Yeah i just had this problem with my ipod video 30 gig and dont enter diagnostic mode. I'll give you a fair warning that I've heard some people got stuck especially in accessorize test. If you ever do find out how to get into those tests and get stuck just hold center and menu for 10-15 seconds. Warning that some people have had theres crash and get stuck on booting up. If anyone wants to go there anyway, turn on the ipod switch hold on then off then hold center and menu until it reboots. when the apple logo appears hold center and rewind. to get out from the menu hold center and play/pause.
And this will work for most not if all ipods with screens

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