shouldnt matter that you are the second owner, you are still covered on the fed warranty, call chevy

 

Is it possible there could be something wrong with the O2 sensor?

 

Why don't you try switching the rear O2 sensors and see if the code follows the sensor to the driver side in which case youll throw a different code. Make sure to reset the pcm before you do this to clear the previous code. If you still throw a code for the passenger side then it is probably the cat.

 

You can try swapping the rear 02's side to side and see if it throws P0420 for the drivers side, but my money is on a bad cat.

 

Federal emission mandate 8 years/80k regardless of owner. Even if you are the 4th owner and fall in any warranty they will do it if the part fails and is subjected to the warranty.

I threw a P0420 code(catalyst efficiency below threshold bank 1) and ended up being the cat as on the highway, the car would lose power and the rpms would raise up.