Don't worry about that light. The light comes on every 30k miles (I think) and is just a reminder to have the O2 sensor checked; it does not mean that the sensor is bad. There is a black box behind the instrument cluster with a cable coming out from it to the speedometer. On that box, there is a button you can push to reset the timer, which will turn the light off until 30k miles down the road.

People often take the bulb out rather than reset the box.
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--Charles
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