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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?
 

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Either a "piss poor" O2 sensor, or a fault in the wiring or connector to it (actually LESS likely).

The catalyst code is related to the relationship BETWEEN sensors, & could show up anytime one of them isn't working right....

What brand sensor did you use? Some have posted poor results with Bosch brand for example....

Luck!
 

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Discussion Starter #3
I did use a bosch o2. But could that really not make any mpg or performance increase in the car at all?
 

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Depends on what is wrong with the car.....

Diagnosis saves parts money....

You've probably got another issue, and we need more info. to start guessing....
 

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First-a lot of guys do not like bosch. I have bosch o2 sensors for both, they work fine. I know you didn't get a code for the rear one, but check it anyway. It is very close to the intermediate axle shaft. Mine had been hit by the axle very slightly just enough to throw a code. Bad 02 sensors will cause terrible mileage. Oh and BTW SVT's are interference engines, so I'd be changing the timing belt asap, unless you know for sure that it has been changed recently.
 
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