Engine Performance problem
1996 Ford Explorer 6 cyl Four Wheel Drive Automatic 62500 miles
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hi. my oxigen sensor totally broke down. i've been driving like that for 3 month. my mechanic ( in ford's service center) said that replacing it will improve my fuel efficiancy.

2 days ago i've replaced my oxigen sensor ( the mechanic put an original part). since then, instead of improoving my fuel consumption, the car is cosuming more fuel than it used to

of course my mechanic says he has nothing to do with it and its not his blame.

what can i do ?

thank you very nuch