The Magnuson-Moss Warranty Act (15 U.S.C. 2302(C)) reads in relevant part as follows:

No warrantor of a consumer product may condition his written or implied warranty of such product on the consumers using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade or corporate name... (15 U.S.C. 2302(C)).

This federal law regulates warranties for the protection of consumers. The essence of the law concerning aftermarket auto parts is that a vehicle manufacturer may not condition a written or implied warranty on the consumers using parts or services which are identified by brand, trade, or corporate name (such as the vehicle maker's brand) unless the parts or service are provided free of charge. The law means that the use of an aftermarket part alone is not cause for denying the warranty. However, the law's protection does not extend to aftermarket parts in situations where such parts actually caused the damage being claimed under the warranty. Further, consumers are advised to be aware of any specific terms or conditions stated in the warranty which may result in its being voided.

This Federal law is also meant to prevent big car makers from monopilizing the market for parts and thereby destroying competition. Think about it: if you could only use parts made by the manufacturer of your car, they would have no incentive to improve them or make them cheaper. It would be a classic monopoly and anti-trust violation.

This law has been used by many modders to save their warranties.