Found out today that my O2 sensor on my Ciera is on the way out, the wires are corroded and falling apart or something. I assume this is what's causing my SES (service engine soon) light to turn itself on an off at differing times. How much of an effect on fuel economy would an essentially non-functioning O2 sensor have? I notice my fuel mileage isn't what I expected it to be, I put $54 in the tank, at 88.9 cents per liter that's just over 60 liters, or 15.8 gallons to you US folks. I only managed to get just over 400 kilometers, 249 miles, to the tank before I decided to refuel. My fuel gauge becomes wildly inaccurate at about the 1/4 tank mark, so who knows, I may have been able to go further on that tank of fuel. Does that sound low to you?