Ford Repair: Ranger P0420 code

Question
Hello,
I have a 04/11 Ranger with this code on:
P0420 B1 catalyst efficiency below limit.
Where should I start,the truck is running OK but the long term data are stuck to 99%,the short ones are in the 3 to 4 % range.
I have access to the OTC EVO scanner and wiring diagram from Mitchell on internet but I'm not very familiar with these tools yet.
Waiting to hear back from you.
Thanks
terry

Answer
Hi, I hate to be the bearer of bad news. But that code comes on for only one reason . The converter has failed. Unfortunately , tune up filters and so on won't fix this problem. I think if you even google this code with your vehicle , you will receive the same answer. I am as honest as I can be with people so,... Let me know if you get a different opinion and I'll work with you further