Here's the legal standing on consumer warranties in the US market:A sticky about warranty and aftermarket parts would be helpful....
Auto Warranties & Routine Maintenance | Consumer Information
Doesn't mean a dealer won't give you a big hassle should you show up for service with aftermarket parts looking for warranty coverage. It's always best to establish your dealer's tolerance for aftermarket parts and make a decision how to proceed. Sure, the law may be behind you, but is it worth arbitration or court action to get a dealer or PCNA to cover major repairs due to something like a catback exhaust?