Its probably a faulty Oxygen sensor..
Usually they will tell you to disconnect the O2 sensor and see if a different code sets,if it does,bad catalytic converter.
Same ,replace the oxygen sensor.
Replace all O2 sensors,i would.
Downstream sensor is the sensor after the cat/con.
The upstream is the one in the exhaust manifold..it reflects the condition of the exhaust gasses as they exit the engine,the downstream reflects the condition of the gasses after they pass thru the converter.
Remember,the Catalytic converters are warranteed for 8 years or 80,000 miles ,whichever occurs first.
So,if your within those parameters and the cat/con is bad,take it to the dealer for replacement.
BTW...replace with OEM (original equipment manufacturer)(dealer) sensors,you will have better luck...