I have been getting really lousy gas mileage in my '97 (16-17mpg, both city and freeway.) I've been wondering if I have a bad sensor or vacuum valve somewhere but I wanted to hook an OBD II code reader to see what I could find. My check engine light has never turned on, however. I was wondering: does the diagnostic system report any codes despite the fact that my "Check Engine" light is not on? If I hook up a diagnostic code reader will it tell me anything at all or do me any good? I'll do anything to fix this gas mileage problem. Thanks.