So I have a used 2003 wrx with 91000 miles. Since I bought it the gas mileage hasnt been great (~19MPG). I changed engine oil, oil filter, transmission fluid, spark plugs, fuel filter, PCV valve and used seafoam along with lucas fuel injector cleaner. Nothing seems to help. I do from time to time get a P0420 code (cat converter). The car has a TBE with one stock cat on there where the O2 sensor goes. Also has a turboxs BOV. I am really confused and out of ideas. Only other thing I can think of is the wide-band O2 sensor. Although apart from the poor gas mileage the car runs great.