Really not up to them. I have a very good friend of mine who is (or was) very much into mustangs. The dealer refused warrenty work based on mods. Called SEMA, and ended up winning a very healthy settlement from the dealership and the svc manager lost his job. Now writes articles for an auto mag. If you don't know better and they tell you that your SOL then you are. If you understand what the law is the dealer that tried to lie to you and violate the law is SOLThat all sounds good in theory but the bottom line is they decide if your mod voids your warranty or not. I have dealt with a few warranty issues at work in and it's very easy to find reason to warranty something or not warranty something! If they think your mod caused the failure I promise you that you are SOL!