OBDII Codes P0171 and P0174: Diagnosis

Updated on March 23, 2017

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)

The MAF could be under reporting 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 into all the same.

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 the 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 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.


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      Chriss 3 years ago

      Sounds like they just didn't want to take the time to do it for you. Goto Autozone. The computer does store codes and the light does not need to be on. If i had to guess i would say there is a crank senosr code stored.

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      Nichol 3 years ago

      it would not have any codes on it? My undestanding was not unklie yours, I thought it had all the codes on it and the light would come on when it reconised the code was interupted? I was told by them it could be the TPS (Throtle possition sensor) I unpluged this while the car was running at idle it did not shut down no did the check light come on?

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      Bonner 3 years ago

      Pleasing you should think of sohtmeing like that

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      joe 4 years ago

      The man knows what he is talking about. I went to autozone they put it on the diagnostic machine and it gave the codes 0171 0174 1131 1151. The guy said it was probably the o2 sensors so I bought them took my truck(which is a 2003 f150 4x4 4.6 liter) to a mechanic and he looked at it and said it was just the pcv hose leaking. Changed that and its running great.