[redsoxsstink]

2002 camro 3.8, pacesetter headers, stock cat/exhaust

i am throwing a P0420 code (catalyst efficiency bank 1) i know it could be either o2 sensors or my cat going bad. i need to pass inspection next month so im trying to figure out what is the easiest way to fix this and still have the car ready for me to drive everyday.

if i were to replace the o2 sensors which would i do? upstream or downstream? or both?

if the sensors dont fix the issue, i can try a o2 sim, which would be the best to go with?

o2 sensor spacers? opinions? do they really work?

im thinking its my o2 sensors is the problem, the cat isnt glowing or making noise so i dont think it would be clogged. i know it still could be bad even without these things. im also getting junky gas millage, i normally get 30mpg highway i just averaged 21 over course of 200miles.

i rather fix it the right way and no use sims and stuff but im not opposed to it just to pass inspection. just looking for some guidance before i just start throwing parts at it