The MAF sensor is only one of many possible causes of this code.
The Ford PC/ED states the following for this code: P0171 - System Too Lean (Bank 1) Description:
The adaptive fuel strategy continuously monitors the fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit. Refer to Section 1, Powertrain Control Software
Fuel Trim for more information. Possible Causes:
- Ethanol content in the fuel
- Damaged or leaking fuel pulse damper
- Fuel filter plugged or dirty
- Damaged or worn fuel pump
- Leaking fuel pump check valve
- Leaking/contaminated fuel injectors
- Low fuel pressure or running out of fuel
- EVAP canister purge valve is leaking when the canister is clean
- Fuel supply line restricted
- Fuel rail pressure sensor bias
Air Induction System:
- Air leaks after the mass air flow (MAF) sensor
- Vacuum leaks
- Positive crankcase ventilation (PCV) system is leaking or the valve is stuck open
- Improperly seated engine oil dipstick
- Air induction turbulence due to incorrect air filter
- Exhaust leaks in the exhaust manifold gasket or mating gaskets before or near the HO2S
- Vacuum hose disconnected on exhaust gas recirculation (EGR) system module (ESM) applications
- EGR valve tube/gasket leak
- EVR solenoid vacuum leak
Secondary Air Injection:
- Damaged secondary air injection system or a mechanically stuck valve
Air Measurement System:
- Damaged or contaminated MAF sensor
View the Freeze Frame Data to determine the operating conditions when the DTC was set. Observe the LONGFT1 and LONGFT2 PIDs. Refer to Section 2, Adaptive Fuel DTC Diagnostic Techniques
for more information and the appropriate pinpoint test for specific concern identification. Application Key On Engine Off Key On Engine Running Continuous Memory
All — — GO to Pinpoint Test H
Adaptive Fuel DTC Diagnostic Techniques
The Adaptive Fuel DTC Diagnostic Techniques help isolate the root cause of the adaptive fuel concern. Before proceeding, attempt to verify if any driveability concerns are present. These diagnostic aids are meant as a supplement to the pinpoint test steps in Section 5. For a description of fuel trim, refer to Section 1, Powertrain Control Software
, Fuel Trim. Obtain Freeze Frame Data
Freeze frame data is helpful in duplicating and diagnosing adaptive fuel concerns. The data (a snapshot of certain PID values recorded at the time the DTC is stored in Continuous Memory) is helpful to determine how the vehicle was being driven when the concern occurred, and is especially useful on intermittent concerns. Freeze frame data, in many cases, will help to isolate possible areas of concern as well as rule out others. Refer to Freeze Frame Data
in this section for a more detailed description of this data. Using the LONGFT1 and LONGFT2 (Dual Bank Engines) PIDs
The LONGFT1/2 PIDs are useful for diagnosing fuel trim concerns. A negative PID value indicates that fuel is being reduced to compensate for a rich condition. A positive PID value indicates that fuel is being increased to compensate for a lean condition. It is important to know that there is a separate LONGFT value that is used for each RPM/load point of engine operation. When viewing the LONGFT1/2 PIDs, the values may change a great deal as the engine is operating at different RPM and load points. This is because the fuel system may have learned corrections for fuel delivery concerns that can change as a function of engine RPM and load. The LONGFT1/2 PIDs will display the fuel trim currently being used at that RPM and load point. Observing the changes in LONGFT1/2 can help when diagnosing fuel system concerns. For example:
Resetting Long Term Fuel Trims
- A contaminated MAF sensor will result in matching LONGFT1/2 correction values that are negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads.
- LONGFT1 values that differ greatly from LONGFT2 values will rule out concerns that are common for both banks (for example, fuel pressure concerns, MAF sensor, etc. can be ruled out).
- Vacuum leaks will result in large rich corrections (positive LONGFT1/2 value) at idle, but little or no correction at higher RPM and loads.
- A plugged fuel filter will result in no correction at idle, but large rich corrections (positive LONGFT1/2 value) at high RPM and load.
