Chevrolet Repair: catalytic converter, oxygen sensors, muffler shop

Question
I have a 1998 K1500 Chevrolet pickup with 5.7 liter engine and the service engine light indicates a code for "low efficiency, catalytic converter, bank #2." The dealer says I need a new converter but this truck has two. They said a muffler shop can tell which one needs replaced.

My question- How can a shop tell which converter should be replaced without cutting apart the entire system, its all welded, no bolt on connections. Is this repair absolutely necessary and if so, how soon should it be done?

Thanks for your help.  

Answer
Hi Bob,
If the truck has oxygen sensors before and after the converters, they compare readings, and if the exhaust coming out of the converter is approximately the same as the exhaust that enters the converter, then it says it isn't doing what it should.
A problem there, though, is if one of the oxygen sensors is bad, or disconnected, the ecm may be getting a false reading.
Before replacing a converter, I think I would swap oxygen sensors from one side of the truck to the other...all 4 of them, and see if the problem remains on the same side, or travels with the sensors....or completely goes away because during the swap you inadvertently corrected a bad connection.
Good luck,
Van