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):
What will help you to diagnose these two codes, is knowing how the PCM sets them. I won't go into any complicated automotive theory, just the basic facts that you need to know:
When the engine is running, the PCM is constantly adjusting the amount of fuel it's injecting.
These adjustments are called Short Term (ST) Fuel Trim and Long Term (LT) Fuel Trim. And you and I are able to see the values that are generated on a scan tool with Live Data capability.
Each engine bank is adjusted separately from one another. For this reason, you'll see a ST FTRM 1 and LT FTRM 1 for Bank1 and ST FTRM 2 and LT FTRM 2 for Bank 2.
This fine-tuning (of the amount of fuel injected) is accomplished mainly via the oxygen (O2) sensors, although every sensor on the engine also plays an important part in adjusting the fuel Trim of both engine banks.
The oxygen sensors that have a direct bearing on how the PCM fine tunes the fuel injection ar O2S11 and O2S21.
O2S11 stands for Oxygen Sensor Bank 1 Sensor 1. This bad boy is located on the exhaust pipe that connects to the bank that houses cylinders #1, #2, #3, and #4. Also, this O2 sensor is upstream from the catalytic converter.
O2S21 stands for Oxygen Sensor Bank 2 Sensor 1. This bad boy is located on the exhaust pipe that connects to the bank that houses cylinders #5, #6, #7, and #8. Also, this O2 sensor is upstream from the catalytic converter.
When the PCM injects too much fuel (causing the air fuel mixture to turn Rich), the oxygen sensor produces a voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.
When the PCM sees this Rich Condition, it starts to inject less.
When the PCM starts to cut fuel, the Short Term (ST) Fuel Trim values to go into a negative value to let you and me know that it is cutting fuel.
As the PCM starts to cut back fuel (causing the air fuel mixture to turn Lean).. it may go too far and not inject enough. Here again the O2 sensor saves the day by reporting a voltage that can go as low as 0.050 to 0.100 Volts.
Voltages in this range let the PCM know that the air/fuel mixture is too Lean and starts to inject more.
When the PCM starts add more fuel, the Short Term (ST) Fuel Trim values to go into a positive value to let you and me know that more fuel is being added.
If all is peachy, the oxygen sensors will switch between a Lean and Rich Condition several times every few seconds the whole time the engine is in operation.
Also, the Fuel Trim values will move up and down between a positive and a negative value. Usually no more than 10% and no less than -10%.
If the O2 sensor stays stuck at a Rich Condition (anything above 0.500 millivolts) for too long, the PCM will try to reduce the amount of fuel to change the O2 sensors values.
Since the PCM is reducing fuel, the LT Fuel Trim (Long Term Fuel Trim) values will go into their maximum negative Value of -20%.
If this Rich condition continues, without switching to a Lean condition, for too long... the PCM lights up the check engine light and reports a P0172 or P0175 (depending on the bank that's being affected).
“Remember, this hobby is supposed to be your therapy, not the reason you need therapy.”