Operating System Version: 7.5 and Above If you are experiencing problems with your Macintosh, start with the system Extensions Off. If it then runs properly, you may have a problem with your system Extensions, control panels, or even the fonts. Using the Extensions Manager control panel you can disable all extensions, then add a few back at a time until the problem returns. If the problem returns, you should replace the item. Key Used (SHIFT) Turn off System Extensions Start or restart the Macintosh. Hold and continue to hold down the Shift key until you see Extensions Disabled on the startup screen. You can now release the Shift key. Extensions are disabled.