for the last 6 months i have been living with a failed o2 sensor. the check engine light went on and stayed on but i had no difficulty with the engine or ride and the gas milage was fine. however when the summer came and when you thought milage should improve it did not. for $290 plus labor i was lothed to have the sensor replaced. am i doing damage to the vehicle? i have 100,000 miles on it now and the fiqures dont add up re: cost benifit ration. any thoughts?