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At 140 k miles (since sold) my 99 Maxima had a CEL for EGR flow. If you are going to clean the EGR passageways, you might as well take a systematic approach and clean everything in the throttle control area. EGR and PCV combine to create a tar like substance in the induction system and this causes most of the problems. I consider it a maintenance issue since you can do nothing to mitigate the accumulation except regular oil maintenance and possibly the use of an effective fuel injection system cleaner. The older pintle type injectors were especially susceptible to accumulations of deposits, but it seems like the advent of more modern designs greatly reduced the deposits.
In Nissans the hot wire MAFs had the same issues as the O2 sensors. Honda seemed to not have the same issues with their MAP sensors. While air mass detection on the intake side determines base mixture ratios, the O2 sensor is the final arbiter or mixture control. In my experience, primarily Nissans, by the time you got a CEL for an oxygen sensor the mileage drop approached 25%. It is not really possible to precisely predict the mileage difference between a new sensor and the unknown quality of an older sensor. It may be possible to determine that with a scangauge, but without a known perfect host for a baseline determination then you are still guessing.
I think the recommendations of others in this thread are probably good. I think my Ranger's sensor is still functioning properly at 125k but suspect it is probably near it's practical limit. I do not know if it is original or not but it looks to be original. I'll probably replace it in the spring when I go to the next step in trying for best mileage compared to the current tank at 33.34 MPG.
In a Honda Civic VX the oxygen sensor is a critical part of the mileage equasion and there still seems to be a difference between OE and any aftermarket sensors.
I always price OE compared to aftermarket but I still get 25% off OE parts so that makes the difference lower and even makes OE parts less expensive in my case.
Hope that helps, but I also hate to recommend any replacement part to anyone unless their is good evidence that the expense is justified.
regards
Mech
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