Rear Heated Oxygen Sensor

REAR HEATED OXYGEN SENSOR (REAR HO2S) (DTC: P136)
The Rear Heated Oxygen Sensor (Rear HO2S), after the three way catalyst, monitors the oxygen level in the exhaust gas. Even if the switching characteristics of the front heated oxygen sensor* shift, the air-fuel ratio is controlled to stoichiometric by the signal from the rear heated oxygen sensor.
This sensor is made of ceramic titania. The electric resistance of ceramic titania drastically changes at the ideal air-fuel ratio. The output voltage of the sensor, depending on its resistance, is approximately 0 to 2.2 volts.
The Engine Control Module (ECM) supplies the sensor with approximately 2 volts and then measures the output voltage. About 0 volts means lean and about 2 volts means rich.
Under normal conditions, the rear heated oxygen sensor is not used for engine control operation.
It is used only for the on-board diagnosis of three way catalyst.

*: Oxygen sensor (M/T models except for California)
Heated oxygen sensor (M/T models for California and A/T models)