I have a 2003 Lancer OZ. How can you tell if your O2 sensors are bad? I've looked at a lot of discussions and saw that replacing broken ones helps gas mileage. Last time I filled up I added 10.3 gallons and I went about 275 miles before that which gives me about 27 mpg (not a terribly bad thing), but I have a friend with an 03 ES/LS who can go 350 miles and fill it up with 11.5 gallons (or so he claims) which gives him about 30.5 mpg. I've put fuel injector cleaner in the gas which has helped some, I think. Or is my gas mileage average? Any comments on the O2 sensors would be appreciated, thanks!