#OBD Codes

P0171 – Fuel trim, bank 1. System too lean

P0171 – Fuel trim, bank 1. System too lean
Back to OBD codes library

Technical definition

Fuel trim, bank 1. System too lean.

What does P0171 mean?

The P0171 trouble code indicates that your vehicle's engine runs too lean on bank 1. This means the air-fuel mixture contains too much air and not enough fuel.

This imbalance can occur for a several reasons, including faulty oxygen sensors, problems with mass airflow sensors, fuel system issues, or vacuum leaks.

What is the fuel ratio? 
The air-to-fuel ratio (AFR) is the proportion of air to fuel in an engine's combustion mixture. Gasoline engines aim for a 14.7:1 ratio for optimal efficiency. The upstream oxygen sensor measures this ratio and provides feedback to the engine control unit (also known as the powertrain control module).

An air-to-fuel ratio graph below illustrates how the AFR varies under different engine conditions. At the ideal ratio of 14.7:1, the engine runs efficiently. Deviating too far, either too lean or too rich, will affect performance and may trigger the check engine light and diagnostic trouble codes such as P0171.
The graph illustrates the relationship between air-fuel ratios, power output, and fuel consumption.

What does "Bank 1" mean?
In engines with two cylinder heads (like V6, V8, V10, W8), "Bank 1" indicates the side with the first cylinder.

How serious is this DTC?

The P0171 code isn't always noticeable and may not seem very serious at first. However, it can sometimes cause irreversible damage to the vehicle, so it can't be ignored.
If left unaddressed, it can cause improper combustion and engine knocking. In addition, a lean mixture burns very hot, which can even melt engine internals and damage the ceramic honeycomb inside the catalytic converter. This extreme heat can severely damage components such as pistons, piston rings, spark plugs, cylinder head, and valves. 
It's important to address this issue immediately to prevent further damage. 

Can I still drive with P0171?

While you can still drive with a P0171 code, your first stop should be a repair shop. Driving with this code can make the problem even worse. 
In the short term, you may notice reduced performance and possible stalling.

Symptoms of this DTC

When your vehicle has a P0171 code, there are several symptoms you might notice. These include:
  • Engine stalling or rough idling 
  • Reduced power and poor acceleration 
  • Limp mode 
  • Difficulty starting the vehicle 
  • Check engine light illuminated 
  • Unusual engine noises, such as knocking or pinging 
  • Overheating engine

Common causes of this DTC

Several common issues can trigger the P0171 code, including:
  • Dirty or faulty mass airflow (MAF) sensor 
  • Vacuum leak in the intake manifold, hoses, or Positive Crankcase Ventilation (PCV) system 
  • Faulty oxygen sensor 
  • Clogged fuel injectors 
  • Dirty fuel filter 
  • Weak fuel pump or low fuel pressure 
  • Faulty fuel pressure regulator 
  • Exhaust leak before the O2 sensor 

How to troubleshoot this DTC

To avoid wasting money fixing components that aren't broken, use these tips:

Scan fault codes

Using a diagnostic scanner, confirm the P0171 code and look for any additional codes that may provide further insights into the problem. Other common trouble codes to watch for include:
  • P0170 – fuel trim malfunction indicates an issue with how the engine adjusts the air-fuel mixture 
  • P0174 – indicates the engine is running too lean on Bank 2. This often appears alongside P0171 and can point to a shared issue affecting both banks. 
  • P0101-P0104 – mass airflow sensor related codes indicating issues with the sensor that could affect air-fuel mixture 
  • P0130-P0135 – oxygen sensor-related codes might signal problems with the sensors responsible for monitoring the air-fuel ratio 
  • P0300-P0312 – misfire codes can suggest fuel delivery, ignition, or air intake issues that might contribute to a lean condition 
Clear the codes and test drive 
Use your scanner to clear fault codes. Then, drive your car for at least 15 minutes to fully warm up the engine – some issues only show up when the engine is hot. Also, try different driving styles; sometimes, this code only shows up with aggressive driving. 
After the drive, scan the engine again to check if the same problems reappear. If the issues persist, this indicates there's a real problem that needs further attention. This step helps determine whether the issue is ongoing or was just a one-time occurrence.  
Check freeze frame data
The freeze frame data captured when the fault code occurs can provide valuable insight. This data includes engine load, temperature, rpm, and other parameters that help you understand the conditions that triggered the code. 
Analyze fuel trim live data readings

For more detailed information, check the short and long term fuel trim live data values. In the OBDeleven VAG mobile app, you'll find these values under the engine control unit details. Normally, fuel trim values range from -5% to +5%.

High positive values indicate the engine is compensating for a lean condition by adding more fuel, while high negative values suggest it is reducing fuel to correct a rich condition. Monitoring these values helps determine if the lean condition is ongoing and how severe it is. 

It's crucial to look at live data parameters only when the engine is warm, as a cold engine will naturally run rich. 
Further reading: How to read OBD live data 
Check other live data parameters

Other related live data parameters can help identify potential causes of the P0171 code, too. Consider looking at:
  • Mass airflow sensor readings. Ensure the MAF sensor is providing accurate airflow measurements. 
  • Oxygen sensor voltage. Check the upstream O2 sensor for proper operation, typically fluctuating between 0.1 and 0.9 volts. 
  • Exhaust Gas Recirculation (EGR) valve position. Check the EGR valve to ensure it is functioning properly and not causing incorrect air-to-fuel ratios. 
Regularly change fuel filter

A fresh fuel filter ensures a steady flow of clean fuel to the engine and prevents contaminants from clogging the fuel injectors. 
It is generally recommended to change the fuel filter every 30,000 to 40,000 kilometers (20,000 to 25,000 miles), but always consult your vehicle owner's manual for specific intervals. 
Clean MAF sensor 
A dirty MAF sensor is a common cause of the P0171 fault code. Cleaning it is a simple process that can restore accurate air measurements. 
Here's how to clean the MAF sensor:
  1. Locate the MAF sensor. It's typically located between the air filter box and the throttle body. 
  2. Disconnect the MAF sensor. Carefully unplug the electrical connector to avoid damage. 
  3. Remove it. Use a screwdriver to remove the screws securing the sensor in place. 
  4. Clean the sensor. Spray the sensor wire only with a specialized MAF sensor cleaner. Oil-based solutions can damage the sensor. Don't touch the wire, as it is very fragile. Let it dry completely. 
  5. Reinstall the sensor. After it dries, reinstall the sensor and reconnect the electrical connector. 
Mercedes MAF sensor


How does a fuel pump cause a P0171 code?

A weak or failing fuel pump can cause insufficient fuel delivery to the engine, especially when driving fast or under heavy load. This lack of fuel causes the engine control unit to detect a lean mixture and trigger the P0171 trouble code.

How does a vacuum leak cause a P0171 code?

A vacuum leak allows unmetered air to enter the engine, resulting in too much oxygen in the air-fuel mixture and causing the engine control unit to detect a lean condition.

How do exhaust leak cause a P0171 code?
If there's a leak in the exhaust system before it reaches the O2 sensor, it can pull in more oxygen. This makes the sensor think the engine is getting more air than it actually isdoes, which triggers the P0171 trouble code. 
Why is it important to use only specified MAF sensor cleaner? 
Other cleaners may contain grease and other substitutes that don't evaporate. This leaves a film on the sensor wire, which disrupts the measurements. Also, the wire is very sensitive to abrasives. 
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.

Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.