Diagnosing Diagnostic Trouble Codes P0171 (System too Lean Bank 1) and/or P0174 (System too Lean Bank 2) can be tough. Why? Well because these Codes do not accuse any specific part as BAD.
In this article, I'm gonna' demystify the process of troubleshooting these two codes on the GM 4.3L, 5.0L, and 5.7L and I'm gonna' offer you my own diagnostic strategy (that I've used for a number of years with a lot of success).
You can find this tutorial in Spanish here: Cómo Probar: Códigos P0171 y P0174 (GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).
Tip 1: To completely take advantage of the tests and tips in this article, you'll need a Scan Tool (that has Live Data capability). You don't need to have a Professional Technician level Scan Tool, since a generic Scan Tool that has Live Data capability will do just fine (Don't have a Scan Tool? Need a Scan Tool? Check out my recommendation: Actron CP9580 Scan Tool).
Tip 2: Some of my testing suggestions involve tests with the engine running, this means you've got the on your toes and alert. Think safety all of the time.
P0171: Fuel Trim System Lean Bank 1
(System Too Lean Bank 1)
P0174: Fuel Trim System Lean Bank 2
(System Too Lean Bank 2)
What gives you and me a tremendous head ache, when dealing with these two Diagnostic Trouble Codes, is that they don't accuse a specific Sensor or part as being BAD. For example, neither of these two Codes are saying that the Oxygen Sensors are BAD.
What they are saying is that the Computer is having a hard time calculating the amount of fuel it can spray into the engine because there's an unmetered amount of air entering the engine. So no matter how much Fuel it dumps (into the engine), it can not compensate for it (usually , but not always, due to a very large Vacuum leak).
What specifically ‘throws a wrench in the works’ is that the Engine may not have a Vacuum leak at all... but the Computer thinks it does. It could be an Oxygen Sensor starting to go BAD, but that hasn't died completely yet or even a very dirty MAF Sensor.
Well, I'm gonna' show you a good diagnostic strategy that should help you get to the bottom of the issue.
Before I jump into specific testing techniques, to troubleshoot a P0171 or a P0174, let me go into some detail about Bank 1 and Bank 2.
As mentioned before, both Codes mean, in layman's terms, that the PCM (Powertrain Control Module= Fuel Injection Computer) has noticed that it needs to dump more Fuel than necessary to compensate for a real or perceived Vacuum Leak that are affecting a specific Bank of Cylinders.
Bank 1: Now, to be a little more specific, P0171 DTC tells you that Bank 1 is the one having the issue. Bank 1 is the Engine Bank that holds the Spark Plugs for Cylinders 1, 3, and 5 (V6 4.3L). Cylinders 1, 3, 5, and 7 (V8 5.0L, 5.7L). This bank is the one on the driver side of the engine.
Bank 2: A P0174 DTC tells you that Bank 2 is the one having the issue. Bank 2 is the Engine Bank that holds the Spark Plugs for Cylinders 2, 4, and 6 (V6 4.3L). Cylinders 2, 4, 6 and 8 (V8 5.0L, 5.7L). This bank is the one on the passenger 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.
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She said if she told me, it would defeat the purpose.”