O2 Sensor / Catalytic Converter

I have a "98" Intrepid that the "check engine" light has come on. The mechanic says that the O2 sensor is detecting that the Catalytic Converter in not doing it's job, and needs to be replaced. The car has 135,000 miles. I was told by someone else that the Catalytic Converter maybe ok, but that the O2 sensors can wear out in that mileage range....and only that may need to be replaced, which would be less expensive then replacing the Catalytic Converter. The car runs great. However, I don't want to get hit up for a $600 Catalytic Converter repair when maybe it's just the sensor. Anyone ever have a problem like this?

i have a 98 intrepid with 137k , i have changed all 4 o2sensors ($70 each) and my check engines still on (but it comes and goes)my code is reading
p0172 sytem too rich in bank 1. my mechanic mentioned a few possible things, clogged cat converter, fuel pump/filter, fuel regulator, open fuel injector, he`s going to test some more to narrow it down