A warranty is very easy to void by a dealer. They can just refuse to work on your car (which they will) and it's then up to you to take them to court. Chances are pretty good you'll lose. As far as the MM act, well, that's not going to help you in the case of a tune. Mods and warranty claim denial- the truth - Mustang Evolution
Incorrect. The Magnuson Moss act very clearly states that it is up to the dealer to prove beyond any reasonable doubt that the modification directly caused the failure that they are trying not to cover. If you put headers on and drop a valve they are still going to cover that under warranty. If you over torque the header bolts when you install them and warp the mating surface that is on you. The dealer can't just say "eh you changed something, no warranty for you" Highly illegal and they have a lot to lose. That said, make damn sure your mods are done correctly and you know what it will effect.