Symptoms

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

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

When Detected

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

Tech Notes

The codes is usually set when there is an intake leak after the air flow meter, before replacing any parts check for loose or broken hoses and intake gasket leak. On Ford some models dirty air filter, and dirty or faulty air flow sensor are the cause of the problem. It's a very common problem for the Ford Windstar. Here a link to a web site with information how to repair: 1999-2003 Fords Windstar Code P0171

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 MIL (2 trip detection logic).