I'm a little confused on which should be replaced the o2 sensors or the catalytic converter , and how many sensors there is,2 on each side or 4 on each side please help, and which side is bank 1 on,I'm getting a po420 code ,and could a bad injector cause this code to come on cause I'm getting po203 code too and a po303 should i fix the first code in order which is 203? please all help needed 2004 cadillac cts 3.6L

thanks xjoex