Oxygen Sensor

Yes it determines how rich or lean the engine is running, and if not reading properly it will run rough. Recomended changes are 25,000-50,000

plus with a good sensor gas milage can increase up to 15% and at the pump that definatly pays for the 02 sensor.

The O2 sensor does not usually work as soon as you start the engine from cold. So the O2 sensor is not in the picture as of yet, the ECM is said to be in open loop. Once the O2 sensor gets hot enough to start producing a voltage the ECM sees this and then goes into closed loop and the O2 sensor controls A/F ratio. Has a lot to do with if you have a 1 wire sensor or a multiple wire (heated sensor) as to how soon it starts working after start up
Darryl.