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Failed local emissions test

(1999 Ford Taurus)
in Mesa, AZ on December 08, 2010
failed emissions diagnostic codes read P0171 & P0174 I replaced both down stream O2 sensors but still same whats common?
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Popular Answer
on December 08, 2010
Roy's suggestions gave you some thoughts as to what could be the problem. And -- only a true diagnosis with proper test equipment can tell what is wrong for sure. You can replace gaskets, fuel pump, MAF sensor and spend a bunch of money and still not fix it. Get a good diagnosis and then if you choose you can repair it yourself and save some cash that way.
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on December 08, 2010
not the o2 sensors. the long term trims are out of spec from a vaccuum leak, low fuel pressure or a bad mass air flow sensor. you already put in 2 parts it did not need, now get it checked for the exact issue then replace what the problem is. in other words, get the diag done and then fix it yourself.

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on December 08, 2010
so where should I start? One guy told me it was the o2 sensors,but this didn't fix the issue. Running lean after replaceing the two rear sensors.
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