DTC P0420: Catalyst System Efficiency Below Threshold (Bank 1): Description

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

This diagnostic test is designed in order to measure the efficiency of the Three-Way Catalytic Converter (TWC) system. Catalytic converter efficiency is a measure of its ability to store oxygen after converting the levels of Hydrocarbon (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx) to less harmful gases. The control module is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past. Once the conditions for running this Diagnostic Trouble Code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued. For example, if the control module were to command a rich mixture, the upstream Heated Oxygen Sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream Oxygen Sensor (O2S). As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2S is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.