since auto zone checked codes and recommended a gas cap I will assume it either has a small or large leak in the EVAP system because that is what they usually recommend for those codes. Check codes and verify first before doing all this. Check the vent valve. unplug it and put power on one terminal and ground to the other terminal. you should hear a clicking sound if not swap power and ground. if no clicking is heard then replace the vent valve. check hoses for a crack(s) in the EVAP system. check the mounting surface for the gas cap for rust. if you can't find the problem then it needs to have a smoke test.

Auto Zone non-mechanics are a waste of time. Get your money back for the gas cap they said was causing the problem.

*The dealer of the car can read the stored fault code and then will be able to tell you *exactly which emissions needs to be cleaned or replaced.

have the codes run again, when you replaced the gas cap did you disconnect the battery for a minimum of 1 hour? you need to disconnect it to reset the codes or the light will not go out, if so, have them run the codes to see what it is now, it may be something different or could be the neck of the gas fill hose (the piece the cap screws on to), if plugged or damaged it will show the same codes!

Retrieving the code(s) from a car is very easy but fixing can be a great deal more difficult.

If you don't know how to fix it, take it to someone ( your mechanic ) that does.

Check gas cap is thigh and the O2 sensor.

Yes

So you could ask a piano tuner or a Starbucks barista, but chances are a mechanic would be a better place to start.

ASK A STOOOPID QUESTION, GET A STOOOPID ANSWER.

pull the mil codes

My check engine light has been on for a little while now. I went to auto zone and they said just to replace the gas cap. I did. Still won't go away or anything like that. And I have been noticing a decrease in my gas mileage. I don't know what to do or where to go. Do I just ask a mechanic to try and find what's wrong?