Catalyst Monitoring

In order to control the emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a Three-way Catalytic Convener (TWC) is used. The convener 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 convened 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 to that of its corresponding post-catalyst HO2S bank 1 sensor 2, bank 1 sensor 3, and bank 2 sensor 2. A pre-catalyst HO2S bank 1 sensor 1 and bank 2 sensor 1 produces an output signal relative to the amount of oxygen present in the exhaust gas entering the three-way catalytic convener. A post-catalyst HO2S bank 1 sensor 2, bank 1 sensor 3, and bank 2 sensor 2 produces an output signal relative to the amount of oxygen present in the exhaust gas exiting the Three-way Catalytic Convener (TWC). The difference of the two values indicates the oxygen storage capacity of the catalyst. This in turn indicates the ability of the catalyst to convent the exhaust gases efficiently. A properly operating TWC 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.