P0171 Code – Meaning, Causes, Symptoms (& How to Fix it)

A guide on how to diagnose & repair problems causing the P0171 trouble code

Obd2 Trouble Code

A P0171 code appears in the engine control unit when the Oxygen sensor on bank 1 recognizes a too lean air-fuel mixture for the sensor to correct it.

Several different things could cause this trouble code. Here is what you need to know about the P0171 code.

Code Definition

P0171 – System Too Lean (Bank 1)

What does the P0171 code mean?

The P0171 code means that the Oxygen (O2) sensor on bank 1 recognized a too lean mixture to correct it.

The Oxygen O2 sensors have the ability to correct a fuel mixture of +-15%. If it needs to correct the air-fuel mixture more than that, the P0171 code will be stored.

P0171 Symptoms

If the fuel mixture is slightly lean, you will often get no other symptoms from the P0171 code other than the check engine light on your dashboard. If the fuel mixture is very lean, you may get any of the following symptoms:

How serious is the P0171 Code?

Moderate – In most cases, the lean mixture is significant, and you will not notice any difference in the engine’s performance. Other times your car engine may actually run so lean that your engine parts are in danger on high load.

It might be the first sign of something that will fail more soon, like a clogged fuel filter or a failing fuel pump.

It is okay to drive for short distances to the workshop, but ignoring the code and continue to drive may cause integral engine parts to take damage.

P0171 Causes

The P0171 tells us that there is a lean air-fuel mixture, but it doesn’t tell us what could cause a lean fuel mixture. There are actually many different things that can cause a lean mixture. The following issues can be caused by the P0171 code:

What repairs can fix the P0171 code?

There are many different solutions for the P0171 code because there can be so many different causes. Here are the most common fixes to the P0171 code. Most common from the top.

  • Replace faulty vacuum hoses or gaskets around the intake
  • Repair other intake leaks
  • Replace PCV Valve
  • Replace the fuel pump/fuel filter/fuel pressure regulator or repair wirings
  • Replace EVAP valve
  • Replace O2 sensor(s)
  • Replace EGR valve
  • Replace MAP/MAF sensor
  • Repair exhaust leak
  • Replace coolant temperature sensor
  • Repair faulty wirings
  • Replace ECM/PCM (rare)

Common Diagnosis mistakes

A common diagnostic mistake is to start replacing parts without making a proper diagnosis. In most cases, the P0171 code is caused by something easy as a vacuum leak or a bad PCV valve. If you start replacing MAF sensors, O2 sensors, etc., you will probably spend a lot of money without results.

A faulty O2 sensor is not a very common cause of the P0171 code, even if many people start to replace it directly when they see this code.

How to diagnose the P0171 Code

Diagnosing the P0171 code is easy if you have the right tools to do it. It is most often not easy to diagnose the P0171 without these tools. This guide is made for professionals with access to these tools. In most cases, you can lend these tools at a workshop if you ask nicely.

  1. Connect an OBD2 scanner to your vehicle. Recheck the trouble codes and find any other related trouble codes that could cause a lean mixture. Follow the diagnose procedure on the other trouble code if you get any.
  2. Check the live data for the MAF sensor, temperature sensors, and figure out if their signals are logical or way out of range.
  3. Connect an EVAP smoke machine to your car’s inlet and check for any intake leaks. Check the PCV valve and the EVAP purge valve. You will find a leak or a faulty one-way valve that is causing the P0171 code in most cases.
  4. Check the fuel pressure. Connect a fuel pressure gauge to the fuel rail and make sure that the fuel pressure is correct. If it is too low, check the fuel filter, fuel pump, fuel pressure, and regulator.
  5. Check the EGR valve. An EGR valve stuck open can cause the air-fuel mixture to stay lean. If possible, try to check the valve inside it while you open and close it with your diagnostic tool.
  6. Clean the MAF sensor. A dirty MAF sensor can cause the air-fuel mixture. Inspect it and check for any dirt on the sensor. After it, you should try to clear the trouble codes and check if the code comes back. It might also be damaged, so check its values on idle and on pressure and replace it if faulty.
  7. Check for exhaust pipe leaks before the O2 sensor. Repair the leaks if you find any.
  8. If the problem still occurs and you haven’t found any problems with the mentioned parts above, it is time to diagnose your O2 sensor. If you find it giving wrong values – replace it.

Estimated Repair Cost

Here are some examples of the repair costs of the P0171 code. The prices are including parts and labor. It does not include diagnosis costs.

  • Vacuum Hose replacement – 10$ to 50$
  • O2 sensor replacement – 200$ to 300$
  • PCV Valve replacement – 20$ to 60$

Common Related Questions

How to fix the P0171 code?

To fix the P0171 code, you need to check and repair vacuum leaks, repair low fuel pressure, or replace any faulty sensor that can cause a lean mixture on bank 1.

What causes P0171 code?

The most common causes of the P0171 code are vacuum or intake leaks, low fuel pressure, faulty PCV valve, or a faulty MAF sensor. Other things can also cause the P0171 code.

What does P0171 mean?

The P0171 code means that the O2 sensor recognized a lean mixture on bank 1. The sensor can only correct the mixture within +-15%, and if it is out of there, the p0171 code will be stored.

How to clear code P0171?

To clear or reset the P0171 code, you need to use an OBD2 scanner. Remember that just resetting the code will not fix the problem causing it from the beginning, and it will most likely come back again.

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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.