Circuit Description

The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the catalytic converter. In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides Of Nitrogen (NOx), a Three-Way Catalytic Converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and the CO that is present in the exhaust gases. The oxidation process converts the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx to nitrogen. The Powertrain Control Module (PCM) has the ability to monitor this process using the Heated Oxygen Sensors (HO2S 1 and HO2S 2). The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 produces an output signal that indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the converter ability to convert exhaust gases efficiently. The HO2S 1 output signal will be far more active than the HO2S 2 output signal when the converter catalyst is operating efficiently. A DTC P0420 will set if the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently.