have a 99 LS with almost 200000 miles (fwy). Runs great. However, my CEL came on and when I checked it i got this:
P0420 Catalyst System Efficiency Below Threshold (bank1)

If I clear the code. And I've done this twice. The light doesn't return until I've driven 500 - 700 miles.

Does this mean I need to replace the cat? Or could it be a O2 sensor? How can I tell?

P.S. the car is all stock.