DTC P0420: 3-Way Catalyst Function: Description

The ECM monitors the switching frequency ratio of HO2S1 and HO2S2. A warm-up 3-way catalyst with high oxygen storage capacity will indicate a low switching frequency of HO2S2. As oxygen storage capacity decreases, HO2S2 switching frequency will increase. When the frequency ratio of HO2S1 and HO2S2 approaches a specified limit value, the warm-up 3-way catalyst malfunction is diagnosed.