96 grand prix se poor gas mileage

I just recently took my car to the mechanic to find out why my car all a sudden is getting sucky gas mileage. dropped from about 20 in the city to 15. Code came up for bad o2 sensor. replaced both. replaced MAF also. Plugs and wires are fairly new and the car runs and idles great but the gas mileage didnt improve. Any ideas?????

Have you changed the gas you use, such as going from regular to using the ethanol based gasoline. I found that both my cars, 2000 GP and 98 Taurus, got better mileage using the regular 87 octane gasoline. When I changed from the ethanol to the regular, both cars got 3 miles to the gallon more. Just a thought.

If it were me, I'd try a compression test to verify integrity of the combustion chambers.

Also I'd look over your EGR valve.

I'd check the cat converter to make sure the brick isn't melted.

Last thing to think about is extra ESC activity (sensing knock) which "wastes heat" by retarding spark and dumping it down the exhaust pipe. The heat could and ideally should be used to drive the piston down when no knock is present.

Good suggestions by both.

Might I add, check tire pressures and verify Trans is going into Overdrive.

(my milage dropped from 27average to 22 average mainly becuase of cold weather warm ups and winter blended fuels)

Also it might be a good idea to change the fuel and air filters if you havn't already.