Catalyst Monitor Heated Oxygen Sensors

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), the system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which converts the HC and the CO present in the exhaust gas into harmless water vapor and carbon dioxide. The catalyst also converts the NOx to nitrogen.

The control module monitors this process using the post-catalyst heated oxygen sensors. The pre-catalyst oxygen sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The post-catalyst oxygen sensor produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the pre-catalyst HO2S signal will be far more active than that produced by the post-catalyst HO2S.

In addition to catalyst monitoring, the post-catalyst heated oxygen sensor has a limited role in controlling fuel delivery. If the post-catalyst HO2S signal indicates a high oxygen content or a low oxygen content for an extended period of time while in a closed loop, the control module adjusts the fuel delivery slightly in order to compensate.