http://www.ftc.gov/bcp/conline/pubs/...#Magnuson-Moss
A dealer cannot deny warranty simply because you have changed a part. Basically, a dealer has to honor the warranty unless they can prove the part you changed caused the fault. e.g. altering your wheels & tires should not effect a warranty claim on the a/c. This sounds straightforward but often things fall into a gray area. For example, supercharging the engine and then trying to warranty a transmission failure. Did the transmission fail because of the extra engine power? Much depends on the relationship with your dealer.