99 Pontiac Grand Prix with poor fuel mileage all of a sudden?

I have a 99 Grand Prix GTP that I normally averaged 27-28 mpg. My check engine light started to come on for maybe a day or two and then go off for weeks before coming back on again and for for the same day or two. After this starting happening pretty consistently, I noticed my fuel mileage dropped to approx. 18 mpg. The dealer scanned the check engine light to say my car was running rich. I could have told him that since I seem to be burning gas like crazy. He suggested changing the air and fuel filters, spark plug wires and ignition coils. I changed everything myself with an outcome of now 19-20 mpg. On my last trip to the dealer, I asked for a Fuel Injector service since I hit 100,000 miles. He stated that he didn't think that would cure the problem and he suggested changing the O2 sensor. I now have found out that my car has 2 sensors and was wondering if anyone has a suggestion as to which one I should try to switch first or if they think this might not even be the problem.
99 Pontiac Grand Prix with poor fuel mileage all of a sudden?
as a rule of thumb if you know for a fact that they have never been changed id just change them both,that may help bring the mileage back up a lot on it,that was a lot of mileage to loose at one time,id replace both of them,their called an up stream and a down stream, so it wont hurt to replace both of them,good luck maybe this will help the mileage.
99 Pontiac Grand Prix with poor fuel mileage all of a sudden?
I have the same car and Im having the same problem... I change the O2 sensor but theres another??

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yeah change both of them . your computer works with both to determine air fuel ratio so you can change one and still have the same problem
u should probably check the sensor closest to the air filter(farthest from the engine).
Seems awfully vague. What fault code(s) did the dealer pull? That will tell you where to start looking for the problem rather than taking shots at a dart board, which is what you are doing now. I certainly would not replace an O2 sensor unless I knew there was a problem with it. If you paid for a diagnostic then O2 sensor function would have been part of the test if the code was for a lean or rich running condition. If they didn't, get your money back and go somewhere else.

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