The Magnuson-Moss Act is great in theory but has serious limitations in practice. Part of the problem is that a dealer can refuse a warranty repair based on an aftermarket part or accessory and it then becomes the burden of the car owner to sue and prove that the modification didn't cause the failure.

This is especially true with a tune. It's easy for an uncooperative dealer to claim almost any driveline failure was the fault of a tune... even things that would appear to be unrelated. For example, a V6 driveshaft failure could be said to have been caused by a tune removing the speed limiter (almost all tunes do that). Still, there's no way a dealer could claim something like a wiper motor failure was caused by a tune.