The best bet for you is to do some reading on the subject of the P0171 code and some of the diagnostics needed to address it.
Here'* a link to a good thread to get you started:
2000 SSEI running lean
Read the whole thread.
To get started on this, check to make sure that one of your rear plug wires isn't touching the O2 sensor wire. If it is, then use a tie-wrap to keep it away. Then check all your electrical connections for tighteness and corrosion. It seems that this code is most often caused by electrical anomalies.
Also, please put your car model, year, and trim level in your sig, so that we don't have to guess, possibly wrongly.
Oh yeah, and if this is your first post, Welcome to BC!
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s