Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item. The upstream replacement--typically done around the time of other items such as trans flush, ECT replacement, filters, spark plugs, etc--often results in slightly higher fuel mileage. (Stating all of that because it obviously doesn't directly implicate the replacement of the o2 sensors as the cause of improvement.)
Which leads to my question: do the downstream sensors ever matter for fuel injection calculation, or are they simply only there to indicate a bad catalyst? This has always been my understanding, and I've gotten up to around 250k miles without replacing downstream sensors.
Which leads to my question: do the downstream sensors ever matter for fuel injection calculation, or are they simply only there to indicate a bad catalyst? This has always been my understanding, and I've gotten up to around 250k miles without replacing downstream sensors.