I think the sky is blue and my monitor is huge! But what "we" think has nothing to do with any of this or your problem.
Its very simple, take the vehicle in, if they "tested" it and couldn't find anything wrong other than your plugs going bad. Then thats that. If the vehicle was stock... new.. old.. doesn't matter, if they can't re-produce or find a problem, they won't work on it... Hard to work on a problem you can't find eh?
Anyway... If, there is a problem, you or they know what it is.. and THEY refused to work on the vehicle because of your "mods" have them put it in writing that they refused warranty service based on "x" you then tell them, you are taking the vehicle to get inspected at serveral other shops. If all those shops agree that your mobs have nothing to do with the problem. You then ask nicely for them to fix it under warranty or you will be forced to take legal action. bla bla bla...
Usually by the time you've had other shops look at it... and tell them you'll be taking legal action to have the warranty honored and legal fees covered... They'll give in.
Or, you could bend over... accept it... remove all the custom mods (which you hope are just bolt-ons)... and take it back in. The customer isn't always right anymore... however the customer.. if making a big enough stink will usually get his/her way still. You notice how those stupid stuck up bitches who cause a fit in line or when ordering food always seem to get "special" treatment... If being nice a polite isn't working... Let them know you're willing to force them to make good on the legal contracts you signed.