Originally Posted by AtlantaLotusLove
I also saw the suggestion of the more advanced diagnostics information that can be obtained using a better OBD dongle with the appropriate app... sounds kinda groovy, but I might just opt for a pro mechanic before going that route...
Is your hesitance due to dollars or lack of expertise? If the former, note that the bluetooth OBD dongles are quite reasonably priced (something like $10-20). If it is the latter, please allow me to share my tin foil hat with you...
I too had the P0171 and could not pin down the root cause. I tried many of the strategies listed above, and with the aid of appropriate OBD data, and decided that the sensors were not faulty. I could not find evidence of an air leak or of the engine control strategy trying to compensate for a lean condition, in spite of the error code. I kept trying different things, clearing the code each time, only to have it come back. Although it is very tempting to just throw money at the problem (i.e. buy new sensors in spite of the lack of evidence that they are faulty), I chose to keep looking for data that there was actually a problem and thereby pinpointing the source. I still have not found any; I recommend a tin foil hat because I lack confidence that a mechanic would serve your best interest in this case- it is far too easy to recommend spending money on parts based on preconceived notions of what causes a lean error code.
Ultimately I just kept data logging, trying to replicate the conditions which tripped the error code while I took data logs. In spite of the recurring error code, I took a long road trip to the D.C. area from MI. By the end of the trip, the code no longer came on (this has been since mid September). Could it have been a long, hot drive affecting the sensors? Could the battery voltage have recovered during the long drive? Maybe the change in gas stations along the way? I don't know what it was, but the end result is consistent with my data which indicated that there was not really a problem.
If you end up taking the car to a mechanic please ask them to show you the data or evidence which motivates their recommendation. If they tell you 'this is what typically causes lean conditions', I suspect that they also did not find strong evidence.