Which Codes Will This Page Cover?
- P0170 Fuel Trim Malfunction (Bank 1)
- P0171 System Too Lean (Bank 1)
- P0172 System Too Rich (Bank 1)
- P0173 Fuel Trim Malfunction (Bank 2)
- P0174 System Too Lean (Bank 2)
- P0175 System Too Rich (Bank 2)
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What Is the P0171 Code?
Symptoms
Causes
Seriousness
Can I Still Drive?
Diagnose
Difficulty of Inspecting
Get Additional Help
A P0171 code is set by your vehicle’s powertrain control module (PCM) when it detects that the air-to-fuel ratio of your vehicle is too lean. This means that the PCM is relaying that the engine is receiving too much air into the combustion chamber and not enough fuel.
The PCM receives signals from the oxygen sensors located in your vehicle’s engine bay and is constantly working to adjust the right amount of air and fuel based on the readings provided by these sensors. When the factory predefined range falls out of line, more specifically, is lower in fuel than the motor needs, the PCM will trigger the P0171 code. The PCM then calls for more fuel to be sent through the fuel injectors and into the combustion chamber of the engine. This action is called a fuel trim, and in the case of a P0171 code, where more fuel is called for, this is a positive fuel trim.
What Else Is There To Know About A P0171 Code?
The P0171 code is set when the PCM can no longer compensate for the low fuel condition by adding fuel and will be accompanied by a check engine light. It’s important to note, however, that this code is almost always paired with at least the P0170 code as this code simply states that an anomaly in fuel trim has been detected.
The P0170 code will also be triggered if you are faced with the P0172 code, which indicates the opposite of the P0171 code. The code is triggered when the PCM detects a negative fuel trim and calls for less fuel to be pushed through the fuel injectors. This means that the engine is running “too rich”.
Another important note with the P0171 and related codes is that they make a distinction between air-to-fuel ratio issues in Bank 1 versus Bank 2. Bank 1 is the side of the engine that contains the #1 cylinder whereas Bank 2 contains the #2 cylinder. However, Bank 2 is typically only found in motors like V6s, V8s, or V10s, and refers to the side of the engine block that contains the #2 cylinder.
Nevertheless, some in-line 4 engines are also split into Bank 1 and Bank 2 and follow the same pattern of cylinder identification.
Finally, the make and model of your vehicle will determine whether Bank 2 will be on the passenger or driver side. So, codes P0170-P0175 are essentially a means of figuring out exactly what sort of fuel trim issue is happening and where the issue is in the engine block.
What Are the Symptoms of a P0171 Code?
A common symptom of a P0171 code, and the P0170 and P0173-P0174 codes, is that the engine will seem to run with a lack of power and may stall at any time. The stalling usually occurs when you stop at a stoplight and the engine cannot maintain a steady idle anymore. If the issue is caused by a vacuum leak, however, you may notice a hissing sound coming from your engine bay. Your engine may have difficulties turning over because it isn’t receiving the right amount of fuel in the combustion chamber. You may also hear an engine knock or pinging under load or when you are accelerating up a hill, as well as a loss of power, because even less fuel is making its way to the right place.
As for codes P0172 and P0175, when the engine is running too rich, you may notice fewer physical symptoms. This is because more fuel is being sent to the engine and your vehicle is running inefficiently because of the extra fuel that it’s burning. You’ll receive fewer miles per gallon and will have to fill up fuel a lot more frequently as a result.
However, you may sometimes notice a physical symptom if the air-to-fuel ratio is too rich. Black smoke or residue starts could start emitting from your vehicle’s exhaust system. You may also experience other problems like misfiring from any of the engine’s cylinders and you will notice a rough idle or lack of power.
What Are Some Causes of a P0171 Code?
The most common cause of the P0171 code – and the P0170 and P0173-P0174 codes – is unmetered air entering the combustion system. Most of the time, this happens due to faulty or dirty mass airflow sensors or weak fuel pumps. You may also find that jammed or unclean fuel filters or fuel injectors can cause unmetered air to enter the system and trigger any of these codes to set. Defective oxygen sensors must also be looked at as a cause of an engine running lean or rich. Exhaust leaks can also be a reason that a P0171 and its related codes are set in your car.
However, it’s important to make the distinction that for codes P0172 and P0175, where the issue lies with the engine running too rich on either the Bank 1 or Bank 2 side of the engine block, the most common cause is a faulty fuel injector. As the fuel injector fails, it will start to leak and push too much fuel into the combustion chamber. A leak in the fuel lines can also make it appear that the fuel injectors are working properly. But, if the fuel lines are operating under factory set conditions, you may find that leaks in the vacuum lines are allowing extra oxygen to escape before it’s mixed with the fuel. This will always cause the engine to run too rich and you will experience loss in gas mileage or power.
