DTCs P0171 and P0174 System Too Lean Diagnostic Aids
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2015 Lincoln Navigator, 2015 Lincoln MKZ, 2015 Lincoln MKX, 2015 Lincoln MKT, and 2015 Lincoln MKS. 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.