DTC P0420: TWC System - Low Efficiency: Description

WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.

A Three-Way Catalytic (TWC) converter is used in order to control 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 process will convert the HC and the CO into a harmless water vapor and CO, and will reduce the NOx, converting the NOx into nitrogen. The catalytic converter also has the ability to store oxygen. The Powertrain Control Module (PCM) has the capability to monitor this process by using a Heated Oxygen Sensor (HO2S) which is in the exhaust stream past the three-way catalytic converter. 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, 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 O2S 1. When the HO2S 2 response is close to that of the O2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be bad and the Malfunction Indicator Light (MIL) will illuminate.