DTC P0420: Three-Way Catalyst (TWC) System Efficiency Low
WARNING: This page is about a different car, the 1997 Suzuki X-90. However, it is still accessible from the selected car via links, so may be relevant.
Check Heated Oxygen Sensors (HO2S) following procedures in DTC P0131 , DTC P0132 and DTC P0136. Repair as necessary. If HO2S is okay, replace TWC. Retest system.