DTCs P0171 and P0174 System Too Lean Diagnostic Aids
WARNING: This page does not describe the selected car, but rather 26 other vehicles, including the 2020 Lincoln Navigator, 2020 Lincoln Nautilus, 2020 Lincoln MKZ, 2020 Lincoln Corsair, and 2020 Lincoln Continental. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
If the system is lean at certain conditions, then the LONGFT PID would be a positive value at those conditions, indicating that increased fuel is needed.
The ability to identify the type of lean condition causing the concern is crucial to diagnosis.