They will be back unless you have resolved the reason they appeared in the first place.
The P0420 and P0430 codes are very common and have been the source of many previous discussions. Poke around (using the SEARCH tool) for more details.
I am coming to the opinion that some of the cat failures are due to the period of time that the upper O2 sensors were defective. I believe that the enrichened exhaust mixture resulting from the weak upstream sensor has been causing the catalyst to fail prematurely not too long after the upstream O2 sensor is replaced.
Perhaps the recent discussion on the rejuvenation of the cats is related to this.
Steve
PS The lower O2 sensors do NOT cause the P0420 or P0430 codes. They have a separate self-diagostic procedure that the PCM executes prior to running the cat efficiency monitor test.