How Serious Is the P0171 Code?
The P0171 code, and its related codes, can certainly range in seriousness. The code by itself doesn’t present any immediate dangers, but you should get it checked out immediately by a qualified technician. If the code exists for a longer period in the PCM it may cause further damage that is costlier to repair.
As we’ve shown, even if no symptoms have presented themselves immediately, this doesn’t rule out the possibility that the vehicle may be running too rich and is ruining the fuel efficiency. If there is a fault with your fuel pump or injectors, this can cause long-term damage if the wrong air-to-fuel ratio is consistently pushed into the system. The factory-set conditions for engine operation are there for a reason and it’s because those ratios have been tested and are what is needed to keep the vehicle running smoother for longer.
Nevertheless, most cars today will take corrective action to avoid severe damage from being caused to the engine by going into limp mode or reduced power mode. As we’ve noted before, running a vehicle in lean conditions can cause pinging or spark knocking and this can have more precarious consequences.
Pinging means that the Oxygen (O2) is being converted to Nitric Oxide (NOx) which is an atmospheric pollutant. And this process can lead to your combustion chamber suffering damage as well as damage to valve surfaces. This leads to the superheating of the catalytic converter to a point where it glows red hot and could catch other parts of the car on fire.
Can I Still Drive with a P0171 Code?
You will be able to physically operate a vehicle if P0171 or any of its related codes are showing on your dash, but it’s strongly not recommended. Even if your vehicle does go into limp mode or reduced power mode, this is further proof that the issue is more serious than you think and you should get the vehicle inspected by a qualified technician as soon as possible.
How Easy Is It to Diagnose a P0171 Code?
The most common mistake made when diagnosing P0171 or any related codes is replacing the air-to-fuel sensor or oxygen sensor immediately and moving on. As long as you are equipped with a sophisticated OBD-II scanner, you should be able to pinpoint exactly where the air/fuel condition is happening and then you can inspect the corresponding parts.
Mass airflow sensors or vacuum leaks should be prioritized when diagnosing any of the related codes because that is often the source of unmetered air entering the combustion chamber. It’s important to read and analyze fuel trims and freeze-frame data to diagnose a P0171 or any related code correctly.
Unfortunately, it’s easy for someone with little experience to misdiagnose this type of problem. A common fix is replacing the air-fuel sensor or the O2 sensor. However, replacing these parts may not solve the issue’s root, since the code could be caused by a dirty or faulty mass airflow sensor or even vacuum leakage. The new O2 sensor may then be forced to read differently in an attempt to compensate for this. This is why it’s critical to read and analyze fuel trims and freeze-frame data to diagnose a P0171 problem correctly. Again, this is best left to qualified technicians with prior experience working on these issues.
How Difficult Is It to Inspect a P0171 Code?
We recommend you contact professional help as the following is for informational purposes only.
The first step to diagnosing and fixing a P0171 code or any of its related codes is to leave this part of the process to a qualified technician to repair any issues with the fuel system. They will want to ensure that the fuel pressure and fuel injector pulse are where they need to be and that the fuel injectors are functioning properly and are not clogged. They will also check your fuel lines for any damage followed by your vacuum lines to see if there are any leaks. They should repair any problems that you discover because these leaks cause air to escape instead of entering the engine’s combustion chambers. Again, while these repairs could be performed by learned car enthusiasts, there is still a lot of room for errors to be made and is why this should be performed by trained technicians with experience.
The next step should be to remove and clean the mass airflow sensor before reconnecting it again. Steps like these are best reserved for qualified technicians and shouldn’t be attempted by DIY enthusiasts. After reconnecting, you should run a diagnostic test again to see if the P0171 code or its related codes are showing. Finally, you should check the exhaust for leaks and make any necessary repairs to the system with professional help. If the problems persist, consider replacing the air-to-fuel sensors and their oxygen sensor counterparts to repair the issue.
Need Additional Help with Your P0171 Code?
Ultimately, you certainly don’t want to risk a P0171 code or any of its related codes causing permanent damage to your car. More than anything, issues with how your engine burns fuel and air should be prioritized. The more you wait with issues like this, the bigger the likelihood that you will be fixing some much more costly repairs is.
For more information on the P0171 code and all of its related codes, or to find a qualified technician to help you with these issues, contact a service center and get your car safely back on the road in no time.
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