> but I've never heard of anyone having say, an alternator go bad, have an aftermarket part blamed for it, then taking the dealer to court over it and winning.

Have you ever heard of someone having an alternator go bad and then the dealer blaming an aftermarket part on it in the first place?

I've never had dealers seek out aftermarket modifications when I get service done at all. They just want to get cars in & out quickly, and they bill the manufacturer for warranty work anyway so why would they care if you had related aftermarket modifications? They only push back if they think they won't get reimbursed by the manufacturer for the warranty work. Which would tend to require "obvious" data showing modifications as a likely suspect.

Tesla changes this by having the service centers also be owned by the same company footing the bill. So this is likely to be way more common of a battle going forward, though.

Blaming aftermarket parts for non related failures is very common for luxury brands especially German ones: Audi & Mercedes are biggest culprits.

Some Audi dealers would void almost your entire power train warranty for simple things like an aftermarket cold air intake.

I think shops generally get better money when they charge you directly than when they get re-imbursed. Otherwise I could only explain their tendency to get out of warranty work as a sense of responsibility and that seems unlikely.

It depends. I believe warranty work is typically done at a flat "book rate" paid by the manufacturer based on their estimate of how long it 'should' take. If they can do it quicker, then it works out in their favor. If you read mechanic forums, they'll talk about their shortcuts for common warranty work. (like Takata airbag replacements)

I mean, I have an aftermarket stero in my car and I went to the dealer to get the blindspot sensors replaced. They honored the warrenty and replaced the sensors at no cost to me.

In your scenario, that's also exactly what small claims court is for, or also contacting the manufacturer directly. In another scenario, a ground wire went faulty and a (shadier) dealer tried to get me to pay for the cost of the diagnostic. I said I can chat with GM about that cost and they very quickly said they would waive the fee.

They can eat it, but there are examples where they don't eat it.

https://www.abajournal.com/magazine/article/sweeter_lemon_la...

my understanding is that it's largely academic. people who mod their cars tend to do so in a way that could conceivably affect many expensive parts of the car. AFAIK, the most common mods are to increase peak output of the engine, change the power curve of the engine, or to change the exhaust sound (which also affects the engine and usually requires a tune). anything that changes the behavior of the engine can be reasonably argued to contribute to failures in any part of the drivetrain. I'd guess you're right in general though. if the dealer wants to argue that they don't have to fix your power window because you're running an aftermarket exhaust, they're in the wrong, but it's probably not worth taking them to court.

In real life, we would just remove the aftermarket equipment and let the dealer sort it out.