With the high gas prices the 02 sensor has been a hot topic of discussion lately. Can it improve your mileage if replaced? Under certain circumstances yes it can. If you have had an antifreeze leak or any gaskets replaced or your vehicle has high mileage it can cause a problem with the 02 sensor. The anti freeze and the silicone gasket material can foul the sensor. It is a very sensitive sensor considering it sits in the exhaust system under high heat conditions. It is one of the items that should be checked during a tune up or if your mileage falls off. Usually if it causing problems it will set a check engine light but not always. A word to the wise if you are getting better than average mileage do not replace the sensor as it may decrease the mileage. Do have it checked because if it is allowing the engine to run to lean it may cause damage The 02 sensor is difficult to test without a lab scope and a special adapter that evens out the sine waves, and a tech that knows how to read them.