Circuit Description

The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the exhaust system catalytic converter. In order to control exhaust emission of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), a three-way catalyst converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx into nitrogen. The powertrain control module (PCM) has the capability to monitor this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. The PCM compares the signal from the HO2S 1 located before the catalyst with the sensor 2 (HO2S 2) located after the catalyst in order to determine if the catalyst has deteriorated. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than the HO2S 1. Both HO2S signals changing at similar rates indicates that the catalyst is deteriorating.