Legally, a vehicle manufacturer cannot void your warranty for installing an aftermarket automotive part unless they can prove that the aftermarket part caused or contributed to the damage directly or indirectly in the vehicle (per the Magnuson Moss Warranty Act (15 U.S.C. 2302(C)). Tesla’s warranty is governed by the Magnuson-Moss Warranty Act of 1975.
Tesla must prove – not just say – that aftermarket equipment caused the need for repairs before they can deny warranty coverage on that basis.
Point out to Tesla that the provision of the Magnuson-Moss Act Require that they explain to you how the aftermarket equipment caused the problem. If they can’t – or their explanation sounds questionable – it is your legal right to demand they comply with the warranty.
Fact: If you are still being unfairly denied warranty coverage, there is recourse. The Federal Trade Commission, which administers the Magnuson-Moss Act, monitors compliance with warranty issues. Direct complaints to the FTC at (202) 326-3128.
This is the actual language of the act:
No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer’s 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; except that the prohibition of this sub-section may be waived by the Commission if:
1. the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
2. the Commission finds that such a waiver is in the public interest.
The district courts of the United States shall have jurisdiction of any action brought by the Attorney General (in his capacity as such), or by the Commission by any of its attorneys designated by it for such purpose, to restrain (A) any warrantor from making a deceptive warranty with respect to a consumer product, or (B) any person from failing to comply with any requirement imposed on such person or pursuant to this chapter or from violating any prohibition contained in this chapter.
— MAGNUSON MOSS WARRANTY ACT —
US Code – Title 15, Chapter 50, Sections 2301-2312
Here is more from the FTC’s website:
Will using ‘aftermarket’ or recycled parts void my warranty?
No. An ‘aftermarket’ part is a part made by a company other than the vehicle manufacturer or the original equipment manufacturer. A ‘recycled’ part is a part that was made for and installed in a new vehicle by the manufacturer or the original equipment manufacturer, and later removed from the vehicle and made available for resale or reuse. Simply using an aftermarket or recycled part does not void your warranty. The Magnuson-Moss Warranty Act makes it illegal for companies to void your warranty or deny coverage under the warranty simply because you used an aftermarket or recycled part. The manufacturer or dealer can, however, require consumers to use select parts if those parts are provided to consumers free of charge under the warranty.
Still, if it turns out that the aftermarket or recycled part was itself defective or wasn’t installed correctly, and it causes damage to another part that is covered under the warranty, the manufacturer or dealer has the right to deny coverage for that part and charge you for any repairs. The FTC says the manufacturer or dealer must show that the aftermarket or recycled part caused the need for repairs before denying warranty coverage.