SW1 L4-1.9L SOHC VIN 8 (1996)
Oxygen Sensor: Description and Operation
Rear Heated Oxygen Sensor
General Description
The heated rear oxygen sensor, mounted behind the catalytic converter monitors oxygen content in the exhaust to determine the efficiency of the catalytic
converter, The HO2S-2 is a 4 wire sensor; two which heat the sensor (B+ and ground), the other two which are the signal reference and ground. The
sensor acts as a electro chemical cell, or battery to produce a voltage in response to the oxygen content in the exhaust. A bias voltage (voltage sent from
the PCM) is between 350 mv and 550 mv which is the voltage the PCM will see when the sensor is inactive or cold. As the heater and the converter
temperature heat the sensor up around 275°C (527°F) the sensor will produce its own voltage to counteract the bias voltage. Low voltage from the sensor
indicates high oxygen content (lean mixture) whereas high voltage indicates low oxygen content (rich mixture). The wires to the sensor carry air
reference which is used to compare differences in oxygen from the atmosphere to the exhaust stream.
The rear HO2S-2 will react slower than the front O2S due to the oxygen storage in the catalytic converter. The rear oxygen sensor is used only for
converter efficiency not for A/F ratio control.
The rear oxygen sensor operates between 275°C (527°F) to 850°C (1562°F).
Circuit Operation
The heated rear oxygen sensor (HO2S-2) consists of a zirconia electrolyte between two platinum plates. When the sensor reaches approximately 318°C
(600°F) it becomes an electrical source that responds to the oxygen content in the exhaust. The PCM produces a bias voltage of approximately 450
millivolts on the oxygen sensor circuit. When the sensor is cold its internal resistance is extremely high, therefore, the PCM recognizes the bias as an
open circuit. As the sensor warms up, the internal resistance decreases. When the sensor reaches approximately 318°C (600°F), it starts producing a
voltage based on the oxygen content in the exhaust stream.
The heated oxygen sensor (HO2S-2) is mounted behind the catalytic converter. This sensor is used to determine the oxygen storage capability of the
catalytic converter. Oxygen storage capability is used as a measure of converter condition, and its ability to perform its function. Due to the location of
the rear HO2S, it requires additional heat to operate properly. This sensor incorporates a heater (12 ohms), powered by ignition voltage which allows the
sensor to remain functional under most operating conditions. In order to determine that the sensor heater is functioning, its function can only be tested
after what is considered a cold start. The PCM will monitor airflow and the time it takes for the sensor to become functional. If a start up is considered to
be a hot restart the heater function cannot be tested due to the amount of heat stored by the catalytic converter. Also if airflow after start up is too high
the sensor will not be tested due to the amount of heat generated by the engine making it difficult to accurately diagnose a heater malfunction.