Horrible gas mileage in a 1999 Infiniti Q45 with 79K miles?

I have a 1999 Infiniti Q45, and love this car. Problem is I am getting horrible gas mileage, like 200mi to the fill-up. I have change the the fuel filter, new spark plugs (NKG's), all new drive belts, made sure tires pressure are good, had an alignment, oil changes on time, and stopped driving excessively fast, still no improvement in gas mileage, but better performance. Check engine light just came on, says its my 02 sensor. Not sure if that would make my gas mileage that terrible, but who knows??? Any suggestion???