If it only happens when the AC is on, there might be another trickier problem hiding in there.
When the PCM determines that the AC is on, I believe that it adds to the calculated load value used in the fuel lookup tables (and even uses a whole different table possibly). IF this sensor is not properly reporting that the AC is on, this would cause the PCM to use the wrong part of the lookup table (and/or the wrong table). IF the AC being on requires more fuel than the lookup reports, the PCM will eventually give that extra fuel by looking at the O2 sensors and adding to the Long Term Fuel Trim. When the LT Fuel Trim gets too high, the lean code is thrown (LT plus Short Term greater than 30% throws P0171).
So, if you buy this idea, you need to check the sensor or switch that reports the AC being on to the PCM. Anyone with a good OBD-II scanner should be able to check that for you.
