DTC P0420: Catalyst System Efficiency Below Threshold: Description
WARNING: This page is about a different car, the 2002 Suzuki Esteem. However, it is still accessible from the selected car via links, so may be relevant.
ECM monitors oxygen concentration in the exhaust gas which has passed through Three-Way Catalytic Converter (TWC) by H02S No. 2. When TWC is functioning properly, variation cycle of H02S No. 2 output voltage (oxygen concentration) is slower than that of H02S No. 1 output voltage due to amount of oxygen in exhaust gas which has been stored in TWC.