Oxygen Sensor: Description and Operation

Primary and Secondary HO2S (Heated Oxygen sensors)
The heated oxygen sensors detect the oxygen content in the exhaust gas, then sends signal to the ECM/PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensors have an internal heater. The primary sensor is installed in the exhaust pipe.

The secondary sensor is installed in the TWC.

By controlling the air/fuel ratio with sensors, the deterioration of the primary sensor can be evaluated by its feedback period. When the feedback period exceeds a certain value during stable driving conditions, the sensor is considered deteriorated and the ECM/PCM sets a DTC.