Circuit Description

Three-Way Catalyst (TWC) is used to control Carbon Monoxide (CO), Hydrocarbons (HC) and Oxides of Nitrogen (NOx) exhaust emissions. TWC promotes a chemical reaction which oxidizes HC and CO, converting them to water vapor and carbon dioxide. NOx emissions are converted to nitrogen. PCM monitors this process using a Heated Oxygen Sensor 2 (HO2S2), mounted downstream from TWC. PCM compares its signal with Oxygen Sensor 1 (O2S1), mounted upstream from TWC. If TWC is operating correctly, HO2S2 signal will be less active than O2S1. DTC P0420 will set when HO2S2 is as active as O2S1 at normal operating temperature.