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.
Contents of this tutorial:
Important Suggestions And Tips
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'.
With this in mind, I recommend you take a look at the section: P0172 And P0175 Essentials and more importantly How Does The PCM Know A Rich Condition Exists? before you start testing.
P0172 And P0175 Diagnostic Essentials
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):
- 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, 5, and 7 (5.0L, 5.7L V8) and cylinders 1, 3, and 5 (4.3L V6).
- Tells you that this bank's air/fuel mixture is too fuel heavy and the PCM is reducing fuel to its maximum limit (without results).
P0175 (System too Rich Bank 2):
- 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, 6, and 8 (5.0L, 5.7L V8) and cylinders 2, 4, and 6 (4.3L V6).
- Tells you that this bank's air/fuel mixture is too fuel heavy and the PCM is reducing fuel to its maximum limit (without results).
How Does The PCM Know A Rich Condition Exists?
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 are 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's 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 for too long without switching to a Lean condition, the PCM lights up the check engine light and reports a P0172 or P0175 trouble code (depending on the bank that's being affected).
TEST 1: Fuel Trim Check
The very first thing you need to do and the most important, after pulling the diagnostic trouble codes, is to check the Fuel Trim values with your scan tool in Live Data mode.
The reason for this is to see if a continuous and nonadjustable Rich condition is truly present.
If the extreme Rich condition that's causing the PCM to cut fuel to its maximum limit is present, you'll see the Long Term Fuel Trim values of one or both banks stuck somewhere around -20%.
Remember, an uncontrollable Rich condition is the one that causes P0172 and/or P0175 DTCs to set.
OK, this is what you need to do:
- 1
Start the vehicle and let the engine up
IMPORTANT: Let it run for about 15 minutes. - 2
Connect your scan tool to the OBD II diagnostic connector.
Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool. - 3
Once connected and powered up, get to Live Data mode.
- 4
Scroll down to the PID that's labeled: LT FTRM1 and LT FTRM2.
LT FTRM1 reads the Fuel Trim values for Bank 1 of the engine and LT FTRM2 reads the values for Bank 2 of the engine. - 5
Is the fuel trim at -20%?
If the Rich condition is present, that's causing the problem Fuel Trim Too Lean code, the LT FTRM1 and/or LT FTRM2 will be at -20%.
You may or may not see them both at -20% and this is OK. Why? Well, it all depends if one or both banks are being affected.
Let's take a look at your test results:
CASE 1: Your scan tool showed one or both LT FTRMs at -20%. This tells you that there truly is a problem causing the PCM to cut as much fuel possible to that bank or those banks.
Your next step is to go to: Rich Condition Is Confirmed.
CASE 2: Your scan tool showed one or both LT FTRMs at between 10% and -10%. These are the normal values that a vehicle without any Fuel Trim problems should have.
This also means that whatever is setting these codes is failing intermittently and intermittent failures can not be diagnosed as long as the problem is not present.
Rich Condition Is Confirmed
OK, after confirming that you do have a bonafide Rich condition on one or both banks, you can start doing some specific tests to find out what's causing it.
What you're looking for is anything that would add fuel to the air/fuel mixture beyond the control of the PCM. The most common problem areas are:
Ignition System Issues
Verify that you do not have a misfire condition. A misfire condition will cause the air/fuel mixture to go Rich. This will have a direct and negative impact on the PCM's fine-tuning of the fuel injected into the engine.
The most common component failures are:
- Bad spark plugs.
- Bad spark plug wires.
- Distributor cap.
- You can find the ignition system tests here: How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04) (at: easyautodiagnostics.com).
Fuel System Issues
- Dirty or clogged fuel injector(s).
- To be more specific: If the fuel that is being sprayed into the cylinder is not atomized correctly, the fuel will not burn completely and this will cause a Rich condition that will force the PCM to trim fuel to the bank to which this injector belongs to.
- Leaking fuel injector.
- Leaking fuel pressure regulator.
- The 'Spider' fuel injector assembly used on these engines are infamous for leaking fuel pressure regulators. What usually happens is the rubber diaphragm (inside the fuel pressure regulator) tears, leaking fuel into the intake manifold and immediately causes the air/fuel mixture to go Rich.
- Failed pressure regulator causing excessive fuel pressure.
- You can find the 'Spider' fuel injector and fuel pressure regulator Tests here: How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
Cooling System Issues
- The thermostat is missing.
- Incorrect temperature thermostat is being used. In other words, you're using a lower temperature rated thermostat instead of the stock 195° F rated thermostat.
- Engine coolant temperature (ECT) sensor is not reporting the correct temperature. More specifically, it's reporting a cooler temperature than normal. The cooler the temperature reported, the more fuel the PCM dumps.
