DTC P0420: Catalyst System Efficiency Below Threshold: General Description

The three way catalytic converter (TWC) is installed in the exhaust system. The 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 TWC adsorbs/releases oxygen during this process. This ability to adsorb oxygen deteriorates as the TWC performance deteriorates. The TWC performance correlates with the storage capacity for oxygen, so the TWC deterioration can be detected by monitoring the storage capacity for oxygen. The storage capacity for oxygen can be monitored according to the inversion cycle of the secondary HO2S (Sensor 2) which detects the oxygen content after passing through the TWC.

The engine control module (ECM) determines fuel feedback control by monitoring the secondary HO2S for a set time period, and then calculates the average time of the inversion cycle of the secondary HO2S, whose waveform alternates between rich and lean. This inversion cycle varies by the amount of the exhaust gas entering the TWC and needs to be regulated by the OSC INDEX (Oxygen Storage Capacity INDEX):

OSC INDEX = The inversion cycle of the secondary HO2S x (times) the amount of exhaust gas during calculation of the inversion cycle.

The ECM calculates the moving average of six drive cycles and compares it to the predetermined threshold in the ECM. If the latest moving average is cleared by resetting the ECM, each threshold is applied according to the number of drive cycles until the moving average of six drive cycles is evaluated. If the calculated value is less than the threshold, the TWC performance is considered to be deteriorated and a DTC is stored.