Oxygen Sensor: Description and Operation

HEATED OXYGEN SENSOR (O2S SENSOR)-PCM INPUT
As vehicles accumulate mileage, the catalytic converter deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors in each side of the exhaust system (4 total). One sensor upstream of the catalytic converter, one downstream of the converter. The PCM compares the reading from the sensors to calculate the catalytic converter oxygen storage capacity and converter efficiency. Also, the PCM uses the upstream heated oxygen sensor input when adjusting injector pulse width.

When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL).

The O2 sensors produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture), the sensors produces a low voltage. When there is a lesser amount present (rich air/fuel mixture) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

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

In Closed Loop operation the PCM monitors the O2 sensor input (along with other inputs) and adjusts the injector pulse width accordingly During Open Loop operation the PCM ignores the 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 all upstream and downstream heated oxygen sensors. The oxygen sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature.

The input from the upstream heated oxygen sensor tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air-fuel ratio by adjusting injector pulse width.

The downstream heated oxygen sensor threads into the outlet pipe at the rear of the catalytic converter. The downstream heated oxygen sensor input is used to detect catalytic converter deterioration. As the converter 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 converter efficiency.

CIRCUIT OPERATION
There are four heated oxygen sensor used on this vehicle. Power for the upstream heated oxygen sensors is supplied on circuit A142 when the contacts in the Automatic Shut Down (ASD) relay are CLOSED. Battery voltage for the contact side is supplied on circuit A14. This circuit is HOT at all times and protected by a 20 amp fuse located in cavity 15 of the Power Distribution Center (PDC).

Power for the downstream heated oxygen sensors is supplied on circuit F142. This circuit is HOT when the contacts in the ASD relay are CLOSED and is protected by a 15 amp fuse located in cavity 1 of the PDC. Power for the fuse is supplied on circuit A142.

Power for the coil side of the ASD relay is supplied on circuit F12. This circuit is HOT when the ignition switch is in the START or RUN position. Ground for the coil side of the relay is controlled by the Powertrain Control Module (PCM) on circuit K51.

When the PCM supplies a ground path for circuit K51 the contacts in the ASD relay CLOSE connecting circuits A14 and A142. Circuit A142 powers the heated oxygen sensors and is spliced internal to the PDC to supply an ASD relay energized input to the PCM.

Circuit K241 delivers the signal from the left upstream heated oxygen sensor to the PCM.

Circuit K41 delivers the signal from the right upstream heated oxygen sensor to the PCM.

Circuit K141 delivers the signal from the left downstream heated oxygen sensor to the PCM.

Circuit K341 delivers the signal from the right downstream heated oxygen sensor to the PCM.

The PCM provides a ground for the left and right heated oxygen sensor signals through circuit K4. Circuit K4 connects to cavity A4 of the PCM connector.

Circuit Z12 provides the ground path for the heater circuits in the sensors.