Oxygen sensors P0420 code replace 02 sensors an cats does a dealer have to do it?

 

Best to start by understanding what this code is telling you. The cat efficiency code is saying the computer is sensing too much of a difference between the front o2 sensor (upstream "lambda" sensor that controls fuel trim) and the downstream o2 sensor after the cat (ie too much o2). This can be caused by a sensor or its wiring, the cat or exhaust leak. Considering the cats can be quite expensive, best to make sure there's no other contributing issues first. Dealers don't have to do the work but keep in mind there's no guarantee an after market part will fit quite right and correct the code. One other option (depending on where you live and your state's emissions rules) is to try a cel-boss spacer. This inserts between the rear sensor and the plug to move the sensor away from the air flow -reducing the amount of o2 detected. If you car is capable of passing emissions as is, this may be a viable solution.

Also, you didn't mention which engine or how many miles on your car. The concern here is whether something causing the contamination in the cat or on the sensors. Any signs of the car consuming oil or coolant?

 

3.2L engine? Bank1 cats need to be replaced is my guess, but it would be good to have a trained tech check it out before forking out that much money.

From direct experience, replacing O2 sensors is not always the solution, the sensors are doing their job to alert a problem. Trying to avoid replacing to source of the problem can be $$$ wasteful.