Oxygen Sensor: Description and Operation

HO2S





The EMS has four HO2S:
^ One upstream of each catalytic converter, identified as LH and RH front HO2S.
^ One downstream of each catalytic converter, identified as LH and RH rear HO2S.

The LH and RH front HO2S enable the ECM to determine the AFR of the mixture being burned in each cylinder bank of the engine. The LH and RH rear HO2S enable the ECM to monitor the performance of the catalytic converters and the front oxygen sensors, and trim fuel.

From 2005 model year, the LH and RH front HO2S are fast light off HO2S. The fast light off HO2S are part of the changes introduced to comply with the Low Emissions Vehicle 2 (LEV2) requirements. The pre and post 2005 model year front HO2S are similar in appearance, but have different coloured electrical connectors, orange on pre 2005 model year and blue on post 2005 model year.

Each HO2S consists of a sensing element with a protective ceramic coating on the outer surface. The outer surface of the sensing element is exposed to the exhaust gas, and the inner surface is exposed to ambient air. The difference in the oxygen content of the two gases produces an electrical potential difference across the sensing element. With a rich mixture, the low oxygen content in the exhaust gas results in a higher sensor voltage. With a lean mixture, the high oxygen content in the exhaust gas results in a lower sensor voltage.

During closed loop control, the voltage of the two front HO2S switches from less than 0.3 volt to more than 0.5 volt. The voltage switches between limits every two to three seconds. This switching action indicates that the ECM is varying the AFR within the Lambda window tolerance, to maximise the efficiency of the catalytic converters.





The material of the sensing element only becomes active at a temperature of approximately 300 degree C (570 degreen F). To shorten the warm up time and minimise the emissions from a cold start and low load conditions, each HO2S contains a heating element powered by a supply from the main relay. The earth paths for the heating elements are controlled by the ECM

On start up, the current supplied to the heating elements is increased gradually to prevent sudden heating from damaging the ceramic coating. After the initial warm up period the ECM modulates the earth of the heating elements, from a map of engine speed against mass air flow, to maintain the HO2S at the optimum operating temperature.

The nominal resistance of the heating elements is 6 W at 20 degree C (68 degree F).

If an HO2S fails, the ECM illuminates the MIL. If a front HO2S fails the ECM also adopts open loop fuelling and catalytic converter monitoring is disabled. If a rear HO2S fails, catalytic converter and front HO2S monitoring is disabled.