Circuit Description

WARNING: This page is about a different car, the 2003 GMC Sonoma and 2003 Chevrolet S10 Pickup. However, it is still accessible from the selected car via links, so may be relevant.

A Three-Way Catalytic Converter (TWC) is used in order to control the exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides Of Nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas. This will convert the HC and the CO into a harmless water vapor and a carbon dioxide, and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also has the ability to store oxygen. The PCM has the capability to monitor this process by using a Heated Oxygen Sensor (HO2S) which is in the exhaust stream past the TWC. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The PCM monitors the catalyst efficiency by first allowing the catalyst to heat up, waiting for a stabilization period while the engine is idling. Then, the PCM adds and removes the fuel while monitoring the reaction of the HO2S. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the Oxygen Sensor (O2S) 1. When HO2S 2 response is close to that of O2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be bad and the Malfunction Indicator Light (MIL) will illuminate.