What Does the P0172 Code Mean?
Diagnostic trouble code (DTC) P0172 stands for “Fuel System Too Rich (Bank 1)”. Your car’s computer will set the code when it determines that there’s too much fuel (or too little oxygen) in the engine’s air-fuel mixture.
NOTE: The oxygen sensor couldn’t care less about unburned fuel in the exhaust stream–the sensor is designed to sense oxygen, not fuel. So, whenever any cylinder misfires due to spark or compression, there will be unburned fuel in the exhaust stream and the oxygen that didn’t unite with that fuel both passing the O2 sensor–but all the sensor will see is the oxygen.
To learn more about air-fuel mixture and how it can cause codes like P0172 to be stored, read our in-depth explanation here.
What Does “Bank 1” Mean in P0172?
The “Bank 1” portion of this code indicates that the problem is concentrated on the side of the engine that includes the #1 cylinder. Since inline four cylinders only have one bank, the one bank they do have is considered bank 1. But inline six cylinder engines may have two banks (3 cylinders per bank) even though the engine is an inline configuration.
What are the Possible Causes of the P0172 Code?
Like any other OBD-II code, a P0172 can be caused by a variety of problems. In other words, there is no easy way to pinpoint what’s causing the problem other than examining the critical parts responsible for maintaining the right fuel mixture.
Here are the different possible reasons why you’re getting the engine code P0172:
- Contaminated engine oil (too long since the last oil change)
- A leaking fuel injector
- Excessive fuel pressure due to restriction along the fuel return line or a faulty fuel pressure regulator
- A saturated carbon canister due to packing the fuel tank.
- Clogged air filter
- Restrictions somewhere in the air intake system
- Clogging due to buildup or physical damage in these exhaust components: catalytic converter, exhaust pipe, and muffler
- Faulty O2 sensor (but O2 sensors more commonly fail by reading lean, not rich).
- Exhaust leak (upstream of the O2 sensor, a leak will allow oxygen to enter the exhaust stream, and this will typically throw the opposite code rather than a P0172)
- Problems with other sensors (e.g., coolant temperature sensor, mass airflow sensor)
- Circuit issues, such as loose connections and damaged wiring
- Issues with the PCM, such as software in need of an update
What are the Common Symptoms of the P0172 Code?
A rich fuel mixture can cause your engine to consume more fuel. Because of the extraordinary measures the ECM/PCM can employ to keep the fuel system balanced, you may not notice any symptoms at all other than an illuminated Check Engine (MIL) light. In extreme cases where some other component has failed, symptoms can range from mild to engine-damaging.
Your exhaust system is also at risk as unburnt fuel reaching the exhaust lines can combust and damage parts like the catalytic converter.
To avoid this kind of headache, look out for these symptoms:
- Illuminated check engine light
- Lack of engine power
- Rough or rolling idle
- Hesitation on acceleration
- Strong fuel odor from the exhaust or inside the cabin
- Poor fuel economy
How to Diagnose the P0172 Code
As outlined above, there are many potential causes for the OBD-II code P0172. Therefore, diagnosing the exact issue that is triggering the trouble code can be difficult.
For an idea of how to troubleshoot the code, check out the video below:
How to Fix the P0172 Code
There are many ways to fix a code P0172, but it would depend on the exact cause. You may end up simply needing to clean a component, but in some cases, you would likely need to replace something. Therefore, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, as outlined above, then perform any necessary repairs.
And of course, remember that all vehicles are different. When troubleshooting and repairing OBD-II trouble codes, make sure to consult the factory repair information for your application.
Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.
An In-Depth Look at Air-Fuel Mixture and How It Could Cause an OBD Code to Be Stored
Ideally, combustion engines should burn a fuel mixture of 14.7:1. This means there should be 14.7 parts air and 1 part fuel in the mixture by weight, not volume. An engine basically uses about 9000 gallons of air for each gallon of fuel. Engineers always measure fuel by weight. Injectors are rated by how many pounds (grams, kilograms, etc.) of fuel per hour they can deliver at a given pulse width.
If the air/fuel ratio is balanced at 14.7:1, the engine is burning as clean as possible, meaning that every hydrocarbon molecule gets “married” to two oxygen molecules. This process forms CO2, which feeds the grass beside the road; think about it – plants need CO2. CO2 is far heavier than air and hugs the ground. And again, the trees and flowers need it to survive.
