DTC P0420 & P0430: TWC System - Low Efficiency - Bank 1 & 2: Description

WARNING: This page is about a different car, the 2001 GMC Savana, 2001 Chevrolet RV Cutaway, 2001 Chevrolet Cutaway, and 2001 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

This diagnostic test is designed to measure efficiency of Three-Way Catalytic Converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting levels of Hydrocarbon (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx) to less harmful gases. Powertrain Control Module (PCM) is able to evaluate catalyst efficiency once vehicle has met code enable criteria and vehicle is at idle instead of steady cruise speeds used in the past. Once code enable criteria for running this DTC are met, PCM commands either a lean or rich air/fuel ratio depending on current state of exhaust oxygen level. PCM issues a rich command if exhaust is currently lean, or a lean command if exhaust is currently rich. After completion of first command, a second and opposite command is issued. For example, if PCM were to command a rich mixture, upstream Heated Oxygen Sensor (HO2S) voltage would increase immediately. Rich mixture is delayed in reaching downstream HO2S due to conversion process occurring within converter. The higher the efficiency, the more delay before rich or lean mixture is detected by downstream Oxygen (O2) sensor. As a result of lower conversion efficiency within a damaged or poisoned catalyst, the delay in rich or lean mixture reaching downstream O2 sensor is significantly shorter. This DTC monitors amount of time required for both upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to rich or lean command.