Oxygen Sensor: Description and Operation

Fig. 6 Heated O2 Sensors:

UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located in the exhaust downpipe (Fig. 6). It provides an input voltage to the Powertrain Control Module (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 convertor (Fig. 6). 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).


CIRCUIT OPERATION
When the Automatic Shut Down (ASD) relay contacts close, they connect circuits A16 and F142. Circuit F142 supplies voltage to the upstream and downstream heated oxygen sensors.

Circuit K41 delivers the signal from the upstream heated oxygen sensor to the Powertrain Control Module (PCM). Circuit K41 connects to cavity A24 of the PCM. Circuit K141 supplies the signal from the downstream heated oxygen sensor to the PCM. Circuit K141 connects to PCM cavity A25.

The PCM provides a ground for the heated oxygen sensor signals (circuits K41 and K141) through circuit K167. Circuit H167 connects to cavity A4 of the PCM connector.

Circuit Z1 provides ground for the heater circuit in each sensor.