DTC P0420: Catalyst System Efficiency Below Threshold: General Description

The warm up three way catalytic converter (WU-TWC) converts hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) in the exhaust gas to water vapor, carbon dioxide (CO2), and dinitrogen (N2). The engine control module (ECM)/powertrain control module (PCM) fluctuates the air/fuel ratio temporarily to detect the performance of the WU-TWC and measures the degree of fluctuation in the secondary heated oxygen sensor (secondary HO2S (sensor 2)) output. If the accumulated value for a set time is too great, deterioration of the performance of the catalyst is detected, the ECM/PCM detects a malfunction and stores a DTC.