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OBDII Codes P0171 and P0174: Diagnosis

Johnny is a longtime online writer and car enthusiast who has expertise in fixing car problems.

An all-too-familiar reminder to drivers everywhere.

An all-too-familiar reminder to drivers everywhere.

If your "check engine" light is on, and you've used an OBDII scanner tool to read the fault codes, and they returned P0171 and P0174, this guide is for you. Here is a discussion of their meaning and their common causes.


P0171: Fuel System Too Lean (Bank 1)
P0174: Fuel System Too Lean (Bank 2)

Common Symptoms in Your Car When These Codes Are Found

  • "Check Engine" or "Service Engine Soon" light is on
  • Rough idle
  • Lack of performance or power when accelerating, or a misfiring engine
  • In many cases, there are NO noticeable signs of engine trouble

Why Have These Codes Been Logged?

Basically, because your engine fuel mixture has too much air and not enough fuel in it (a lean mixture).

Your engine operates at an optimal fuel/air mixture that is controlled by the Engine Control Module. A mixture that isn't optimal may be called:

"Rich" Mixture: Too much fuel, not enough air
"Lean" Mixture: Too much air, not enough fuel

The Engine Control Module measures the amount of oxygen in the exhaust system via the oxygen sensors and makes adjustments by adding more or less fuel to the mixture. When the adjustments become too large to maintain the proper mixture, the fault codes P0171 and P0174 are logged. They are logged together, since in V8 and V6 (and more rarely in some 4-cylinder and straight 6-cylinder cars) the cylinders are split into two separate groups, or banks.

Common Causes of P0171 and P0174

1. Vacuum Leaks

This is the most likely cause of too much air being taken into the system. The source of a vacuum leak could be one of many things, including (but not limited to) cracking, wear, or a hole in any of the following:

  • Vacuum hoses
  • Air intake boots (see image example below)
  • PCV hoses
  • EGR valves
  • DISA valves
  • A loose dipstick (or one with a broken seal)

If any of these parts are not forming a perfect seal (have a slit in them) then unmetered air will be entering the engine. Try to listen for a whistle or hiss under the hood with the engine running and visually inspect all hoses and connections.

Example of cracks in an air intake boot causing P0171 and P0174 codes

Example of cracks in an air intake boot causing P0171 and P0174 codes

2. Mass Airflow Sensor (MAF)

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The MAF could be underreporting the amount of air passing through the intake. This could be caused by a dirty or fouled MAF or the MAF could be defective.

3. Engine Computer Software Needs to Be Updated

The Powertrain Control Module (PCM) software may need to be updated. As an engine wears, the PCM Fuel Map may not properly compensate. This seems rather unlikely but worth checking out.

4. Fuel System Obstructions

These could include:

  • Clogged fuel filter
  • Weak fuel pump
  • Clogged fuel injectors

This is a case in which air flow is normal but not enough fuel is being delivered. Due to a possible loss of fuel pressure or flow, the ECU cannot provide enough fuel to attain the correct mixture.

5. Intake Manifold Gasket Leaking

If the engine is a V8 or V6 and only one side or bank is reporting a fault code it could be a leaking intake manifold gasket or leaking manifold. If you have logged both codes, however, it is unlikely that a leaking manifold would be the issue since both manifolds would have to be leaking.

What About the Oxygen Sensors?

It's possible but highly unlikely that P0171 and P0174 codes are the result of faulty O2 sensors. If O2 sensors are causing both P0171 and P0174 banks to report a lean mixture, that would mean that both O2 sensors are misreading the amount of oxygen in the exhaust. Not a likely scenario. Also, the computer will run validation tests on the readings of the O2 sensors before looking at fuel trim adjustments. Only then would a too-lean mixture be returned and logged as a fault P0171.

Research What These Codes May Mean for Your Make and Model

Be sure to search for other information about these codes that might be specific to your car's make and model. There may be other very specific (or common) issues that other motorists have experienced with your car. Just Google the codes followed by your car's make and model, for example: "P0171 and P0174 Ford Expedition."

As one example, many report that a common cause of the P0171 and P0174 lean mixture fault in the BMW X3 is a faulty DISA valve.

How to Read Your OBDII Computer Codes

Read this tutorial on how to use one particular OBDII Bluetooth code reader.

Additionally, an economical solution that doesn't require a smartphone or dealing with wireless connecting is an all-in-one code reader.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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