I just bought my 2003 focus svt a few weeks ago. It has 110k miles on it. It had a CEL on when i bought it but I hooked a scanner up and it said p0133 which is o2 sensor bank 1, sensor 1. So I bought it thinking it was the o2 sensor. I retested it a couple days later and got p0133 again as well as p0420 for the catalyst efficiency low. I replaced the first o2 sensor and now I have p0171 (System adaptive fuel too lean (Bank 1)). I cant figure out what the problem is. I've been getting terrible gas mileage (19 mpg with 70% city and 30% highway) while driving very conservatively. Any thoughts on what the problem could be?