I have a 2001 Millenia with a 2.5l engine. I was wondering how often does the o2 sensors go out? It seems that my cel comes on at least once a year maybe twice a year and it reads p0421 Warm up catalyst below threshold bank 1, and p0431 Warm up catalyst below threshold bank 2. Now keep in mind that I have replaced all 4 o2 sensors twice and the cat closest to the radiator once.