DTC P0420: Catalyst System Low Efficiency: Description

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), a 3-way catalytic converter is used. Catalyst within converter promotes a chemical reaction which oxidizes HC and CO present in exhaust gas, converting HC and CO into harmless water vapor and carbon dioxide. Catalyst also reduces NOx by converting NOx to nitrogen. Converter also has ability to store excess oxygen and release stored oxygen to promote these reactions. This Oxygen Storage Capacity (OSC) is a measurement of catalysts ability to control emissions. Powertrain Control Module (PCM) monitors this process using a Heated Oxygen Sensor (HO2S) located in exhaust stream past 3-way converter. When catalyst is functioning properly, HO2S 2 is slow to respond to a large change in HO2S 1 signal. When HO2S 2 responds quickly to a large change in HO2S 1 signal, OSC and efficiency of catalyst are considered to be degraded and MIL will be illuminated if subsequent tests also indicate a failure.