2002 Dodge Durango catalytic converter

Engine Mechanical problem
2002 Dodge Durango Four Wheel Drive Automatic 70,000 miles

Our repair soon light came on and our mechanic said the catalytic converter is not working properly. The oxygen sensor was replaced before we bought the car - within the last month. The dealer and our mechanic say this is a $1,200 repair. One mentioned muffler centers. They say it will be about $300.00. We don't know what to do.

5.9 liter engine