2003 Mitsubishi Montero Service Engine Lights ON?

Hello i have 03 mitsubishi montero limited the service engine lights on and i bring it to autozone to check whats wrong and they said it was the oil changer sensor something, how do i fix this?Is this need to be reset or change the sensor, pls help and how much you think will it cost me if i bring it to shop...Pls help!!!!