What Trouble Code P0171 Means for Ford F150 Models


May 3, 2016
While DTC code P0171 and its closely related cousin, P0174, are two of the most common codes to afflict Ford F150 models, almost all Ford models are equally likely to suffer the effects of these codes at some point, and particularly Ford applications that have done more than about 60 000 miles or so. In this article, we will take a closer look at what this code means exactly, as well as explore possible repair options that won’t cost an arm and a leg.

In technical terms, codes P0171 and P0174 are defined as “System too Lean (Bank 1), and “System too Lean (Bank 2) respectively, but in both cases, the word “lean” refers to a condition where the air/fuel mixture deviates from the ideal 14.7 : 1 ratio by an amount that exceeds a predefined limit for that particular application.

In practice, the “ideal air/fuel mixture” on gasoline refers to 14.7 parts of air to one part of fuel being available for combustion. In this ratio (also known as Lambda =1), all the fuel is combusted using all the air. Thus, when a condition arises where the air /fuel mixture contains more than 14.7 parts of air to one part of fuel, the mixture is said to be “lean”.

NOTE: “Bank 1” refers to the bank of cylinders that contains cylinder #1; while “Bank 2” refers to the bank of cylinders that does not contain cylinder #1. Therefore, code P0174 does not apply to engines that do not have two banks of cylinders.

What causes code P0171 / P0174?
There are many possible causes for these codes, but in cases where there are no other codes present, the most common cause is vacuum leaks that cause unmetered air to enter the engine, with defective, or fouled/contaminated MAF (Mass Airflow) sensors following close behind.

How do engine vacuum leaks cause codes P0171 / P0174?
Since the PCM needs to know exactly how much air is entering the engine at any given moment in order to be able to calculate appropriate fuel delivery strategies to suit the driving conditions at any particular moment, it depends on the MAF sensor to measure the volume of intake air very accurately at all times. Note that the PCM also obtains input data concerning the temperature and therefore, the density of the intake air, in order to be able to calculate appropriate fuel delivery strategies that always come as close as possible to maintaining the ideal air/fuel mixture at all times.

Thus, if any air enters the engine through a vacuum leak downstream of the MAF sensor, the PCM cannot include this added volume of unmetered air into its fuel delivery calculations, since it cannot determine the exact volume of the additional air. Moreover, since the PCM’s ability to compensate for the presence of unmetered air is severely limited, it will set code P0171 or P0172 (or both) depending on the application, and the site of the vacuum leak.

How do MAF sensors cause codes P0171 / P0174?
The purpose of a MAF sensor is to monitor/measure the amount of intake air that enters the engine. Thus, if the sensor is defective, or if the sensors’ sensing element is contaminated / fouled by a layer of oil or other contaminant, the sensor is as likely to under report the amount of air that passes over it as it is to over report the amount of air that passes over it.

If the MAF sensor under reports, the PCM (Powertrain Control Module) will register a rich running condition since the PCM sees less than 14.7 parts of air to one part of fuel. However, the sensor is more likely to over report the amount of air; if this happens the PCM sees more than 14.7 parts of air to one part of fuel (a lean condition), and it will set either code P0171 or P0174, or both, depending on the application.

What are the most common symptoms of code P0171 / P074?
Typical symptoms of this code include the following-

· Stored trouble code and an illuminated warning light

· Rough, or erratic idling

· Poor performance

· Misfires on one or more cylinders

· Engine may hesitate or stumble upon acceleration

NOTE #1: Take note that in some cases, one or more symptoms may only appear under certain conditions, such as when the engine is hot.

NOTE #2: Be aware that the symptoms listed here apply to vacuum leaks; other possible causes of these codes can produce similar symptoms that may vary in intensity, depending on both the nature of the problem and the application.

Can I repair code P0171 / P0174 myself?
While diagnosing and repairing these codes should not present the average DIY mechanic with undue difficulties, it should be noted that there are other possible causes of these codes, besides vacuum leaks and defective MAF sensors.

Therefore, it is advisable to obtain a diagram of the affected application’s engine vacuum system, as well as a code reader that can monitor live data streams. If other codes are present, it may become necessary to monitor the operation of oxygen sensors, the functioning of the EGR (Exhaust Gas Recirculation) valve, long-term fuel trims, as well as the operation of the fuel system, including monitoring the actual fuel pressure during a range of engine speeds. In some cases, using a smoke machine to trace vacuum leaks is the quickest way to find leaks that may not be readily visible.

IMPORTANT NOTE: In view of the above, it should never be assumed that P0171 and/or P0174 is always the result of an engine vacuum leak. Note that since neither code identifies a particular part, component, system, or sub-system as a probable cause of the problem, it is essential that the affected application’s fault memory be scanned for the presence of EGR or fuel system related issues that can also cause these codes to be set.

How to fix code P0171 /P0174 if no other codes are present
If no other active or pending codes are present, it is safe to assume that the problem involves a vacuum leak. Here is how to find it-

Step 1
Make sure the engine is cold to prevent burning or scalding yourself.

