http://www.obdii.com/articles/Replacing_Oxygen_Sensors_Preventive_Maintenance.pdf "A good oxygen sensor is essential for good fuel economy, emissions and performance. If a vehicle with a sluggish O2 sensor is only averaging 18 mpg, and is driven 12,000 miles a year, replacing the sensor can save $100 or more a year in fuel bills if the new sensor improves fuel economy 10 percent to 15 percent (which it often can). It’s not a big savings, but there are other benefits, too. As we said earlier, bad O2 sensors are a major cause of emission failures, as well as a leading cause of catalytic converter failures. Replacing an aging O2 sensor for preventive maintenance, therefore, is recommended not only to restore peak fuel efficiency and to minimize exhaust emissions, but to also prolong and protect the life of the converter, too."