In the past few months, my fuel economy has been dropping off. Everything superficial checked out to be fine (tire pressure, spark plugs, hand brake...). Today I hooked it up to my OBD diagnostic system and some numbers didn't look right. First, when the car wasn't warmed up, the external temp read 187 F. After the engine warmed up, the short term fuel trim read ~5%, and from what I have read, it should be around 50%. All of the sensors are not sending trouble codes. Does anybody know what is wrong?