Rear Heated Oxygen Sensor

The rear heated oxygen sensor (Rear HO2S), after three way catalyst, monitors the oxygen level in the exhaust gas. Even if switching characteristics of the front heated oxygen sensor are shifted, 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 electrical 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 2.2V.
Under normal conditions the rear heated oxygen sensor is not used for engine control operation.

ON-BOARD DIAGNOSIS LOGIC
The rear heated oxygen sensor has a much longer switching time between rich and lean than the front heated oxygen sensor. The oxygen storage capacity before the three way catalyst causes the longer switching time. To judge the malfunctions of rear heated oxygen sensor, ECM monitors the sensor's voltage value and the switching response during the various driving condition such as fuel-cut.