Diagnosing Trouble Codes: P0172 (System too Rich Bank 1) and/or P0175 (System too Rich Bank 2) can be a challenge on your GM 4.3L, 5.0L, 5.7L equipped GM pick-up, van, SUV or mini-van.
What complicates getting to the bottom of what's causing these two codes to pop up, is that they don't accuse one specific component as being BAD. For example, most folks mistakenly believe that these codes point to a failed Oxygen Sensor and that's the first thing that gets replaced (among many) that do not solve the issue.
To help you in your troubleshooting process, I'll go into the basics of what these Trouble Codes mean and how to confirm that there really is something causing the PCM to set these codes. I'll also offer you a testing strategy to find the root cause of them.
Tip 1: You will need to use a Scan Tool with Live Data Capability to test Diagnostic Trouble Codes P0172 and P0175.
You don't need an expensive Professional Technician level Scan Tool (that cost around $2K to $5K [US]). A simple Generic Scan Tool with Live Data will do (Don't have a Scan Tool? Need a Scan Tool? Check out my recommendation: Actron CP9580 Scan Tool).
Tip 2: Most of the tests needed to troubleshoot codes P0172 and P0175 involve the engine running, this means you've got the on your toes and alert. Think safety all of the time, your safety is your responsibility.
Tip 3: To successfully diagnose codes P0172 and/or P0175 you have to brush up on some operating theory of how the PCM adjusts (fine-tunes) the amount of Fuel the Engine consumes. Otherwise, testing becomes a 'grope in the dark'.
As mentioned earlier, you can not assume the Oxygen Sensors have failed right off the bat. Remember, these two codes don't accuse any specific component as having failed.
What these codes are really saying is that no matter how much the PCM tries to fine tune the Fuel Injection... something is causing too much Fuel to mix with the incoming air the Engine is breathing. In a nutshell, when too much Fuel is mixing with the available amount of incoming air, this is known as a Rich Condition.
And so, to effectively diagnose the issue that's causing this Rich Condition, you need to understand how the PCM fine tunes the amount of Fuel Injected into a running Engine. You'll also need to know what terms like Rich, Lean, Bank 1, Bank 2, Fuel Trim, O2S11, and O2S21 mean.
Don't worry, none of this stuff is hard to grasp, and in this article... I'll explain it all in plain English.
Specifically, this is what each code means:
P0172 (System too Rich Bank 1):
P0175 (System too Rich Bank 2):