The law is the Magnusson-Moss Warranty- Federal Trade commission Improvement Act of 1975. Under the Magnusson-Moss Act, aftermarket equipment that improves performance does not void a vehicle manufacturer’s original warranty, unless the warranty clearly states that the addition of aftermarket equipment automatically voids your vehicles warranty, or, that the aftermarket device is the direct cause of the failure. The easiest way to be sure is to look in your owner’s manual under, "what is not covered".