My Bank 1 02 sensor threw a P0420 code two days ago. I changed the plugs about two weeks ago, 4 of them were original, and two of them had been replaced within the past couple of years. I've had the truck for about a year, and it now has 129k miles. One of the plugs and coils were covered in rust. Back to the code.. The engine light came on during highway traffic. The guy at Orielly's wanted me to buy a new 02 sensor, but that's dumb.. And with the truck running so well I don't think it's the catalytic converter. For the past couple of months, I've noticed a clunk/jolt at low rpm right after 1-2 gear change. Mostly in parking lots and traffic. I figured it was the rear diff, but I'm now suspecting it has been a misfire, (which would also throw the code). Where should I begin in diagnosing this problem? I'm most likely going to have the code cleared and see if I can get it to turn back on with purposely making it misfire. Any help would be greatly appreciated, thanks!!