1999 Oldsmobile Silhouette V6 3.4, P0420 code

Question

The check engine light keeps coming back after it was erased and the car is driven for about 200 miles. Apparently, the car works and runs great, like nothing is wrong wit it. However, the code P0420 shows up with Catalytic convertor efficiency bellow trasholder Bank 1. Had already replaced the cat, coils, sparks(all new), Mass airflow sensor. Any other suggestions, please ? Thank you very much. Any help is greatly appreciated.

Answer 1

check for exhaust leaks first 

you may have a bad oxygen sensor or wiring