General Information:
“Starting in 1996, all vehicles use at least one Upstream O2 sensor and one Downstream O2 sensor (fig. 16). An O2 sensor provides the PCM with a voltage signal (0-1 volt) inversely proportional to the amount of oxygen in the exhaust. If the oxygen content is low, the voltage output is high; if the oxygen content is high, the output voltage is low. The PCM uses this information to adjust injector pulse-width to achieve the air/fuel ratio necessary for proper engine operation and to control emissions.”
“The downstream sensor is located just after the catalytic converter, or between the catalyst biscuits, and produces a similar signal input to the PCM that is used for two purposes. One function is to verify catalytic converter efficiency as a requirement of OBD II diagnostics. The other function is to provide fuel correction information based on actual catalytic converter output.”
Downstream O2 Sensor:
“As mentioned previously, the downstream O2 sensor has two functions. One function is measuring catalyst efficiency. This is an OBD II requirement. The oxygen content of the exhaust gases leaving the converter has significantly less fluctuation than at the inlet if the converter is working properly. The PCM compares the upstream and downstream O2 sensor switch rates under specific operating conditions to determine if the catalyst is functioning properly.
Another function is to help establish the upstream O2 sensor's goal voltage. While the upstream O2 sensor input is used to maintain the 14.7:1 AF, variations in engines, exhaust systems and catalytic converters may cause this ratio to not be the
most ideal for a particular catalyst and engine. To help maintain the catalyst operating at maximum efficiency, the PCM fine-tunes the A/F ratio entering the catalyst based on the oxygen content leaving the catalyst. This is accomplished by modifying the upstream O2 sensor goal voltage.
In the past this goal was a preprogrammed fixed-value based on where it was believed the catalyst operated most efficiently. With the new downstream O2 sensor fuel control, the upstream O2 goal is moved up and down within the window of operation of the O2 sensor. If the oxygen content leaving the catalyst is too high (excess oxygen) the PCM moves the upstream O2 goal up, which increases fuel in the mixture causing less oxygen to be left over. Conversely, if the oxygen content leaving the catalyst is too low (not enough oxygen) the PCM moves the upstream O2 goal down which removes fuel from the mixture causing more oxygen to be left over.”
