Great concept for a sticky - unfortunately, much of it is wrong because the OP doesn't understand the MM act.
This is what I posted on another board a few years back:
Let me try to explain mods and warranties in logical layman's term. I'm sure that some will come up with all sorts of nebulous arguments but they are not real world.
1. The oft misquoted MM act has nothing to do with performance parts. It was enacted to keep manufacturers from requiring the use of a specific brand of replacement part in order to keep the warranty intact.
Accordingly, the use of a "substantially similar" replacement is OK and the manufacturer must show that the "substantially similar" part caused the failure if they refuse warranty service for that failure.
In practical terms, this means that you can substitute a Osram H7 bulb for the Bosch H7 that the car originally came with.
2. Let's look at performance parts for a moment. By definition, performance parts are not "substantially similar" as they are intended to alter the performance characteristics of a system. Based upon this, performance parts fall outside the scope of the MM act.
In order to refuse warranty service for a failure, the manufacturer (legally) merely needs to show that a part that is not "substantially similar" was used and that this part could have caused the failure. At this point, the burden of proof shifts to the consumer to prove that the part did not cause the failure.
Going back to out lightbulb example, if you have an electrical failure and you've replaced your 55w bulbs with 85w "hyperwhites" or LED's then the dealer is under an obligation to the manufacturer to refuse warranty service on the failure if they believe that the performance bulbs caused the failure.
Simple - right?
Now, let's talk warranty a moment.
3. The term "void the warranty" is often bandied about by both consumers and dealers alike.
Very few things will actually "void the warranty." This was alluded to by other posters but not fully explained. If a car is so badly abused, or modded to the point where virtually every system failure can be traced back to a mod, then the manufacturer can "void the warranty." A flood damaged car or one that was in a severe accident might also be candidates for the manufacturer to void the warranty on the entire car.
Mitsubishi voided the warranty on a bunch of cars that they saw were used at racetracks.
What is commonly referred to as "voiding the warranty" actually refers to a dealer refusing warranty service on a particular service.
Quite simply, if a dealer believes that a modification, chip, performance part, abuse or whatnot caused (or substantially contributed) to a failure, then he has an obligation to the manufacturer to refuse warranty service on that failure.
Accordingly, using our lighting example again, the dealer could refuse warranty service on any electrical failure. This could actually extend to ECUs or the entire electrical system.
Farfetched? Not really. A few years back a guy on VWVortex decided to paint his sidemarker bulbs silver on his brand new Jetta.
The bulb heated up, melted the paint and dripped into the housing. The housing melted and shorted out parts of the wiring loom. Since the Jetta uses a CAN-BUS, the whole thing needed replacement $2500 later the car was on the road again.
4. Why doesn't the dealer give the customer the benefit of the doubt? The manufacturer often asks for parts back to determine the cause of the failure. They are extremely vigilant for failures that often are seen on modded cars.
Fell free to argue until you're blue in the face - but - like it or not, the above is the way it is.
You can always find a lawyer who will argue a different viewpoint, but that doesn't mean it is right.
So, what is the bottom line?
ANY mod may cause warranty woes. Consider ALL the implications of any mod you are contemplating and be mentally and monetarily prepared to pay for the consequences.
Read your owner's manual carefully. It specifically warns against engine (ECU) and suspension mods as examples of mods that will cause problems.
Feel free to mod away, but remember that you might have to pay to play. The other posters who say "just do it" aren't the ones that will have to foot the bill.
As a clarification - there is some heated discussion about the manufacturer's burden of proof regarding the use of "performance parts" but the bottom line remains the same.
In addition, if the manufacturer allows a case to go to court then there is a good bet that he has sufficient evidence to back up their claim.