DTC P0420: Three-Way Catalyst (TWC) System Efficiency Low
WARNING: This page is about a different car, the 1997 Suzuki Sidekick. 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 , P0132 and P0136. Repair as necessary. If HO2S is okay, replace TWC. Retest system.