As stated above, this is just bad info. The rear O2 frequently causes this code, and if you know the age of the sensor to be over 80K, changing it makes perfect sense. If you end up putting a new cat in, any decent shop will insist on new sensors anyway, so it's a good idea to try this first, as a likely low-cost fix. This is especially true if you have had to change your front O2 in recent history, as they go bad first, but not by a whole lot. It certainly makes more sense to do this than to start chasing phantom misfires, etc. I cured my P0420 just by changing the original rear O2 in 2009 at 112k. No P0420 since except for one Sunday in a crowded park where I drove for almost an hour at 25mph or less. Cleared it and never came back.
As you stated above, cats usually don't die of old age, something kills them--physical damage, excess unburnt fuel, etc. But sensors do die of old age, and any original sensor out there is beyond end-of-life.Buy a new Sensor on Amazon, and you will spend about the same as asking someone to hook up a scanner. Plus, if he thinks he can sell you a cat, he may well try to if you need it or not.
Many aftermarket cats are junk, but some members have had success. I know one guy on here is happy with his Eastern Catalytic cat ( [MENTION=56021]Nelsonmd[/MENTION] ).
