P0171 OBD-II Trouble Code: System Too Lean (Bank 1)

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.

P0171 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 Trouble Code 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 symptoms down below.

• Check Engine Light On
• Rough Idle or Acceleration
• Loss of Power
• Misfires
• Low/High/Jumping Idle
• Hard Starting Condition
• The engine may die when driving

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.

Causes of the P0171 Code

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; here are some of them.

• Air intake/vacuum/boost pipe leak (Most common)
• Faulty PCV Valve
• Low fuel pressure (Caused by a bad fuel pump, filter, or fuel pressure regulator)
• A faulty EVAP purge valve
• Faulty O2 sensors
• Faulty EGR valve
• Faulty MAP/MAF sensors
• Exhaust leak (Before Front O2 sensors)
• Coolant temperature sensor
• Faulty sensor wirings
• Faulty ECM/PCM (Rare)

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 P0171 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 P0171 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$