Apple computers ship with a pre-installed suite of hardware diagnostic tools, known as Apple Hardware Test (AHT). In principle you can start them by holding the d key while booting. Newer models support holding option d to load AHT over the internet. If however, you reinstalled an older computer from scratch, the diagnostic tools might no longer be available. Unless you have the original disks that came with your computer, there seems to be no way to restore the AHT. Apple provides disk images with AHT for most computers, but does not make the links publicly available. Various blogs and forums have gathered a list of download links. The rest were obtained by scraping http://download.info.apple.com/Apple_Hardware_Test/ for all the links of the form 0(18|22)-\d{4}-A.dmg.
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