my buick uses alot of gas what could be the problem? on 1996 Buick LeSabre

my buick gets about 15 miles to the gallon. i would like to know what the problem could be?

Asked by for the 1996 Buick LeSabre
Specs: 3.8 L engine 17 city 27 hwy. Recheck mpg!
Check eng. light on?
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3 more answers
have live data read for fuel trim issues and ck fuel pressure
have you had a tune up recently?
My 1995 Buick Century did this when the thermostat failed and stuck wide-open. The engine would never heat up as hot as normal, and the heater would take forever to finally make warm air. It caused the engine to eat fuel like mad trying to warm itself up. Dropped from my regular 23mpg down to 13mpg. The only reason I noticed is that the temperature gauge was lower than normal. Changed the thermostat (took about 10 minutes) and the car heats right up properly and fuel mileage went back to normal.