Know your rights.
There is something called the Magnusson-Moss Warranty. You can read it all here:
http://www.ftc.gov/bcp/conline/pubs/buspubs/warranty.htm
Here's a little blurb on it that I picked up off the net:
No aftermarket part or accessory that is properly installed on your vehicle will entirely void your original vehicle manufacturer's warranty. According to the provisions in the Magnuson-Moss Warranty - Federal Trade Commission Improvement Act, it is the dealership's responsibility to prove that your modification directly caused or affected the failure you are trying to resolve.
With that said, some dealerships are more performance-oriented than others and are more willing to work with customers. After all, if they won't comply then you have no choice but to take them to court. Not entirely convenient.
Warranty-related materials, published by the new vehicle manufacturers, usually state that the installation of aftermarket parts have no effect on the warranty unless such equipment causes the problem that leads to the warranty claim. Check your warranty to be sure this is the case.
Nonetheless, some dealers have improperly denied warranty claims even though the claim had nothing to do with aftermarket parts. I.E., just because you install an exhaust system, your suspension failure warranty claim cannot be denied. If you experience a problem with an uncooperative dealership, point out the provisions of the this "FEDERAL ACT".
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