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.
You can find this tutorial in Spanish here: Cómo Probar: Códigos P0171 y P0174 (4.0L Ford) (at: autotecnico-online.com).
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: System Too Lean Bank 1.
P0174: System Too Lean Bank 2.
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:
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.
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.
“For instance, on the planet Earth, man had always assumed that he was more
intelligent than dolphins because he had achieved so much —the wheel,
New York, wars and so on —whilst all the dolphins had ever done was
muck about in the water having a good time. But conversely,
the dolphins had always believed that they were far more
intelligent than man —for precisely the same reasons!”
The Hitchhiker's Guide to the Galaxy