported problems with display output, including blank screens, odd output patterns, and unexpected color output on their displays. Display problems are nothing new for computer systems old or new, and can stem from faults with the monitor and its connections, the OS version or other software configuration, and hardware malfunctions.
Possible problems and options
If you're experiencing screen output problems, one possible reason would be a fault in your computer's hardware, which may happen if the GPU or a supporting hardware component (video RAM or controller chips) gets damaged. Several factors can contribute to hardware malfunctions, including physical damage and overheating, but it can also happen if card firmware has become corrupted. Static discharges from improper grounding can cause this, especially if you are manually servicing your computer, so be sure to properly ground yourself when servicing your systems.
Another source of hardware problems is if you have a graphics card that shares resources with other incompatible or faulty components, which may especially be true for older Mac systems where people have used PCI-based video cards in conjunction with other PCI expansion cards on the same bus. Motherboard faults also can interfere with a GPU's processing, so be sure to check other components such as audio, FireWire, USB, and other I/O behavior to see if they are working as expected (focusing on noises, pauses, and laggy performance).
In the event of a hardware problem, you will usually see distinct speckled patterns of pixels in odd colors on the monitor. The patterns may change color while staying in one position, or they might move around onscreen, especially in response to input or other computer activity.
The patterns and color can appear overlaid on your desktop and other applications, but also can appear to be embedded in various interface elements such as Finder icons or window borders. In these cases, as the images for these elements get processed in the GPU, the faults will corrupt the image and the elements will be displayed incorrectly.
In addition to odd display patterns, hardware faults can also result in blank output that is either consistent or that may happen intermittently and randomly.
Software configuration faults also can result in problematic display output. This can happen due to an unexpected change in driver code between OS updates or upgrades (especially if you are using a third-party video card), or a faulty installation of an OS upgrade or restoration. In these instances, you usually will experience odd color outputs such as washed-out colors, reversed or grayscale colors, or output resolution being stuck at a low settings such as 800x600 or 1,024x768 pixels.
The final option is that your monitor itself is having troubles. In this situation you may see either dead or stuck pixels onscreen, but these usually do not display in patterns and will not change onscreen or between restarts. In addition to stuck or dead pixels, you might see black output if the display's inverter or backlights are not working properly.
Testing and tackling the issue
Any time you are experiencing problems with your system, the first thing to try is to minimize your system to see if the problem goes away. Unplug any peripheral devices from your computer and then boot into Safe Mode by restarting the system while holding the Shift key. Sometimes hardware problems are based in how the GPU is handling high-level computations that are disabled in a minimal boot environment in which only basic video acceleration is used, and booting to Safe Mode can help uncover this.
In addition to Safe Mode, you can try booting to an alternative boot volume such as the OS X installation DVD, or better yet a full OS X installation on an external volume, to see if the problem persists there. This is perhaps the easiest way to isolate a software fault from a hardware malfunction, but keep in mind that you may need to try booting the system to fresh installations of multiple OS versions (the latest and most fully updated one, the one your system was shipped with, and so on) to see if an intermediate OS update contributed to the problem.
Beyond alternate boot environments, try enabling screen sharing on your system and then connecting to it from another Mac. Screen sharing processes the video output independently of the graphics card, which can help determine whether the problem lies in software or hardware.
Since different applications will handle the GPU and other video hardware differently, try launching different applications such as a full-screen game, a DVD player, or an image editor to see if the behavior changes with the use of these programs. If these show different behavior then you may be able to tell if the problem is a more general issue or if it happens only when the computer is performing certain computations.
To tackle software faults, you can try reapplying the latest Combo system updater for your version of OS X in addition to performing a general maintenance routine to clear system and user caches and other temporary items. You can also try removing the hidden global preference files for the user and the system, which hold a number of display settings such as ColorSync profiles. These files begin with the name .GlobalPreferences but are hidden, so open the Terminal and enter the following two commands to show hidden files in the Finder (perform these actions from an administrator account):
defaults write com.apple.finder AppleShowAllFiles TRUE
After these commands are run, the Finder will be restarted and you will see all the normally hidden files. Then go to the following directories and remove the global preference files within them, and restart the system:
When you are done removing the global preference files, you can repeat the Terminal commands listed above, but replace TRUE with FALSE to hide the hidden files again and keep the filesystem looking clean.
Moving further into tackling software configuration issues, try adjusting your monitor's settings in the Displays system preferences to see if any changes cause the problem to disappear. Adjust the resolution and color profile to see if that helps, but in addition go to the Universal Access system preferences and see if you have inverted colors, high contrast, or grayscale settings enabled, and disable these if they aren't desired.
When troubleshooting your monitor hardware, consider trying the monitor on a different computer or with a different display adapter if you use one. If the monitor supports multiple inputs (such as DVI, HDMI, VGA, and DisplayPort) then try using an adapter that uses a different input to see if the problem persists. Try a different display with your system for monitors that show black output, but if you have an iMac or a MacBook then you can troubleshoot a black display by shining a bright flashlight on the screen at an angle or through the Apple logo at the back of the display (for laptops). This may well generate enough reflected light for you to see if the LCD component of the display is working properly. In this case you should be able to detect faint OS X interface elements in the blackness of the display when the system is booted.
How to communicate the problem
If you are having problems with your display output, it may help to first contact other Mac users and technicians to see if they have any advice on the issue. One common way to do this for screen output problems is to use screenshots; however, screenshots do not always capture display output problems and even if the image file looks faulty on your system it may appear normal on others (see this discussion thread as an example of this). Therefore, if you decide to take a screenshot of your problem, you should preview it on another device to be sure that it portrays your problem before using it. Alternatively, you can use a digital camera to take a picture of your monitor showing the display problems.