DTC P0420: Catalyst System Efficiency Below Threshold: Condition

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.

DTC is set when value from Heated Oxygen Sensor (HO2S) No. 2 becomes closer to value from HO2S No. 1. MIL is illuminated when Engine Control Module (ECM) detects malfunction during 2 consecutive drive cycles. Possible causes are:

Exhaust system malfunction.
Catalytic converter malfunction.
HO2S No. 2 malfunction.