I 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.

 

maybe it's the o2 sensor try to switch the one from header to the cat and see what happens just to save you time wasting money to buy this and that..