I need someone who REALLY knows the Ford Mustang Engine?

I have a 2001 Ford Mustang. It is a V6 3.8L automatic. The only problem is... I CANT GET IT TO P EMISSIONS!!!! When the slip printed out, the OBD code was P1131, i think, and when i asked a mechanic, he said it was an airflow/O2 sensor. I dont know how to fix it or how much it will cost but its making me very upset that it wont p. Plus, my check fuel cap light is on. I just bought a brand new one and the light didnt go out. Someone help, PLEASE?