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.

 

is that the only code thats coming up? most likey youll have to replace your cat. but give it some time and hope it goes away

 

could be a good excuse to get a high flow cat.