does a 2006 sentra 1.8 have a in line fuel filter?
check for intake and exhaust leaks first if none are found most likely the oxygen sensor on bank 1 sensor 1 is bad. after a sensor is replaced perform the mixture ratio self-learning procedure (easy way to do it is disconnect the battery for 30min or so)
http://nissanhelp.com/diy/sentra/projects/nissan_sentra_clear_self_learning.php
if none of those help fuel pressure should be checked which should be 51 psi at idle
the fuel filter is with the fuel pump assembly
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