Oxygen Sensor: Description and Operation

Oxygen Sensor:

Oxygen Sensor (O2S 1)
The Oxygen Sensor (O2S 1) is mounted in the exhaust system where it can monitor the oxygen content of the exhaust gas stream. The oxygen content in the exhaust reacts with the sensor to produce a voltage output. This voltage ranges from approximately 0.1 volt (high oxygen - lean mixture) to 0.9 volt (low oxygen - rich mixture). This voltage can be measured with a scan tool.

By monitoring the voltage output of the O2S 1, the PCM will know what fuel mixture command to give to the injectors.
- Lean mixture-low O2S 1 voltage rich command
- Rich mixture-high O2S 1 voltage lean command


Rear Heated Oxygen Sensor (HO2S 2)

Rear Heated Oxygen Sensor:

The Rear Heated Oxygen Sensor (HO2S 2) is used to monitor the oxygen storage capability of the catalytic converter. Similar to the O2S 1, the HO2S 2 reacts with the oxygen content in the exhaust stream to produce a voltage signal, only after the catalytic converter.

This voltage ranges from approximately 0.1 volt (high oxygen-lean mixture) to 0.9 volt (low oxygen-rich mixture). Unlike the O2S 1, with a good catalytic converter, the HO2S 2 is normal when its activity appears lazy or inactive. The PCM compares readings from both the front O2S 1 and the HO2S 2 to determine the catalyst efficiency and will illuminate the MIL if this efficiency falls below a calibrated level.