Theory Of Operation

For an aged Upstream O2 Sensor, the response rate to the air/fuel change is slower than when it was new. The O2 Sensor tends to move less with the same air/fuel changes in a given time frame. Therefore by observing the activity of voltage readings from the Upstream O2 Sensor, the quality of the O2 Sensor can be detected. The Downstream O2 Sensor is located in the exhaust path behind the Catalytic Converter, and is monitored for proper response to assure optimum Catalytic Converter efficiency. The Downstream O2 Sensor response monitor is intended to diagnose a Downstream O2 Sensor that is not moving or stuck in a voltage window and to insure accurate information for catalyst monitor diagnosis.