DTC P0420: 3-Way Catalyst Function: Description

The ECM monitors the switching frequency ratio of Heated Oxygen Sensor (HO2S) 1 (front) and 2 (rear). A 3-way catalyst with high oxygen storage capacity will indicate a low switching frequency of HO2S2 (rear). As oxygen storage capacity decreases, HO2S 2 switching frequency will increase. When the frequency ratio of HO2S 1 (front) and 2 (rear) approaches a specified limit value, the 3-way catalyst malfunction is diagnosed.