P0171 and P0174: What are they?

Error code P0171 is defined as System too lean Bank 1, and P0174 is System too Lean Bank 2.

Find more about these two codes in this excellent article below. You will find out how these codes are very similar as well as many other great tips.

Symptoms, causes and costs to repair are all covered in the guide below.

What is a P0171 and P0174?

For an efficient combustion system, there is a standard air to fuel mix ratio of 14:7 that ensures the engine’s proper functioning. By monitoring the oxygen sensors on the vehicle, the Powertrain Control Module (PCM) has the ability to detect if oxygen content in the exhaust is correct.

When the PCM notices that the levels are too lean, the code P0171 or P0174 is set.

These codes are very similar as they set under the same conditions, however there is one difference between them.

P0171 is set when a lean condition is noticed on Bank 1 and P0174 is set when a lean condition is present on Bank 2.

There can be numerous reasons for a lean condition to exist. If more air is introduced to the system, there can be a vacuum leak. Too little fuel in the air/fuel ratio can also cause a lean condition.

The Powertrain control module can make necessary adjustments in the system. However, when the adjustments exceed the limit, the P0171 or P0174 code is triggered.

Symptoms of A P0171 Code

In typical cases you will notice that the check engine light is on. You may also notice any issues while driving.

There are many symptoms that can be noticed because of this lean condition. They will most likely be noticed as a check engine light, but you will also notice poor fuel economy.

If the engine does not have a proper air/fuel mixture provided to the engine, complete combustion does not occur. This will cause a decline in engine performance as well as fuel mileage.

  • Improper functioning of the engine
  • The check engine light is illuminated
  • Rough idling
  • engine misfire
  • Lack of power while accelerating


Like any other code that you find, you need to ensure you find the root cause to properly repair.

 Try to find the minor causes and if you notice any larger issues during your diagnosis, shift your focus on them.

A few common causes of the P0171 code are:

  • A damaged vacuum pump/vacuum hose
  • A faulty fuel pressure regulator
  • Low fuel pump output
  • A fault in the Powertrain Control Module
  • Plugged or misfiring fuel injectors
  • A fault in the oxygen sensors
  • Faulty mass airflow sensor

How to Repair the Issue

Vacuum leaks, low fuel pressure, and faulty sensors are the significant causes of a lean mixture. These usually trigger the P0171 or P0174 code.

After carefully inspecting the root cause indicating the code, you have two choices. You can tackle the repair yourself or take it to an experienced automotive technician.

Here are some of the more common repairs that have repaired P0171 or P0174 error codes.

  • Replacing the fuel pump
  • Replacing the fuel filter
  • Replacement of the power train control module
  • Changing the fuel pressure regulator
  • Replacing faulty oxygen sensors
  • A replacement of the Mass Air Flow sensor
  • Repairing vacuum leaks

May I drive my vehicle with P0171 or P0174?

These codes can result in other vehicle concerns. When the air fuel ratio is lean it can cause many different drivability concerns.

You may notice a lack of power and reduced fuel economy. Which, with today’s fuel prices are reason alone to make sure you repair as soon as possible.

Cost to repair P0171 or P0174

The estimated cost of repairing the P0171 or P0174 code:

  • Diagnostic fee $100 (depending on the time taken).
  • The broken vacuum hose repairing $50-$150
  • The clogged fuel filter repair $100-$200
  • A new fuel pump $600 to $800
  • A new oxygen sensor $400 to $500
  • Fuel pressure regulator $200 to $400
  • Mass Air Flow Sensor replacement $100-$300

These prices vary due to vehicle make and model. They are rough estimates to give you a good idea of the repair costs expected when you need this type of repair.

Other Codes That Are Similar to the P0171 Code

While in most cases, the P0171 code may appear alone. But in some cases, it can be accompanied by a P0174 code. This would indicate that the entire engine is running lean and not just one cylinder.

This can always help in diagnosing the issue that is present.  

Staff Writer
Staff Writer
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