Wow I didn't think gas mileage would ever dip that low with failing O2 sensors. My check engine light comes on from time to time, and I think the O2 sensors are going bad or have gone bad. Last summer the best gas mileage I got was 22mpg and that was on bad plugs. I just recently changed them and the mileage is slowly getting back up there. I think it does take time for the computer to readjust to a new installment of O2 sensors. If you take it up to an Auto Zone they will just plug a little sensor into your car and tell if you if have another bad or failing O2 sensor. They may charge you unless you know someone there. It read mine was fine, but the gas mileage dipped big time to around 14 mpg in the winter. Having an open filament air filter might have a little to do with that as well. I would recommend changing the spark plugs and maybe having the fuel filter cleaned or changed if it's been a while for either of those. Some new spark plug wires couldn't hurt.
