DTC P0420: TWC System - Low Efficiency (2.4L): Description

A 3-way catalytic converter is used in order to control exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and oxides of nitrogen (NOx). Catalyst within converter promotes a chemical reaction which oxidizes HC and CO present in exhaust gas. This converts HC and CO into harmless water vapor and carbon dioxide, and reduces NOx, converting NOx into nitrogen. 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 the exhaust stream past 3-way catalytic converter. HO2S produces an output signal which indicates oxygen storage capacity of catalyst. This in turn indicates catalyst's ability to convert exhaust emissions effectively. PCM monitors catalyst efficiency by first allowing catalyst to heat up, 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, HO2S2 response to extra fuel is slow compared to O2S1. When HO2S2 response is close to that of O2S1, oxygen storage capability or efficiency of catalyst is considered to be deficient and MIL will illuminate.