I got me an OBD2 adapter and have it working with Car Scanner. How do I know if I have a bad O2 Sensor? I'm not familiar with these numbers / app on how to read it. What are the numbers suppose to be like? I'm still getting that P0420 On Bank One with the first scanner i got from amazon but it doesn't tell me if is sensor one or two. I want to make sure that my sensors are good to go before changing anything else that I have already, since I have change everything from Cats to sensors to exhaust spark plugs you name it and also got burned by auto shops by a couple of grands and still getting the code and every time i go to a shop the first thing they say is change the cat with no further diagnostic. I know about the O2 extenders but Not quite sure I wanna go that way maybe just temporarily to pass emissions but not permanently. Any expert on subject matter?

Thanks

Jay