When this “marriage” in the combustion chamber between HC and oxygen happens (triggered by the ignition spark), heat is produced very suddenly so that the inert nitrogen (which is 78% of the total air charge) expands, and the piston is driven down to power the crankshaft.
If the air/fuel ratio isn’t balanced and there is too much fuel, each hydrocarbon molecule may only get “married” to one oxygen molecule, forming CO, which is poison. Too much fuel and not enough air is considered rich – too much air and not enough fuel is considered lean. The P0172 code indicates that the ECM/PCM is or has detected a rich condition.
The ECM/PCM and the O2 sensor are all about keeping things balanced in the combustion chamber. If the mixture drifts too far out of balance, a code is stored.
If the air-fuel mix is far enough out of balance, the small amount of oxygen will be used up and there will be lots of HC, which is unburned fuel that makes sooty deposits on the spark plugs.
Can Downstream Oxygen Sensors Be Used to Calculate Air-Fuel Ratio?
The computer determines the air-fuel ratio by monitoring the upstream oxygen sensor/s but if the ECM/PCM determines that the upstream oxygen sensors have become untrustworthy the downstream sensors behind the front catalytic converter will be used for air/fuel ratio calculations. Many mechanics don’t know this and may not even believe it, so be aware of that.
What is Fuel Trim?
Fuel Trim is the ECM/PCM response to the oxygen sensors when the system is in what is called “Closed Loop” fuel control, meaning it is using O2 sensor input as a factor in performing constant Fuel Trim adjustments. The fuel control system drops into “Open Loop” at Wide Open throttle, and starts out in Open Loop with a cold engine or if the ECM/PCM detects a serious malfunction in some other part of the system.
To begin with, there is “Short Fuel trim and there is Long Fuel Trim. Fuel Trim is the ECM/PCM’s fuel mixture adjustment indicator that is visible on a scan tool live data feed. O2 sensor signals can be measured with a scope or a digital voltmeter, but Fuel Trim cannot be measured in any other way except by way live data on a scan tool. Fuel Trim is the ECM/PCM’s programmed response to perceived rich or lean conditions as reported by the Oxygen Sensor(s).
About Long Term and Short Term Fuel Trim
Short term and long term fuel trim are usually displayed as a percentage; the scale is laid out like a number line, with zero in the middle, positive numbers on the right, and negative numbers on the left.
Proper fuel trim on a healthy system should run in the single digits just above and below zero and it should always be moving slightly up and down above and below zero.
The O2 sensor switches rapidly from rich to lean when things are working right, with 0.5 volts being the center of the voltage swing. And as the O2 sensor swings, so does the fuel trim.
When you have the scan tool display a histogram of the Short Term Fuel trim just above the O2 sensor trace for comparison, you’ll notice that the short fuel trim looks like a “saw tooth” pattern and the O2 pattern looks like “camel humps;” on a healthy system, the O2 sensor will switch 2 to 5 times per second, usually more at higher rpm.
As the O2 is going up, the fuel trim is going down – when the O2 pattern changes direction, so does the Fuel Trim, which is always moving opposite the direction the O2 signal is going. Again, the fuel trim is responding to O2 sensor input and activity, which is an after-the-fact indicator of combustion.
If the O2 swings high (rich) and stays there, the fuel trim will respond by beginning to correct to the negative side of zero and continue to correct until the O2 sensor indicates that the negative correction has returned the O2 sensor to normal activity. The ECM/PCM will set the P0172 code in this case. In extreme circumstances you may see fuel trim at -30%. The ECM/PCM performs fuel trim Correction by modifying injector pulse.
Here’s a Tip When Diagnosing P0172
Long term fuel trim is a coarse adjustment factor that will slowly ratchet in the same direction as short fuel trim until short fuel trim returns to the zero range.
Thus, when you go looking for the cause of a P0172, if you’re using a scan tool that shows live data, check long fuel trim first, because it’s an indicator of where the short fuel trim went as soon as the problem occurred.
If you think you have repaired whatever was causing the problem, remove the battery cable to clear the adaptive (fuel trim) memory so that it all starts back at zero – as soon as the scan tool screen indicates “Closed Loop,” the short fuel trim should remain near zero – if it begins to depart from the zero range as soon as the ECM/PCM enters Closed Loop Fuel Control, you still have a problem.
Any fuel trim values beyond 10 or negative 10 means there’s a problem. The farther beyond 10 in either direction, the bigger the problem. When the LTFT drops between negative 20 and 25 (this varies slightly from one manufacturer to another), code P0172 and/or P0175 will likely be set.
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