Yes the law stated dealer must demonstrate aftermarket part caused the failure. But we all know how this goes. Dealer denies service and it becomes your responsibility to get a lawyer and force their hand unfortunately.
Some dealers are honest and know what they are doing and some blow. Haven’t had a need for a dealer visit but when I drove GM I had a relationship with the service advisor and mechanics. I would frequently visit them working on my trucks, shoot the bull, and flip him a $20 or so. I never had a drivetrain warranty denied and all my vehicles have been modded while under warranty.
Having said that I never requested warranty coverage when I blew my heavily modded CTSV motor at Road America because it was pushing 800rwhp
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