Oxygen Sensor: Description and Operation

OXYGEN SENSOR (O2S)-PCM INPUT
Two heated O2S sensors are used. The 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.


In Closed Loop operation, the PCM monitors the O2S 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 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.

The Automatic Shutdown (ASD) relay supplies battery voltage to both the 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.

UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located near the inlet end of the catalytic converter. It provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune the air/fuel ratio by adjusting injector pulse width.

DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located near the outlet end of the catalytic converter 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.

When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Emission Control Systems.

CIRCUIT OPERATION
When the Automatic Shut Down (ASD) relay contacts CLOSE, they connect circuits A16 and A142. Circuit A142 splices to supply voltage to the heated oxygen sensors.

Circuit K141 delivers the signal from the center heated oxygen sensor to the PCM. Circuit K141 connects to cavity A24 of the Powertrain Control Module (PCM). Circuit K341 delivers the signal from the rear sensor and connects to cavity A25 of the PCM.

Circuit K241 delivers the signal from the right heated oxygen sensor to the PCM. Circuit K241 connects to cavity A26 of the Powertrain Control Module (PCM). Circuit K441 delivers the signal from the left sensor and connects to cavity A25 of the PCM.

The PCM provides a ground for the heated oxygen sensor signals (circuits K141 and K341) through circuit K4. Circuit K4 connects to cavity A4 of the PCM connector.

Circuit Z1 provides a ground for the heater circuit in the sensors.