DTC P0420: TWC System - Low Efficiency (Bank 1): Description

WARNING: This page is about a different car, the 2002 Pontiac Sunfire, 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, and 2002 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 converter promotes a chemical reaction which oxidizes HC and CO that is present in exhaust gas. This process will convert HC and CO into a harmless water vapor and CO2 and will reduce NOx, converting NOx into nitrogen. The catalytic converter also has ability to store oxygen. PCM has capability to monitor this process by using a Heated Oxygen Sensor (HO2S) which is in exhaust stream past the TWC converter. HO2S produces an output signal which indicates oxygen storage capacity of catalyst. This in turn indicates the catalyst's ability to convert the exhaust emissions effectively. PCM monitors catalyst efficiency by first allowing catalyst to heat, waiting for a stabilization period while engine is idling. Then, PCM adds and removes fuel while monitoring reaction of HO2S. When catalyst is functioning properly, HO2S 2 response to extra fuel is slow compared to the O2S1. When HO2S 2 response is close to that of HO2S 1, oxygen storage capability or efficiency of catalyst is considered to be bad and MIL will illuminate.