P0171 CODE: Causes and Fixes, System Too Lean (Bank 1)

Everything that you need to know about the p0171 code, and more! That is what you can expect in this article. As a technician myself, I have received a lot of inquiries about this code. Knowing how to solve this problem starts with understanding what this is all about. Let me share with you what I know about it, and guide you through the process.

What Does Diagnostic Code p0171 Mean?

One thing that you need to know is that there are a number of misconceptions surrounding this error code. There are also some ‘shady’ tips that others give to fix it. My job as a technician is to solve issues like this.

The diagnostic code p0171 happens when a lean mixture is detected by the front O2 sensors. This lean mixture can either be short term or long term. If the lean mixture is a short term one, it means that the fuel mixture is usually very lean. Otherwise, for long-term lean mixture, the code p0171 is registered according to an indication of the lean mixture through time.

By design, these O2 sensors typically come with a +/- 15% range. This is for the fuel mixture adjustment. If the fuel mixture is beyond this range, it is not possible for the O2 sensor to adjust the mixture. An error code is then triggered in the engine control unit. This error is then stored in the memory. If there is a lean mixture, the diagnostic code p0171 is then triggered.

What are the Causes of OBD2 Code p0171?

There are a number of engine parts or sensors which may cause the trigger of the p0171 code, along with a lean mixture. Aside from simply focusing on p0171, it is also best to look for other sensor-related codes. Doing so will be able to provide an indication as to where you can start searching for solutions.

Here are some of the most common causes of this error:

  • Most common – a leak in the vacuum, air intake, and boost pipe
  • Low fuel pressure (this usually comes as a result of a faulty fuel pump, faulty fuel pressure regulator or filter)
  • Faulty O2 sensors
  • Faulty EVAP value
  • Faulty EGR valve
  • Faulty PCV value  (This is a common issue among VAG cars, such as VW, Audi, Skoda, and Seat)
  • Faulty MAF sensors
  • Faulty MAP sensors
  • Faulty wirings (sensors)
  • Faulty ECM/PCM
  • Faulty coolant temperature sensor
  • Exhaust leak

  • How to Solve Error p0171 Code?

    When it comes to addressing error p0171 code, there are a number of solutions that are available. I will list down the possible solutions for the issues that were mentioned above.

    Issue: Most common – a leak in the vacuum, air intake, and boost pipe
    Possible Solution: Replace the faulty vacuum hoses, gaskets surrounding the intake, as well as other possible intake leaks.

    Issue: Low fuel pressure (this usually comes as a result of a faulty fuel pump, fuel pressure regulator or filter)
    Possible Solution: Replace fuel pump, filter, fuel pressure regulator, or repair the wirings if needed.

    Issue: Faulty O2 sensors
    Possible Solution: As needed replace the O2 Sensors

    Issue: Faulty EVAP value
    Possible Solution: Replace EVAP value

    Issue: Faulty EGR valve
    Possible Solution: Replace EGR valve

    Issue: Faulty PCV value (This is a common issue among VAG cars, such as VW, Audi, Skoda, and Seat)
    Possible Solution: Replace the PCV valve

    Issue: Faulty MAF/MAP sensors
    Possible Solution: Clean or replace the MAF/MAP sensors

    Issue: Faulty wirings (sensors)
    Possible Solution: Repair the faulty wirings

    Issue: Faulty ECM/PCM
    Possible Solution: Replace the ECM/PCM

    Issue: Faulty coolant temperature sensor
    Possible Solution: Replace coolant temperature sensor

    Issue: Exhaust leak
    Possible Solution: Repair exhaust leak 

    How to Diagnose Code p0171?

    Since the error code p0171 means the presence of a lean mixture in a vehicle, various sensors may cause the code. In order to diagnose this code, do the following:

    Step 1: Use a car battery charger and connect it to your vehicle

    This is part of the troubleshooting process, with which you turn on ignition while not wanting to achieve low voltage. The latter may result in other fault codes, which can give you incorrect values and confuse you while troubleshooting. A low voltage may damage the electrical components of your car, which is why using a car battery charger is very important.

    This is easier to do if you have an OBD2 scanner. Check the values of the coolant temperature, MAF/MAP, intake temperature sensors, boost pressure, and make sure that they are appropriate. Replace the ones that are defective, delete the codes, and do it again.

    Step 3: Check other components

    After checking all sensor parameters, you can then work on checking all other parts, including intake leaks, fuel pressure, EGR valve, exhaust leaks, MAF/MAP sensor. You can also perform a thorough diagnostics of the sensors, or to check if there are damaged parts inside. For this, you need the help of a diagnostic technician for thorough coverage.

    2 thoughts on “P0171 CODE: Causes and Fixes, System Too Lean (Bank 1)”

    1. Hi James Davis
      I used the Auto Zone purchase of the Innova 3040 Diagnostic DIY tool
      I GOT : PO171 System Too Lean Bank (1) Global OBD2
      I ended up cleaning the Air Flow Sensor with the proper cleaner.
      After about a year…. the engine light came on. I checked
      again, with my Innova…And got the exact same codes.
      Today I just unhooked the Negative cable from battery for 1min.
      And Engine light is off for now.
      Any input you can give me…. would great!
      Thanks Barry


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