I recently had to get the one of the o2 sensors replaced in my CSVT, as I was getting horrid gas mileage, about 9-10MPG. This time I fill up the tank, it's a lot better, but still not very good. (about 20MPG with a very light foot) Does it take time for the o2 sensors to ajust or something? Or is another o2 sensor going out on me?