Circuit Description

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), a Three-Way Catalytic (TWC) converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx, converting these chemicals to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This Oxygen Storage Capacity (OSC) is a measurement of the catalysts ability to control emissions. The Powertrain Control Module (PCM) monitors this process using a Heated Oxygen Sensor (HO2S) located in the exhaust stream past the TWC converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst is considered to be bad. The Malfunction Indicator Light (MIL) will be illuminated if subsequent tests also indicate a failure.