Why has gas mileage has dropped from 21 to 15 run great and air filter is clean on 2007 Chevrolet Uplander

Why has my mileage dropped? Dealership says everything looks good and supposedly checks out, sir filter is clean and the engine runs great.

Asked by for the 2007 Chevrolet Uplander
have fuel trim ck it could be cause by a number of things and scan for codes
What exactly is fuel trims and what kind of codes should my mechanic check for? Thanks and God Bless
It may not have any fault codes....Fuel trims are a way that the technician may be able to tell if for some reason the fuel control system may be making the system run a bit on the lean side due to perhaps a vacume leak or on the rich side due to a lazy oxygen sensor or a dirty fuel injector. This requires a good technician to dig for possible make sure you really have an issue first by checking your fuel mileage accurately on a road trip.
1 more answer
Are you checking your mileage in around town driving or a long road trip? Driving habits have more to do with than one would think sometimes.
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