Long term fuel trim corrections are reset by resetting the keep alive memory (KAM). Refer to Resetting The Keep Alive Memory (KAM)
in this section. After making a fuel system repair, the KAM must be reset. For example, if dirty/plugged injectors cause the engine to run lean and generate rich long term corrections, replacing the injectors and not resetting the KAM will now cause the engine run very rich. The rich correction will eventually be learned out during closed loop operation, but the vehicle may have poor driveability and high CO emissions while it is learning. P0171/P0174 System Too Lean Diagnostic Aids Note:
If the system is lean at certain conditions, then the LONGFT PID would be a positive value at those conditions, indicating that increased fuel is needed.
The ability to identify the type of lean condition causing the concern is crucial to a correct diagnosis. Air Measurement System
With this condition, the engine will run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. One possibility is that the mass of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine will run lean at higher RPM because the PCM will deliver fuel for less air than is actually entering the engine.
Examples: MAF sensor measurement is inaccurate due to a corroded connector, contaminated or dirty connector. A contaminated MAF sensor will typically result in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel). Vacuum Leaks/Unmetered Air
With this condition, the engine will run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition is caused by unmetered air entering the engine, or due to a MAF malfunction. In this situation, the volume of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. Vacuum leaks will normally be most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that the concern occurred at idle, a check for vacuum leaks/unmetered air is the best starting point.
Examples: Loose, leaking, or disconnected vacuum lines, intake manifold gaskets, or O-rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket PCV valve, unseated engine oil dipstick. Insufficient Fueling
With this condition, the engine will run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition is caused by a fuel delivery system concern that restricts or limits the amount of fuel being delivered to the engine. This condition will normally be apparent as the engine is under a heavy load and at high RPM, when a higher volume of fuel is required. If the freeze frame data indicates that the concern occurs under a heavy load and at higher RPM, a check of the fuel delivery system (checking fuel pressure with engine under a load) is the best starting point.
Examples: low fuel pressure (fuel pump, fuel filter, fuel leaks, restricted fuel supply lines), fuel injector concerns. Exhaust System Leaks
In this type of condition, the engine will run rich of stoichiometry (14.7:1 air/fuel ratio) because the fuel control system is adding fuel to compensate for a perceived (not actual) lean condition. This condition is caused by oxygen (air) entering the exhaust system from an external source. The HO2S will react to this exhaust leak by increasing fuel delivery. This condition will cause the exhaust gas mixture from the cylinder to be rich.
Examples: Exhaust system leaks upstream or near the HO2S, cracked/leaking HO2S boss, malfunctioning secondary air injection system. P0172/P0175 System Too Rich Diagnostic Aids Note:
If the system is rich at certain conditions, then the LONGFT PID would be a negative value at that airflow, indicating that decreased fuel is needed.
System rich concerns will be caused by fuel system concerns, although the MAF sensor and base engine (for example, engine oil contaminated with fuel) should also be checked. Air Measurement System
With this condition, the engine will run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. One possibility is that the mass of air entering the engine is actually less than what the MAF sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine will run rich at idle because the PCM will deliver fuel for more air than is actually entering the engine.
Examples: MAF sensor measurement inaccurate due to a corroded connector, contamination/dirt. A contaminated MAF sensor typically results in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel). Fuel System
With this condition, the engine runs rich of stoichiometry (14.7:1 air/fuel ratio), if the PCM is not able to compensate enough to correct for the condition. This situation causes a fuel delivery system that is delivering excessive fuel to the engine.
Air Inlet System
- Fuel pressure regulator causes excessive fuel pressure (system rich at all airflows), fuel pressure is intermittent, going to pump deadhead pressure, then returning to normal after the engine is turned off and restarted.
- Fuel pulse dampener diaphragm ruptured (fuel leaking into the intake manifold, system rich at lower airflows).
- Fuel injector leaks (injector delivers extra fuel).
- EVAP canister purge valve leak (if the canister is full of vapors, introduces extra fuel).
- Fuel rail pressure (FRP) sensor (electronic returnless fuel systems) concern causes the sensor to indicate a lower pressure than actual. The PCM commands a higher duty cycle to the fuel pump driver module (FPDM), causing high fuel pressure (system rich at all airflows).
A restriction within any of the following components may be significant enough to affect the ability of the PCM adaptive fuel control.
- air inlet tube
- air cleaner element
- air cleaner assembly
- clean air tube
Engine oil contaminated with fuel can contribute to a rich-running engine.