System extensions are small programs that add to, or extend, the capabilities of your computer's system software. These programs reside in one of two folders in the System Folder:

•	Extensions folder
•	Control Panels folder

Note: The term "extension" refers to items in either folder.

If your computer does not start up properly or behaves erratically, you may have an extension that is not compatible with another extension, or with an application program or system software.

Verify the problem

To verify that you have a compatibility problem:

1	Make sure the Caps Lock key is not pressed, then restart your computer while holding down the Space bar.
2	Release the Space bar when the Extensions Manager control panel opens.
3	Make a note of the name of the currently selected set of extensions in the Selected Set pop-up menu so that you can return to it later if you want.
4	Open the Selected Set pop-up menu and choose Mac OS 9.0 Base.
5	Click the Continue button to continue startup.
6	When the computer is ready, try the same actions that caused the problem.

If the problem no longer occurs, you probably have one or more extensions or control panels that don't work with the version of system software installed on your computer. Continue with the steps in the next section.

Test individual extensions and control panels

To test extensions and control panels:

1	Make sure the Caps Lock key is not pressed, then restart your computer while holding down the Space bar.
2	Release the Space bar when the Extensions Manager control panel opens.
3	Click the Duplicate Set button to create a new set of extensions. In the dialog box that appears, type a name for the set (for example, "Test Set") and click OK.
4	Turn off all the extensions in the new set by opening the Select Set menu and choosing All Off.
5	Turn on the item you want to test by clicking its checkbox.
6	Click the Continue button to continue startup.
7	Try the same actions that caused the problem.
	If the problem occurs again, the item is probably incompatible. Remove the item from the Extensions or Control Panels folder or turn the item off using the Extensions Manager.
8	Restart your computer while holding down the Space bar to open the Extensions Manager. Turn on the next item you want to test. Repeat this step until you've located the extension that is causing the problem.

If you find that a piece of software is incompatible, contact the manufacturer of the software to see if an updated version is available.

Tip: A recently installed extension or control panel might be the problem. If you have a lot of extensions and control panels, you may want to "work by halves." Turn on half the extensions and control panels and test for the problem. If the problem disappears, repeat the process, turning on half of the remaining items. If the problem persists, repeat the process, turning off half of the items that are turned on. Continue to work by halves like this until you locate the conflicting extensions or control panels.

Related topic

If your computer crashes, freezes, or won't start up