If you suspect your Mac machine might have a hardware failure, you can use the bundled Apple Hardware Test that came with your Mac to do a diagnostics.

Apple Hardware Test is a diagnostic tool for detecting issues with your computer’s internal hardware components such as the logic board, memory, modem, video RAM, and Apple’s AirPort card. It does not check externally connected hardware components such as USB drives or Firewire devices; it does not check non-Apple devices such as PCI cards from other vendors.

Apple Hardware Test does not check your computer for issues with the Operating System (OS) or other software related issues such as application or extension conflicts.

These are 2 way to run the Apple Hardware Test

1) To run Apple Hardware Test from the internal hard drive

• Restart your computer while pressing and holding the ‘D’ key on your keyboard during startup.

Note: If Apple Hardware Test was removed by reformatting the drive, or if you cannot open it from your hard drive, you can still use Apple Hardware Test from the ‘iMac OS X Install Disc 1’ install DVD that came with your computer.

2) To run Apple Hardware Test from the Install DVD

• Insert the ‘iMac OS X Install Disc 1’ install DVD into the optical drive of your computer.

• Restart the computer while pressing and holding the ‘D’ key on the keyboard during startup.
• It takes a minute or so for Apple Hardware Test to start up and inspect your hardware configuration. While this is taking place, an icon appears on the screen:
  

• select your language and click the right arrow

• select your language and click the right arrow

• The Apple Hardware Test console appears. You can choose which sort of test or tests to perform:
  • To perform all of the basic tests, click the Test button or press the “T” key or the Return key.
  • To perform a more thorough diagnostic test, select the “Perform extended testing” checkbox under the Test button before you click the Test button.

• After the Hardware test, a fault code will be display on screen if there is a hardware fault. Once you get your fault,call Apple and tell them your fault code to get help from them.

Besides that, Apple Hardware Test also consists of a version of the Apple System Profiler, and allows you view your computer’s memory configuration, video card info, active communication ports, and basic system info such as your serial number, Boot ROM version, and model number.