Hold down the "D" Key on boot.

The Apple Hardware Test (AHT) is a diagnostic program that tests the basic functionality of your Mac’s hardware.

All MacBook Airs and MacBook Pros (2008 later) have the AHT loaded on the hard drive, other Macs will list the AHT version on one of their restore disks.

While the AHT is not always accurate in determining a hardware issue, it’s a great first troubleshooting step.

The AHT tests basic functionality like the GPU, processor, HD and RAM and may provides error code(s) that can be useful in determining an issue.

To run the AHT, disconnect any peripherals like printers or keyboards. Connect your Mac to power and either:

Insert the restore disk that contains the AHT and hold down the “d” key on boot.

or

If you have a late model MacBookPro or Air, you may just need to hold down the “d” key on boot.

Note: The AHT can take up to an hour to run.