2002 Cadillac Seville

How long does it take to reset the p0420? My question to you is, are you going with a budget converter from an exhaust shop? On the average, those may get a shorter period of good performance, and that is what keeps the cost down. The p0420 is relatively straight-forward. If the pre-cat oxygen sensor is transitioning in regards to fueling strategy, and the post-cat sensor is making minimal changes, then the catalyst is deemed to be functioning. If the pre-cat, and the post-cat sensors are basically shadowing one another, then the catalyst is not performing well, i.E. Not oxidizing HC and CO emissions. If the P0420 is the only code you are getting, you would need to compare the O2 readings on a graph, to determine if the cat is functioning. If you are getting lean or rich codes, in addition, or at times, you may be creating a condition to cause the cat to have a shorter life.