Troubleshooting P0171 and P0174
(Ford 4.0L V6)

How to Test Trouble Codes P0171 and P0174 (Ford 4.0L)

This article will help you to diagnose diagnostic trouble codes P0171 (System too Lean Bank 1) and/or P0174 (System too Lean Bank 2).

One of the biggest misconceptions, about these two diagnostic trouble codes, is that they accuse the oxygen sensors as being BAD... and this is just not true. That's right, these two codes do not identify the oxygen sensors as BAD.

In this article, I'll go into the basics of what causes these codes and I'll also offer you a testing strategy to successfully diagnose P0171 and P1074 diagnostic trouble codes.

Important Suggestions and Tips

Tip 1: You'll need a scan tool (that has Live Data capability) to take advantage of my tips and suggestions in this article.

Now, having said that, I've geared this article towards the person that owns a generic scan tool with Live Data capability Actron CP9580 Scan Tool). You don't need to have a professional technician level scan tool (the scan tool in my photos is just a generic scan tool with Live Data).

Tip 2: Working around and engine, especially when it's running requires that you be alert and take all necessary safety precautions. Your safety is your responsibility... so think safety all of the time.

P0171 and P0174 Essentials

P0171: System Too Lean Bank 1.

  1. PCM's Adaptive Fuel Strategy has reached its Rich calibrated limit for bank 1.
  2. In plain English, this means that the PCM is adding fuel like crazy to even out the Air/Fuel Mixture because it has detected a vacuum leak in bank 1.
  3. The vacuum leak may be real or it may be a “perceived vacuum leak” caused by some failed sensor.

P0174: System Too Lean Bank 2.

  1. PCM's Adaptive Fuel Strategy has reached its Rich calibrated limit for bank 2.
  2. In plain English, this means that the PCM is adding fuel like crazy to even out the Air/Fuel Mixture because it has detected a vacuum leak in bank 2.
  3. The vacuum leak may be real or it may be a “perceived vacuum leak” caused by some failed sensor.

As I pointed out at the beginning of the article, these two Trouble Codes don't point to a specific sensor and accuse it of having failed... and this can sometimes cause a headache... if you don't have a troubleshooting strategy.

The thing that's gonna' help you nail down the problem (and thus the solution) is to remember that these two Trouble Codes are only describing a condition in which the Computer is seeing an unmetered amount of air entering the engine, that no matter how much fuel it dumps (into the engine), it can not compensate for it.

Now, in case you're wondering what the heck is unmetered air:

  1. Unmetered air is any air sucked into the engine after the mass air flow sensor (remember, the MAF sensor's job is to meter [measure] the air so that the PCM can inject the right amount of fuel for that amount of metered air).
  2. For example: if the intake manifold gasket is bad and creating a vacuum leak... the air being sucked into the engine at this point (which is after the MAF sensor) is considered unmetered air.

Usually this unmetered air, that the PCM thinks is entering the engine, is usually due to a very large vacuum leak... but not always and this is what complicates things a bit. Several things can fool the PCM into thinking there is a vacuum leak, like a failing oxygen sensor or even a very dirty MAF sensor.

In this article, I'm gonna' explore several of the things and conditions that cause a Lean Condition. More importantly, I'll offer you a diagnostic strategy that will help you to troubleshoot the issue.

Where are Bank 1 and Bank 2?

You may be wondering where bank 1 and bank 2 are located on the 4.0L Ford V6, well, using the illustration in the image viewer as a guide:

Bank 1 is the engine bank that holds the spark plugs for cylinders 1, 2, 3. This bank is the one on the passenger side of the engine.

Bank 2 is the one having the issue. Bank 2 is the engine bank that holds the spark plugs for cylinders 5, 6, 7. This bank is the one on the driver side of the engine.

The main sensors responsible for telling the PCM that there's a problem with the Air Fuel Mixture are the pre catalytic converter oxygen sensors. These two are also known as the upstream oxygen sensors.

Ford Vehicles:

  • Aerostar 4.0L
    • 1993, 1994, 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003

“The secret to success is to go from mistake to mistake without losing your enthusiasm.”
Anonymous

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