2007 Nissan Maxima P0420 code

They replaced the cat under your car for that code?For that code it only show's if it was a bad cat causing that code the 3 way manifold cat on the engine not the cat under the car.

Catayltic converters rarely fail and even when they do its normally because of something else such as a faulty oxygen sensor. I wouldn't be surprised if the oxygen sensor(s) are not at fault here causing the trouble code to appear.