Need ideas on what to change beside O2 sensor,was worse,failed for hi nox in low and high range(failed last year too in high range only).Put new o2 sensor on it and took off egr and looked how dirty it was and pulled map sensor and checked it too but didnt change em.Guy at test place said catalytic convertor,this is second one on it,has 160,000 on it.May have to take to non emission state and sell it cause it still runs great,has 5.2 v-8 extended cab.Any techs feel free to chime in,thanks in advance.(91 dakota if that matters)