Engine Code P0420

I have a 2000 Lincoln LS V8 with 91,000 miles on it. And it is reading the engine trouble code P0240. I had autozone check it out and they said it could be an O2 sensor, or the cats, or a misfire. I looked at the Valve cover gaskets and they look like they have been changed already. Also if it were a faulty O2 sensor, wouldnt it run rough? My Lincoln runs fine. Im wondering if it is an emission problem. I belive when I bought the car, the dealership used regular gas instead of premium gas. Could that have set this code off? Any advice would be much appreciated. Can I clear this code without going to a dealer? Oh yea, and I LOVE my Lincoln.

I assume that the code listed in the subject is correct rather than the P0240 mentioned in the text of the post.

That being the case, as you are probably aware, the code definition is Bank 1 Catalyst System Efficiency below threshold. According to the factory service manual, likely causes are:

Use of leaded fuel
Damaged HO2S
Malfunctioning ECT
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Oil contamination
Cylinder misfiring
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timingAlso per the service manual, you can verify the cause by the following:
Compare HO2S upstream and downstream switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set.

-Rod

So im guessing its just a damaged or bad cat. seeing how it has 91000 nikes on it, its probley time to change them. Thanks for the advice.