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How to use Apple Hardware Test and Apple Diagnostics

Troubleshooting 101 -- at its core -- involves identifying a problem on a computer and using that information to resolve any issue(s) stemming from said problem. While most troubleshooting takes place at the OS level, there will inevitably come a time when the hardware itself will be the cause of a system's instability.
Many 3rd-party tools exist to root out hardware trouble, sometimes heuristically, even before the component has failed. But many Mac owners do not know that Apple bakes in a utility specifically to test hardware found in Apple computers of all sizes.
There are two utilities, as a matter of fact, and depending on when your device was manufactured, you'll access Apple Hardware Test if your computer was manufactured prior to June 2013. If the computer was manufactured after that date, then Apple Diagnostics is the utility that will load. They're both very similar in what they accomplish -- a self test of the hardware, complete with scans of internal storage devices.
The look of the utilities will differ, since Apple Diagnostics is newer and aesthetically keeps the same design principals as those found in OS X Mavericks. Also, the inclusion of internet connectivity, which adds access to Apple's online support pages to assist in troubleshooting, is a huge benefit -- and it eliminates the need for a secondary device to perform research while trying to tackle a problem on the primary unit.
Let's take a look at how to invoke the utilities when needed:
  1. The computer must be turned off. If it is not, please shut it down prior to beginning.
  2. Press the power button to power on the computer.
  3. Immediately after pressing the power button, press and hold the [D] button on the keyboard. Continue to hold down the key until the Apple Hardware Test (AHT) icon appears or the Apple Diagnostics (AD) language selection menu loads.
  4. At this point, the next step will depend entirely on the version of the utility that loads. For AHT, go to step 4a; For AD, go to step 4b.
    • 4a. Once the console is fully loaded, using a mouse or keyboard click the Test button or type in T to begin the self-test. If you wish to perform a more thorough test, check the "Perform extended testing" check box before beginning the test. This examination takes longer to complete, but it provides in-depth checking of RAM addresses and Hard Drive sectors for errors.
    • 4b. Once the language is selected, the computer will begin the self test, which takes 2-3 minutes on average. When the test is complete, any issues detected will be listed on the subsequent screen. Helpful links to resolving many of these issues may be included, directly from Apple's support site. Additionally, AD uses reference codes to identify specific issues pertaining to hardware and potential failures. These codes can be used to further breakdown a problem or contact Apple technical support directly from AD.
Elegant, yet simple are words I use to describe Apple and the approach they bring in everything they create -- from hardware to software. The sleek polish combined with ease-of-use shines through every single time.
This lays in stark contrast to potentially difficult situations that occur while troubleshooting software. With all its variables and the general lack of replacement parts on-hand, testing hardware can become quite daunting and very expensive, especially with newer Mac computers and their all-in-one layout, where each component is soldered, glued, or both to the logic board.
Luckily, Apple has once again made it extremely easy for end users to determine which version of the hardware utility to use or how to make sense of error messages and alerts displayed by Apple Hardware Test.


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