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 promotes a chemical reaction which oxidizes the HC and the CO present in the exhaust gas, converting these chemicals to harmless water vapor and to carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. Each catalyst monitor HO2S, or the post-catalyst HO2S, is always located downstream of the catalytic converter.

The PCM can monitor this process using the post-catalyst HO2Ss. The pre-sensors produce an output signal, which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The post sensor produces an output signal which indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity indicates the catalyst's ability to efficiently convert the exhaust gases. If the catalyst is operating efficiently, the pre-HO2S signal will be tar more active than the signal produced by the post HO2S.

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