Emission Control Systems: Description and Operation

In order to control the emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The converter changes the HC and CO present in the exhaust gas into harmless water vapor and carbon dioxide through a reaction with the catalyst elements. Similarly, NOx is converted into nitrogen and oxygen.

The VCM monitors this process by comparing the activity of a pre-catalyst (HO2S Bank 1 Sensor 1 and Bank 2 Sensor 1) oxygen sensor to that of its corresponding post-catalyst (HO2S Bank 1 Sensor 2, Bank 1 Sensor 3, and Bank 2 Sensor 2) oxygen sensor. A pre-catalyst (HO2S Bank 1 Sensor 1 and Bank 2 Sensor 1) oxygen sensor produces an output signal relative to the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. A post-catalyst (HO2S Bank 1 Sensor 2, Bank 1 Sensor 3, and Bank 2 Sensor 2) oxygen sensor produces an output signal relative to the amount of oxygen present in the exhaust gas exiting the three-way catalytic converter. The difference of the two values indicates the oxygen storage capacity of the catalyst; this in turn indicates the ability of the catalyst to convert the exhaust gases efficiently. A properly operating three-way catalytic converter's pre-catalyst oxygen sensor produces a far more active signal than that of its corresponding post-catalyst oxygen sensor.

Fuel trim values are based upon the HO2S sensor values received by the VCM during closed loop. The VCM adjusts the fuel trim in order to ensure that the fuel delivery is correct for catalyst monitoring.