Is p0171 o2 sensor code upstream or downstream?

This code is triggered by the first downstream (front) O2 sensor. There are many things that can cause this code. They are:

The MAF (Mass Air Flow) Sensor is dirty or faulty Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry. There could be a vacuum leak downstream of the MAF sensor Possible cracked vacuum or PCV line/connection Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1) Sticking/plugged or failed fuel injector Low fuel pressure (possible plugged/dirty fuel filter!) Exhaust leak between engine and first oxygen sensor

P0171 is a lean code for the Bank 1 cylinder head. It is not an 02 sensor code. That code will be an intake manifold vacuum leak or a bad MAF sensor. What idiot told you it was an 02 sensor code? Type the code and Ford into your search line and read the results or look up OBD II codes and go down the chart.

P0171 is a lean code, not an oxygen sensor code.