New to the forum and cant seem to find any info on this code, I will go back and look again but when I searched on here I got google search results. My car went for a smog with the codes wiped off the night before. When I got the smog it passed with flying colors out the pipe but could not complete the smog because the sensors were not reset yet. The code is p0420, all roads lead to the cat but my question is why would it be blowing clean air if bad. Any advice will be greatly appreciated.