Downstream Oxygen Sensor
WARNING: This page is about a different car, the 2006 Chrysler Crossfire. However, it is still accessible from the selected car via links, so may be relevant.
The downstream heated O2 Sensor signal is used to detect catalytic convertor deterioration. As the convertor deteriorates, the signal from the downstream sensor begins to match the upstream sensor signal except for a slight time delay. By comparing the downstream heated oxygen sensor signal to the signal from the upstream sensor, the PCM calculates catalytic convertor efficiency. This calculation is also used to establish the upstream O2 goal voltage (switching point).