O2 Sensor: Notes

Separate controlled sensor ground circuits are run through PCM for upstream O2 sensors. As vehicles accumulate mileage, catalytic convertor deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic convertor deterioration, fuel injection system uses two heated oxygen sensors. One sensor upstream of catalytic convertor, one downstream of convertor. The PCM compares reading from sensors to calculate catalytic convertor oxygen storage capacity and converter efficiency. Also, PCM uses upstream heated oxygen sensor input when adjusting injector pulse width.

When catalytic converter efficiency drops below emission standards, PCM stores a diagnostic trouble code and illuminates malfunction indicator lamp (MIL). The O2S produce voltages from O-1 volt, depending upon oxygen content of exhaust gas in exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture, can be caused by misfire and exhaust leaks), sensors produces a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture, can be caused by internal engine problems) it produces a higher voltage. By monitoring oxygen content and converting it to electrical voltage, sensors act as a rich-lean switch.

The oxygen sensors are equipped with a heating element that keeps sensors at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows system to enter into closed loop operation sooner. Also, it allows system to remain in closed loop operation during periods of extended idle.

In Closed Loop operation PCM monitors O2S input (along with other inputs) and adjusts injector pulse width accordingly. During Open Loop operation PCM ignores O2 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

The Automatic Shutdown (ASD) relay supplies battery voltage to both upstream and downstream heated oxygen sensors. The oxygen sensors are equipped with a heating element. The heating elements reduce time required for sensors to reach operating temperature. The PCM pulse width modulation ground side of heater to regulate temperature.