That code has been known to drive people crazy.
While the O2 sensor seems likely it doesn't seem to be the cause very often. You may have better luck - go with a Delco sensor.
The Maf sensor is another possibility. Unplugging it was already mentioned. You should also make sure there is no debris on the screen in front of the Maf.
Vacuum leaks are another possibility. Try visually checking all the vacuum lines & fittings. Try spraying throttle body cleaner or propane around all the vacuum lines, intake gaskets, throttle body gasket & fuel injector O rings. If the idle RPMs increase then you found a leak. You can also disconnect & plug the vacuum line to the brake booster. Might want to do that on a lonely country road & see how much pedal force is required before you get up to speed.
Fuel pressure & volume also need to be checked. Some people have used a fuel pressure gauge with extension & taped the gauge to the windshield to check pressure at wide open throttle.
That covers most of it, the EGR might also come into play with p0171.
You have to reset the code to see if anything helped.