Downstream Oxygen Sensor 1/2

The downstream heated oxygen sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic convertor efficiency. If the oxygen leaving the catalyst is too lean (excess oxygen), the PCM increases the upstream O2 goal, which increases fuel in the mixture, causing less oxygen to be left over. Conversely, if the oxygen content leaving the catalyst is too rich (not enough oxygen) the PCM decreases the upstream O2 goal down which removes fuel from the mixture causing more oxygen to be left over. This function only occurs during downstream closed loop mode operation.