Manifold/header/muffler/tail Pipe problems of the 2010 Mazda MAZDA3

Within a week after purchasing my 2010 Mazda 3 s sport sedan in October, 2018 from a used car dealer in midvale, ut, the check engine light came on. Using a code reader, I consistently get a code of p0421, which indicates that the catalyst efficiency is below threshold. I have had 3 auto repair shops look at my car, and they all say that the catalytic converter is not operating properly and needs to be replaced. Despite the fact that Mazda warrants California emissions/pzev vehicles (which mine is) for a period of 15 years or 150,000 miles on all emissions related components, they will not repair my car under warranty because I currently do not live in a state that uses CA emissions standards when testing vehicle emissions. I feel that since my car is not performing as it was designed and may be releasing my pollutants into the atmosphere that it should be repaired by the manufacturer at no charge to me, regardless of where I currently live. In addition, when I start the car on a cold engine, I can smell exhaust fumes inside the cabin, which leads me to believe that there may be a leak at the exhaust manifold gasket which could be dangerous to anyone driving the car if they inhale the exhaust gases that may enter the passenger area through the ventilation system. The check engine light continues to go on daily (unless I reset it, and then it goes back on again later after driving) and my car will not pass its next emissions inspection without this issue being fixed.