If you're having trouble with your Mac and think the cause could be hardware-related, you can run the Apple Hardware Test. It's a set of tools by Apple which perform basic and extensive tests on your hardware to see whether they are still okay. These tools are available for anyone with a Mac, and are either preinstalled on your system, or available through your OS'es installation discs.

Running the Apple Hardware Test

Through Apple Hardware Test CD

Applies usually to older models of Mac.

You can download the AHT disc images for a number of older Mac models through here: http://www.info.apple.com/support/aht.html.

1. Insert the Apple Hardware Test disc into the CD drive.
2. Upon starting/restarting your Mac, immediately press and hold C on your keyboard. Keep it pressed down until you see the Apple Hardware Test appear on your display and see that it is loading.

(Newer) PowerPC-based Mac

For newer (G4, G5) PowerPC-based Macs, the hardware test is usually located on Install Disc 1.

1. Insert the Install Disc 1 DVD into your drive.
2. Upon starting/restarting your Mac, immediately press and hold Option (this is the Alt key) on your keyboard. Keep it pressed down until you see the Startup Manager.
3. Startup Manager will scan for other boot volumes which will take some time. During this period, the mouse pointer will be a wristwatch and you will not be able to select anything. When it is complete, the pointer will be a regular arrow, and you will be able to select Apple Hardware Test volume, then click the right-pointing arrow button.

Intel-based Mac

Depending on version of Mac OSX, the hardware test is located:

• Before Mac OSX v10.5.4: located on Install Disc 1;
• From Mac OSX v10.5.5: located on Install Disc 2.

Notice

Macbook Air users have their hardware test integrated into the system, and normally don't need an installation disc. If you are unable to start the hardware test, try starting it remotely from another computer. For more details, see: http://support.apple.com/kb/HT2644.

To start the hardware test, perform the following steps:

1. Insert the appropriate disc into your drive.
2. Upon starting/restarting your Mac, immediately press and hold D. Don't let go until the icon of the hardware test appears on screen.
3. The hardware test will first inspect your hardware. When done, you will be able to choose the tests you want.

Running the test in loop mode

To find out whether you have intermittent hardware problems or finding the cause of the occassional kernel panic, try running the hardware test in loop mode. To use it, before starting the Extended Test, press Control-L. A message will appear on screen, indicating the selected test will run in loop mode.

Notice

Not all versions of the Apple Hardware Test support loop mode. If you do not see a message appear when pressing the key combination, it is most likely not available in the version you are using.

It is recommended to let the test run overnight to find possible problems. This is especially the case when trying to discover RAM problems; it may take many loops for the error(s) to appear.

To abort the test loop, you can do so by one of the following:

• Press the combo Command-. (period).
• Click the cancel or stop button, if you are able (it's difficult in loop mode).
• Manually restart your Mac by turning it off via the power button.

What if your Mac predates the Apple Hardware Test

If your Mac does not have a compatible Apple Hardware Test available, you can use Micromat TechTool Pro 5, a comprehensive suite of various hardware tests, including memory and disk analysis. If you have an AppleCare Protection Plan, you may have received a TechTool Deluxe CD with some of the hardware tests. For a small price, one could then upgrade to TechTool Pro.

Alternatively, to check for memory problems, you can also use the free Memtest OSX software.