Will an aging after-cat o2 sensor affect MPG?

[Yuta]

i have a 02 boxster... had ECL on. The code was P0139 which means aging of after cat o2 sensor... From what I read, the pre-cat sensor adjust the mixture while the after cat sensor only check if the cat is functioning. However, my MPG is reduced significantly. Before I had around 25, then it was 18 to 22. I am confused.... which sensor should I change? Or maybe change all of them??

A few days after the ECL went on, my battery died... I had to change the battery.. after new battery was installed, the ECL was gone... However, the MPG was stil arund 20-22..... I am kinda certan the o2 sensor problem still exists,....

[jwgn777]

Hi your check engine light will most likely come on very soon again. I had the same code and checked the functions of all the sensors on my KTS 500 Porsche tester and found most of the O2 sensors slow. Depending on you mileage if its up there I would replace all the sensors. Do the job one time. I replace all the O2 sensors on my 911 because I knew I would end up under my vehicle every time I got a code for it. Mileage problems could be from the O2 sensors however many other problems can contribute to poor mileage such as clogged air filter stuck open thermostat dirty injectors and so on. JW 1999 Boxster and 2000 Carrera 911