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

In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), the engine controls system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The Powertrain Control Module (PCM) monitors this process using Heated Oxygen Sensor (HO2S) bank 1 sensor 2. HO2S bank 1 sensor 2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S bank 1 HO2S 2 signal will be far less active than that produced by HO2S bank 1 sensor 1. This indicates that the three-way catalytic converters oxygen storage capacity is operating at a threshold considered acceptable. The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:

1. The PCM captures the current rear HO2S rich-to-lean status.
2. The air/fuel ratio transitions from rich to lean or lean to rich depending on the capture rear HO2S rich to lean status.
3. The air/fuel ratio transitions a second time opposite the first air/fuel ratio transition.
4. The PCM captures the response time, the time the HO2S goes from below 300 mV to above 600 mV and from 600 mV to below 300 mV, of the front and rear HO2S when the air/fuel ratio transitions occur. The HO2S response time goes from less than 300 mV to more than 600 mV, and from more than 600 mV to less than 300 mV.
5. The PCM measures the time necessary for the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time necessary for the front HO2S voltage to cross the same rich-to-lean threshold. The difference between the front HO2S time and the rear HO2S time is the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold.