Code P0171 - System Too Lean: Symptoms, causes, common problems, diagnostic

The OBDII trouble code P0171 - System Too Lean (Bank 1) means that the engine air-fuel mixture is too lean, or there is too much air and too little fuel. Driving a car with the code P0171 can cause more damage to your vehicle over time, as when the engine runs lean, the combustion temperature is much higher.

• Symptoms
• Causes
• Diagnostic
• What is a vacuum or intake leak?
• Common problems causing the code P0171
• Basic car knowledge: how the air/fuel ratio is adjusted
• Long and Short fuel trims
• What do terms "Bank 1" and "Bank 2" mean?


Typical symptoms of the lean air fuel mixture include:
- Lack of power
- Rough idle
- Hissing noise from the engine (vacuum leaks)
- Stalling
- Difficulty starting
- Engine knock or detonation (pinging) under load
- Engine running hot


Common causes include:
- Intake or vacuum leaks
- Clogged or faulty Mass Air Flow sensor (MAF)
- Faulty PCV valve or other parts of PCV system
- Stuck open purge valve/solenoid
- Faulty oxygen sensor
- Improperly installed aftermarket intake
- Low fuel pressure
- Clogged-up catalytic converter

How the code P0171 is diagnosed:

With a good scan tool that shows the live data diagnosing the code P0171 is fairly easy. Without a scan tool, the only way to find the problem is to check the basics and research common problems with your particular car make, year and model. We have done some research and posted common problems for some cars, please read further. The best place to take your car for diagnostics is your local dealer - their technicians are factory-trained and know the common problems with the cars they service.
Basic checks under the hood include: is the air filter box properly closed? Does the intake boot have any cracks? Are any of the vacuum lines or hoses disconnected?
If you have a scan tool, check the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings at idle and at higher rpms (Read more about fuel trims below). Normally, both the LTFT and STFT should be close to zero. If the STFT rises higher than plus 15-20% at idle, but drops at higher rpms, it's most likely you have a vacuum leak. If the STFT is close to zero at idle, but elevates to 15-20% at higher rpms, the problem could be caused a faulty mass air flow sensor, clogged catalytic converter or low fuel pressure. Read more about using a scan tool here.

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What is Intake or Vacuum leak?

An air leak anywhere between the engine intake and the mass air flow sensor is often called a 'vacuum leak' or 'intake leak'. Common examples of vacuum leaks include ripped or cracked intake boot, leaking intake manifold gaskets, stuck-open or defective PCV valve or ripped PCV hose, cracked or disconnected vacuum line. Read more: Vacuum leaks: common sources, symptoms, repairs. Here are few examples of intake or vacuum leaks:

Cracked intake boot Cracked intake hose Disconnected vacuum line Leaking intake manifold gaskets

Common problems causing the code P0171 in different cars:

One way to find a common problem with your make and model is to search for technical service bulletins (TSBs). For example, the Toyota Technical Service Bulletin (TSB) EG045-07 for the 2003-08 Toyota Corolla and Matrix with the 1ZZ–FE engine mentions a leaking intake manifold gaskets in subfreezing temperature as a possible reason for the code P0171 along with some other codes. If the leaking manifold gasket is confirmed following the diagnostics steps, it must be replaced.
Many Ford vehicles, including Explorer and F150 had a common problem with leaking gaskets between upper and lower intake manifolds causing the codes P0171 and P0174. Ford has issued a technical service bulletin on this issue. Another known issue in Ford vehicles is a vacuum leak from a ripped-through rubber elbow on one of the vacuum lines at the back of the intake manifold. Some V6 Mazda vehicles (Tribute, MPV) also have this issue where the rubber elbow at the back of the intake manifold rips through, causing a vacuum leak and the code P0171.
A cracked or ripped intake boot is very common reason for the code P0171 in many BMW vehicles.

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