i replaced the sensor and soon after i get the same code... any ideas?

What engine and how many miles? Which sensor did you replace and why did you replace the sensor in the first place?

zetec Mtx, threw a p0171 i posted it and was told to replace the o2 sensor... so i did

i put a mil eliminator on the 2nd o2 sensor but i dont know why it would cause the other one to throw a code.

129500 miles

Did you replace the Upper sensor for the P0171 code? If you did, it could be a vacuum leak. Did you also check the other items previously suggested?

Originally posted by Tony2005:

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Also check for vacuum leaks at gaskets, hoses and hose clampes.

And clean the MAF sensor
http://www.contour.org/mods/mods.php?s=howto&displayid=36
and
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=zetecmaint&Number=989728&page=&view=&sb=5&o=&fpart=1&vc=1
and
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/1b/cb/25/0900823d801bcb25.jsp

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Edit. From TSB,
P0171 - System Too Lean (Bank 1) The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The code is set when the adaptive fuel tables reach a rich calibrated limit.

Fuel System:
�· Contaminated fuel injectors
�· Low fuel pressure or running out of fuel (fuel pump, filter, fuel supply line restrictions)
�· Vapor recovery system (VMV)

Induction System:
�· MAF contamination
�· Air leaks between the MAF and throttle body
�· Vacuum leaks
�· PCV system concern
�· Improperly seated engine oil dipstick

EGR System:
�· Leaking gasket
�· Stuck EGR valve
�· Leaking diaphragm or EVR

Base Engine:
�· Exhaust leaks before or near the HO2S
�· Secondary air concern

Powertrain Control System:
�· PCM concern