I've gotten the P0420 error a few times intermittently over the last couple of months, the latest being this morning. When I first received the error, I replaced the upper and lower Oxygen Sensors, mainly because I had around 240,000 miles on the car (2004 PT Cruiser) and figured they were due anyway. The car did run better and the error code didn't return for a while.
Now I have over 247,000 miles on the car and it is the original cat converter. I am assuming I am due to replace the converter. The car is running fine. The only thing I noticed was a couple of weeks ago, while driving from Florida to New Jersey and back, is that after stopping to refuel or at a rest stop, the car seemed to not have the acceleration it normally had when merging back onto I-95.
I'll probably be leaning towards changing the converter, but wanted to see if anyone had other thoughts.