Ford code

P0171 on a Ford

P0171

In plain words: The engine is getting too much air or too little fuel on bank 1.
Standard name: System Too Lean (Bank 1)
System: Fuel and air mixture

General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.

What the computer saw

P0171 on a Ford is the standard generic code: bank 1 is running lean, with more air than the fuel can match. On a Ford V6 or V8 it often arrives with P0174, the same fault on bank 2.

Bank 1 on a Ford

On a Ford V6 or V8, bank 1 is the side with cylinder 1. On a four-cylinder engine there is only bank 1.

P0171 and P0174 together say both sides are lean, which points to something both sides share: the intake, the airflow sensor or the fuel supply.

Likely causes, most common first

A vacuum leak. Cracked PCV or vacuum hoses and worn intake gaskets are common sources on Ford engines, as on most.
A dirty or failing mass airflow sensor. If it under-reads the air coming in, the engine gets too little fuel.
Low fuel pressure. A weak pump or a clogged filter.
Leaking or clogged injectors. Less common, and usually on one bank.

What you can note yourself

Note whether P0174 is also stored.
Note when it runs worst: idle, cold start or highway.

What to tell the shop

All the codes, and whether P0174 came with it.
Ask whether they will smoke-test for vacuum leaks before replacing sensors.

This page gives no verdict on whether a car is safe to drive; a mechanic can.