I recently bought a 2006 with 109,000 miles on it. One the cats was replaced under a 2000 mile warrenty(now expired) after the check engine light came on. Now the light is back on. I pulled the code P0420 Bank 1. The cat on the passenger side was replaced. Could the light be telling me that the other cat is bad? It is ideling a little rough but no loss of power and the gas mileage is down also.

 

Rough idle, but no struggle under accel. could be a possible clog beginning. Have your truck inspected for a rich running condition, you need to have the reason it burned out the cats issued before repairing the catalyst for it to only re-occur. These cats should last far past 200k miles.