have an 02, check engine light is on, gas mileage is kind of bad.

If an oxygen sensor is reading incorrectly it can dramatically affect gas mileage. Oxygen sensor test the engine's computer is left unburned in the exhaust and trims fuel mixture accordingly.

hi dear friend, I think you must read out the code first. I hope this information can help you.

I don't know about the bad gas mileage but make sure your gas cap is in good condition and never take it off unless you intend to fill the tank.

No, check engine light has nothing to do with gas milage. My first car was an '02, I recieved it with the check engine light on, and gave it up the same way. That car in general does not have good gas mileage.