As the iPod boots up, press the following buttons while the Apple logo is displayed: REW and Center/Select for new iPods or REW, FFW and Center/Select for older iPods. Hold the buttons simultaneously for a few seconds, a chirp noise should sound and the Apple logo will appear backwards.
Navigate the Diagnostics menu with the REW and FFW buttons. The scroll wheel will be disabled but you can still select with the Center/Select button. For a complete rundown of available diagnostic tests--what the tests do and how to properly run them--see the Resources section.
If ever your iPod starts to act a little funny or displays a warning icon upon start-up, it's probably best to give it a little checkup. Apple hid a secret Diagnostics Mode within the iPod that can be accessed via pressing a particular set of buttons upon boot-up. Use these diagnostic tests to determine and repair anything that may be off within your iPod before deciding whether or not to bring the iPod into Apple.
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