What you may not be aware is that Apple dumped AHT this past June. New Macs, introduced in June 2013 or later, no longer include the software. Instead, they have an entirely new diagnostic application called, appropriately enough, Apple Diagnostics. You access Diagnostics exactly the same way as you do AHT: Hold down the D key at startup.
As far as I can tell, the main difference between the two applications is that Apple Diagnostics has a more refined and verbose interface than AHT. If a problem is found, Apple Diagnostics also offers detailed advice as to how to proceed. This is consistent with Apple's ongoing strategy to make its computers more consumer-friendly and easy to use, even for such "techie" tasks as diagnosing hardware problems.
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