Code P0171 - System Too Lean

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 knowlege: how the air/fuel ratio is adjusted
• Long and Short fuel trims
• What do terms "Bank 1" and "Bank 2" mean?

Symptoms:

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

Causes:

Common causes include:
- Intake or vacuum leaks
- Clogged or faulty Mass Air Flow sensor (MAF)
- Faulty PCV valve
- Stuck open purge valve/solenoid
- Faulty oxygen sensor
- Improperly installed aftermarket cold intake
- Low fuel pressure
- Clogged 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, 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 15-20% at idle and drops at higher rpms, 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.

What is Intake or Vacuum leak?

An air leak anywhere betwen the engine intake and the air filter is usually called vacuum or Intake leak. Common examples of intake 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. In a more technical jargon, vacuum or intake leaks are called "unmetered air", as the air enters the intake system through a damage intake boot or other leak is not metered by the mass air flow sensor and therefore the the engine computer cannot calculate the amount of fuel injected properly, causing the engine to run "lean". Here are few examples of intake or vacuum leaks:

Cracked intake boot Cracked intake hose Disconnected vacuum line

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, a 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.

Please note: the information provided in this article is for informational purposes only and cannot substitute for the advice of professional mechanic or authorized dealer. Consult your mechanic, and check your car's owner's manual for safety measures, precautions, warnings, tips and recommendations.