[INF3RN0666]

If your old O2 sensor set a code in the computer or is giving a high O2 reading, then your car would have poor gas mileage as it is. But if your computer hasn't noticed any fluctuations in the O2 sensor reading, then the sensor is most likely fine. What happens is that the sensor starts spitting out random numbers that are out of the possible range and the computer will clue in. Once the check engine light comes on, your computer will start using the standard fuel mixture in all modes of acceleration. THEREFORE, you will get bad fuel mileage. Now if your current sensors are fine, then the only way these new sensors will give you better gas mileage is if the indicate a lower O2 reading in the exhaust. What happens then is that the ECM will decrease the fuel going to the engine. However, that will shorten the life of your engine because you're running lean.