Oxygen Sensor: Description and Operation

IMPORTANT: Only early model year production are equipped with a Pre-Converter (B1S2) Heated Oxygen Sensor (HO2S). It equipped, this HO2S is connected to the PCM and vehicle electrical system but is no longer monitored or diagnosed.

The three or four oxygen sensors are mounted in the exhaust system where they monitor the oxygen content in the exhaust stream. There is an oxygen sensor mounted in each exhaust manifold and one on the downstream end of the catalytic converter. Some early production vehicles also have an oxygen sensor located at the front of the catalytic converter (Pre-converter). The oxygen sensor located in the front exhaust manifold is the Bank 2 Sensor 1 (Front bank) sensor. The oxygen sensor located in the rear exhaust manifold is the Bank 1 Sensor 1 (Rear) sensor. The oxygen sensor located in the Y-pipe ahead of the catalytic converter is the Bank 1 Sensor 2 (Pre-converter) sensor. The oxygen sensor located in the catalytic converter outlet is the Bank 1 Sensor 3 (Post-converter) sensor.

An oxygen sensor acts like a battery because it creates its own signal voltage once it reaches operating temperature. This voltage is produced when the oxygen content in the exhaust stream is different than the oxygen content in the atmosphere. A lean condition (high oxygen content in exhaust) will produce a low voltage (near 0 volt) and a rich condition (low oxygen content in exhaust) will produce a high voltage (near one volt). The PCM provides a reference signal voltage (0.45 volt) and a ground to the sensor. The PCM reference voltage is necessary because the oxygen sensors do not provide their own voltage until they reach operating temperature.

The oxygen sensors also incorporate a heating element inside of the sensor housing. This heating element is energized with the ignition On and allows the sensors to reach operating temperatures quickly. The PCM can then use oxygen sensor information sooner after engine startup.

The PCM uses oxygen sensor information during Closed Loop operation to constantly adjust fuel control to reduce exhaust emissions. Because the oxygen sensors provide information sooner after engine startup, exhaust emissions are reduced. Faulty oxygen sensors will cause various driveability conditions and will set a DTC.