failure code po420 obd11

failure code po420 obd11 what and were is it? thanks
   
DTC P0420 indicates bank one catalyst system efficiency is below acceptable threshold.
DTC P0430 indicates bank 2 catalyst system efficiency is below acceptable threshold. Possible causes are:
1 Use Of Leaded Fuel
2 Faulty HO2S
3 Faulty ECT Sensor
4 High Fuel Pressure
5 Damaged Exhaust System Component
6 Oil Contamination
7 Cylinder Misfire
8 Downstream HO2S Sensor Improperly Connected
9 Retarded Spark Timing
Hope this helps you out