Before we go too far, I wonder, based on the posts I've seen, if anyone besides myself has read Magnuson Moss end to end.
First, the following - which was posted above as being part of the Act, is not part of the Act but someone else's quasi-introductory text which was posted on another Web site.
US Code - Title 15, Chapter 50, Sections 2301-2312
Legally, a vehicle manufacturer cannot void the warranty on a vehicle due to an aftermarket part unless they can prove that the aftermarket part caused or contributed to the failure in the vehicle (per the Magnuson Moss Warranty Act (15 U.S.C. 2302(C)) . For best results, consider working with performance-oriented dealerships with a proven history of working with customers. If your vehicle manufacturer fails to honor emission/warranty claims, contact EPA at (202) 260-2080 or www.epa.gov. If federal warranty protection is denied, contact the FTC at (202) 326-3128 or www.ftc.gov. For additional information, check out the following links:
1. Consumers Bill of Rights
2. What You Can Do If Your Warranty Is Denied
3. Federal Warranty Laws
4. Vehicle Manufacturer Warranty Contact Phone Numbers
So now that we've gotten that out of the way, I'll explain the Act and why the Act itself has nothing to do with the subject of modifications (if you don't believe my statement, other than what I quoted above is the actual text of the act - there is no language contained therein that addresses this).
Magnuson Moss is the federal law that governs consumer warranties.
What seems to confuse 99% of the population is that it covers tie-in sales provisions, specifically not allowing them. Tie-in sales provisions have little in common with mods.
A tie-in sales provision would mean that the seller requires the purchaser of the warranteed product to buy an item used with or for the product FROM the seller or a specified company in order to be eligible to receive a remedy under the terms of the warranty.
If company x sells photocopy machines, and company x requires you to use ONLY company x toner to ensure continuing warranty coverage, that is a tie-in. Such a tie-in is not permissible. Company x cannot restrict your use of toner from third parties. However, if you use the wrong toner and the machine breaks (and this is NOT covered by the Act, this is just common sense), you have no warranty to speak of.
A mfr. can indeed void a warranty if the purchaser modifies whatever it is they have purchased. Not allowing the mfr. to void the warranty would be inane. But in practice it generally has to be shown that the modification in some way precipitated or was connected with the failure. The example someone gave of putting blackline lights on a car that then has an engine failure is a great example of this, but it has nothing to do with the Act.
My father was involved in the development of Magnuson Moss. The warranty from his company (Spiratone) was used as one example of a a good warranty and he gave testimony in open Congressional hearings on the subject.