I could be misinformed or remembering this wrong, but I believe the O2 sensor just before the cat. provides metering data, while the one after the cat is used as a reference against the front one, ensuring resonable validity in data presented. My font sensor was showing a slow response time referenced to the rear, enough so as to trigger a fault code. After a while, these sensors will degrade and require replacement as a poster in this thread noted as 150k miles. I don't think OBD will present a code if both sensors degrade within allowences of eachother, thus the mileage recommendation.