My check engine light is on. My car is stuttering. Is it my O2 sensor?

What is your question?

My check engine light came on. My car stutters when accelerating but doesnt last for more than a few seconds. I brought it to Advanced Auto Parts and they plugged it into the code reader. The code said my downstream O2 sensor. I brought it to a mechanic to get it put in and they said that they werent certain that it was the O2 sensor so they cleaned the connectors. They told me if my light came back on then to bring it back in. Does it sound like my O2 sensor or something else. Im not really burning gas like crazy or anything, I usually get 420 miles to the tank and this time I got 384.