Your mechanic will do many of the things I listed, typically starting with checking if there are any diagnostic fault codes. Next he will likely look at the short and long-term fuel trim numbers. If they are high positive, the computer is adding fuel beyond what was programmed in at the factory in response to something. He has to figure out what that something is. If the numbers are high negative, the computer sees there's too much fuel going into the engine and is trying to reduce that amount without success. The question then is why did the computer lose control of fuel metering or how is that extra fuel getting into the engine?
If the fuel trim numbers are close to zero, the computer is happy with the way the engine is running and there is some other cause for the poor fuel mileage. Most commonly that is due to a dragging brake or an automatic transmission's torque converter that isn't locking up at highway speeds. A misadjusted brake light switch can cause the torque converter to unlock repeatedly. You can watch for that if your car has a tachometer. With the engine warmed up, in third or fourth gear, and above 45 - 50 mph, the torque converter should be locked up. Maintain the car's speed and hold the accelerator pedal steady, then tap the brake pedal with your other foot. You should see engine speed increase about 200 rpm for a couple of seconds, then go back down when the torque converter relocks. If the engine speed doesn't change when you tap the brake pedal, the torque converter is not locking up and fuel mileage will drop. Try holding the brake pedal up with your toes. If that makes engine speed drop, suspect the brake light switch is out of adjustment.
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Wednesday, September 4th, 2013 AT 3:07 AM