This is a question that is asked often and is a major concern for many people as they start down the road of modifying their vehicle. Search the web and you will find plenty of answers from internet “experts”, but as usual most of them totally incorrect. So, what is the correct answer?
Despite all the bad information out there, it is pretty cut and dry because there is federal law on the matter. The Magnuson-Moss Warranty Act (15 U.S.C. 2302(C)) regulates warranties and protects the consumer. This legislation was enacted in 1975 in response to widespread deceptive warranty practices in many industries. Here is how SEMA summarizes things:
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. The law states in relevant part: 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)).
So in short, this means a service department or automotive manufacturer can not void your warranty simply because you have installed aftermarket parts or have modified your vehicle. They can, however, deny a warranty claim if they can prove the issue in question was a direct result of the installed part or modification. Unfortunately, plenty of people have stories of going to a dealership service department with a warranty issue and being denied because they have modified their vehicle. There are definitely shady service departments out there who will try to get over on their customers if they can, but this is illegal and it is up to the customer to know the law and stand up.
Here is an example of what can not be denied:
You install a supercharger in your vehicle and a month later your drivers side window stops working. The supercharger has no impact on your electric window motor, therefore, the repair should be covered.
Here is and example of what could be denied:
You install a supercharger in your vehicle and a month later your clutch is toast. The additional power created by a supercharger can easily overpower a stock clutch and lead to the repair not being covered under warranty.