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