If a scanner with live data stream monitoring is available, connect it to the data link connector, and start the engine. Select the PID (Parameter Identifier) data menu, and set the monitoring function to fuel trims; if a vacuum leak is present, the fuel trim value will be positive. If the value is positive, raise the engine speed to about 2 500 RPM to see if the fuel trim value drops down to level closer to normal, which should happen because the leak will have a smaller effect on the overall air/fuel mixture at higher engine speeds.

NOTE#1: Fuel trim values differ between applications, so refer to the manual for the values that apply to the affected application.

NOTE #2: Using a scanner to confirm or eliminate vacuum leak as the cause of the problem is the quickest way to diagnose the root cause. However, if a scanner is not available, and it is known that no other codes are present, there are other ways of diagnosing the problem- as described below-

Step 2
Start your inspection of the inlet tract by checking that all air/vacuum hoses are present, and that all joints are properly secured by proper hose clamps.

Step 3
If all clamps are tight, check all vacuum hoses and vacuum lines for visible signs of damage such as chafing, rubbing, tears, spits, and dry rot on soft rubber connectors. Pay particular attention to the soft rubber fittings that connect the PCV (Positive Crankcase Ventilation) valve to the inlet manifold via a hose or hard plastic line. Make repairs or replace parts as required.

Step 4
Verify that the line connecting the brake booster (gasoline engines) to the inlet manifold is free of damage, and not split, cracked, or perforated. Replace the hose and/or any connectors /fittings that are in a less than perfect condition.

Step 5
If the vacuum system contains, or is fitted with vacuum check valves or vacuum operated actuators, check that all vacuum valves allow air to flow through them only in the direction indicated on the valve- usually indicate by an arrow. Also, make sure that all vacuum operated actuators are in working order- check that these components contain a vacuum when they are activated. Replace components as required; some defective vacuum check valves cam allow unmetered air to enter the engine, as can some vacuum actuators.

Step 6
If thorough inspections of the engine vacuum system did not produce an obvious leak, suspect leaking inlet manifold gaskets- proceed as follows, but do NOT use flammable aerosols such as brake or carb cleaner to prevent the possibility of an engine fire-

Start the engine and allow it to idle. Spray some inflammable spray such as WD-40 or similar onto and around all pipe joints, as well as onto all mating surfaces where the inlet manifold(s) attach to the engine block. If a leak is present (and assuming the spray reaches the leak) the quality of the idle will change immediately, albeit temporarily, as the spray seals the leak. Note that although professional technicians use this method to trace a vacuum leak, it is not always possible to reach all possible leak sites.

If this happens, one other way is to use a length of garden hose as a stethoscope. Cut a length of about 24 inches or so, and hold the one end against your ear while inserting the other end into all the nooks and crannies in and around the inlet manifold that you could not reach with the spray.

The large volume of the hose will amplify the “sucking” sound of the leak, and you will hear a loud “whooshing” sound when the open end of the hose is at, or close to the site of the leak. Once you have identified the site of the leak, refer to the manual for the affected application for details on the correct procedure to follow to disassemble and or remove what needs to be removed to gain access to the leak.

Note though that repairing some manifold leaks involve partial disassembly and or removal of major engine components; If you are not comfortable with the idea of doing this, refer the vehicle for professional diagnosis and repair to avoid the possibility of causing harm or damage where there was none before.

Step 7
If the problem persists but there are no other codes present and the steps outlined above do not resolve the problem, it is entirely possible that a leak path opens up only under very specific operating conditions.

The most likely sites of these types of leaks are inlet manifolds that deform very slightly when the engine is hot. Thus, check and verify that all inlet manifold retaining bolts/studs/nuts are present and tightened to the recommended torque values. However, bear in mind that in many cases, no amount of retightening can repair a damaged gasket, which damage usually occurs when the gasket leaks for the first time.

If retightening the manifold bolts/studs/nuts does not resolve the issue, the only reliable, long-term remedy may be replacement of the inlet manifold gaskets, or in rare cases, the inlet manifold itself along with the new gaskets.

Other possible causes of code P0171 / P0174
On Ford applications, and particularly on F150 models, the most likely cause other than a vacuum leak is a defective DPFE (EGR differential pressure) sensor. These sensors are known to have a very high failure rate, and when they do fail, they allow excessive amounts of exhaust gas to enter the engine at the wrong time, which when it happens, mimics the symptoms of a vacuum leak. Note however, that it is highly unlikely for EGR related codes not to be present when the DPFE sensor has failed.

Other possible causes of code P0171 / P0174 could include the following, but take note that all of the possible causes listed below will almost certainly produce dedicated additional codes-

· Defective PCV (Positive Crankcase Ventilation) valve

· Low fuel pressure due to a restricted fuel filter, defective fuel pump, defective fuel pressure regulator, or defective fuel pressure sensor(s).

· Clogged, dirty, or defective fuel injectors