P0171 – Fuel trim (FT) system too lean, bank 1 (Ford)

By Reinier (Contact Me)
Last Updated 2018-07-09
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0171 Fuel trim (FT) system too lean, bank 1 Intake/exhaust leak, AIR system, MAFNAF sensor, fuel pressure/pump, injector(s), HO2S

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What Does Code P0171 Mean?

OBD II fault code P0171 is a generic code that is most commonly defined as “System Too Lean- Bank 1”, and is set on gasoline engines when the PCM (Powertrain Control Module) detects that the air/fuel mixture is too lean. In this context, “lean” refers to a condition where there is too much air relative to fuel in the mixture. As a rule of thumb, an air/fuel mixture is considered ideal when there are 14.7 parts of air to one part of fuel in the mixture. Note that while “Bank 1” in this definition refers to the bank of cylinders that contains cylinder #1, code P0171 also applies to in-line 4 & 6 cylinder engines.

NOTE:  On V-type engines, code P0171 is often accompanied by code P0174, which refers to the same problem, but on Bank 2. If both these codes are present, the PCM has detected a problem that affects both banks of cylinders.

On all applications, the PCM monitors the quality of the air/fuel mixture on a continuous basis not only to ensure efficient engine operation at all times, but also to improve fuel economy and reduce harmful exhaust emissions. In practice, the PCM uses input data from the oxygen sensors to monitor the amount of oxygen in the exhaust stream, since this value correlates directly with the composition of the air fuel mixture prior to combustion.

As a practical matter however, the PCM uses input data from the MAP/MAF sensor, barometric pressure sensor (where fitted), throttle position sensor, engine speed sensor, intake air temperature sensor, boost pressure sensor (where fitted), and engine coolant sensor, among others, to calculate how much fuel to add to, or subtract from the intake air to achieve an air fuel/mixture that consists of 14.7 parts of air to one part of fuel.

If the engine is in good condition, and there are no exhaust leaks, engine vacuum leaks, or misfires present, all sensors are working as designed, and all oxygen sensors and catalytic converter(s) are in closed-loop operation, the PCM uses input data from the oxygen sensor(s) located upstream of the catalytic converter to alter the pulse widths (ON time) of the fuel injectors. Altering the pulse width of the injectors (as required) has the effect of either adding fuel to the mixture, or subtracting fuel from the mixture to maintain a 14.7: 1 ratio between air and fuel.

If no defects of any kind exist on the engine, the PCM has a limited ability to compensate for slight wear on moving parts of some sensors, as well as for the loss of sensitivity of most sensors that are associated with air/fuel metering to reduce the incidence of false positives. However, in practice, the PCM makes adjustments called “fuel trims” to adjust the fuel mixture several times per second, and when it detects that a lean condition (too much air relative to fuel), exists that it cannot compensate for, it will recognize that it cannot control the air/fuel mixture effectively. When this happens, the PCM will set code P0171, and illuminate a warning light as a result.

Why does code P0171 affect Ford applications more than others?

While no gasoline engine is immune to the causes that set code P0171, many Ford applications have been particularly susceptible to P0171 for many years, with engine vacuum leaks that lean out the fuel mixture being by far the most common.  See the “Location” section for more details.

Two other major causes of code P0171 on Ford applications are dirty, and/or oil-contaminated MAF sensors and leaking intake manifold gaskets- particularly on 3.8L engines. The most common cause of the leaks at this site appear to be excessive vibration that causes the inlet manifold gasket to fail, and/or premature shrinkage of the rubber compound used in the manufacture of the gasket.

Where is the P0171 sensor located?

The image above shows a cracked rubber elbow (circled in red) in the PCV hose on a Ford Ranger. For reasons that are not entirely clear, the rubber elbows in the PCV hoses of almost all Ford applications are prone to premature rotting, splitting, cracking, and general degradation, and replacement of these elbows generally do not provide a reliable, long-term solution, even if they are replaced with OEM parts. However, this writer has achieved lasting results by dint of replacing the damaged rubber elbow with copper plumbing fittings (soldered together to fit the application), and connected to the rest of the hose system with suitable hose clamps. Note though that some professional assistance might be required to make this modification work.

Be aware that in the case of leaking intake manifold gaskets on 3.8L engines, substantial disassembly of the intake manifold itself, and other components is required. This procedure is therefore NOT recommended for non-professional mechanics who do not have the required skills, or the required tools/equipment, and technical manuals available.

What are the common causes of code P0171?

It should be noted that neither P0171, nor its most closely related code (P0174), are necessarily indicative of oxygen sensor failures/issues. If oxygen sensor issues are present, these will invariably be indicated by codes that relate to one or more oxygen sensors directly. Thus, in the absence of any other codes, the most likely causes of P0171/P0174 will involve engine vacuum leaks.

Nonetheless, some other common causes of code P0171 could include the following-

  • Defective, dirty, or contaminated sensing element in the MAF (Mass Airflow) sensor
  • Engine vacuum leak almost anywhere in the engine vacuum system, with defective PCV valves and or PCV hoses, leaking intake manifold and valve cover gaskets being the most common sites of leaks
  • One or more defective, clogged, or dirty fuel injectors
  • Insufficient fuel pressure
  • Exhaust leak upstream of the #1 oxygen sensor
  • Engine oil level exceeding the “FULL” mark
  • Defects in oxygen sensors and/or their control/signal/heater circuits

NOTE: Almost all of the possible causes of code P0171 listed here will be indicated by codes that relate to each possible cause. Thus, if there are no additional codes present (both active and pending), the most likely cause of this code by far is a leak in the engine vacuum system.

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