Possible causes

– Intake air leaks
– Faulty front heated oxygen sensor
– Ignition misfiring
– Faulty fuel injectors
– Exhaust gas leaks
– Incorrect fuel pressure
– Lack of fuel
– Faulty Mass Air Flow (MAF) sensor
– Incorrect Positive Crankcase Ventilation (PCV) hose connection

Tech notes

If the P0171 is combined with the P0174 code, it’s very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced.

It’s a very common problem for the Ford Windstar. Link to a website with information on how to repair:

1999-2003 Fords Windstar Code P0171

When is the code detected?

– Fuel injection system does not operate properly.
– The amount of mixture ratio compensation is too small. (The mixture ratio is too lean.)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Excessive Fuel Consumption

P0171 FORD Description

With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios.
In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the Malfunction Indicator Light (MIL) (2 trip detection logic).

FORD Technical Service Bulletins for P0171 code