Hyundai - Elantra :: 2007 - Check Engine Light Turned On

I have a Hyundai Elantra 2007 with 123,000 miles on it. Three weeks ago the engine About two weeks ago the engine light turned on. I took it into an auto parts store so they could use the digital code reader, it said the upstream O2 sensor needed to be replaced. I replace it and still have bad gas mileage, the second time I had it read with the code read it said both of the O2 sensors were bad so I replaced the downstream one week later. There was still bad gas mileage and the code reader still said it was both of the O2 sensors, the auto part's representative said when the engine light is on that the computer turns off the O2 sensors so he recommended to erase the codes on my car with the code reader (I pushed the button and he looked away). Still nothing changed so I disconnected the negative terminal on the battery to restart the computer. Still horrible gas mileage with no engine light anymore. Gas mileage averages about 15 mpgs in city and 32-35 on the highway. The highway seems about right but I've never seen any mpg below 30 for city mpg before this problem. I also cleaned the mass air flow sensor with mass air flow cleaner after dealing with the O2 sensor. All preventive maintenance is kept up by me and I have had no problems with this car before. Water pump replacement, timing belt replacement, and transmission flush have occurred in the last ~50,000 miles.