P0420

[PO42O] The scan tool indicates Diagnostic Trouble Code (DTC) PO42O: Catalyst system efficiency below threshold.

NOTE: If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot those DTCs first, then troubleshoot DTC P0420.

P0137, PO137: Secondary HO2S (Sensor 2)
PO141: Secondary HO2S (Sensor 2) Heater

Possible Cause
^ TWC Deterioration
^ Leakage in exhaust system