After about 2-3 of my knock sensor malfunctioning I finally found out how easy it was to replace. After I replaced it I noticed I was getting better gas mileage. Well after about 2 months it went back to having poor gas mileage. I've had the maxima checked for codes and there wasn't any. I've put B12 in my tank to clean fuel injectors etc. Still nothing. What is it that could be making my car have bad gas mileage?

 

It could be the MAF, those things don't always throw a code. Another thing is check the rubber collar between the MAF and the throttle body. If it is cracked and allowing air in, the air fuel mixture will go crazy.