DTC P0420: Catalyst System Efficiency Below Threshold: General Description

The three way catalytic converter (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 powertrain control module (PCM) fluctuates the air/fuel ratio (A/F) temporarily to detect the performance of the TWC and measures the degree of fluctuation in the secondary HO2S output. If the accumulated value for a set time is too great, deterioration of the performance of the catalyst is detected.