I just dealt with the P0420 issue on my 2000 OBW successfully. My car has the equivalent of 130,000 miles on it (bought at 125,000) and came with aftermarket cats/Bosal brand exhaust. My code was persistent and came back every 3-400 miles.
I was advised that many P0420 catalyst efficiency below threshold codes are caused by an exhaust leak upstream of the second 02 sensor. I listened for a leak and couldn't hear anything. I replaced the PCV. I replaced the O2's. The code still came back. I used ether on the vacuum lines to test for a leak, nothing found.
Finally, I used Seafoam for the significant amount of smoke it produces to test for exhaust leaks. With the car up on ramps, I added a small amount of seafoam to the vacuum line at the brake booster and watched underneath. The smoke revealed a very small leak in the front passenger side corner of the cat, adjacent to the front 02 sensor. It was such a small leak, I couldn't hear it, I thought that it couldn't possibly be the source of my P0420 code. Nevertheless, I welded the seam where the leak was, tested it with more seafoam to verify the leak was sealed, cleared the code and have not have a return in the last 2 tank fulls.
I'm by no means a fan of seafoam for its cleaning ability (it spends far too little time in the combustion chamber to be effective in my opinion), but it is invaluable for testing in this capacity.
For the cost of visually verifying the absence of exhaust leaks in this manner, I would highly recommend this $8 test prior to throwing any parts at it, like I did.
