Originally Posted By: 92saturnsl2
I always wondered this, glad I saw this post & feedback.
Due to cost, I usually wait till I see a sign of failure (CEL primarily). I think modern fuel injection (the computer) is a lot better at narrowing things down than it was 15+ years ago. I used to hear the recommendation that you should replace them every 50k, but that was years ago and coming from auto parts stores and mechanics.
FWIW, the rear sensor simply monitors emissions performance (what's coming out after the cat) and has little or nothing to do with how the ECU/EFI behaves. The upstream sensor on the other hand, directly tells the ECU whether the mixture is rich or lean--- I would replace the upstream sensors first, if I felt I needed to include oxygen sensors as part of preventative maintenance.
I really would prefer to catch it before it causes a misfire so that I can avoid contaminating the catalytic converter with a rich condition.
I'd only have to replace the upstream sensor for the reasons you mentioned. My Buick only has two oxygen sensors, which makes it a lot more economical to replace than some of the other cars that have four oxygen sensors.