P0171 Code – Symptoms, Causes, and Fixes
Chandler is an automotive expert with over a decade of experience working on and modifying cars. A couple of his favorites were his heavily modded 2016 Subaru WRX and his current 2020 VW Golf GTI. He’s also a big fan of American Muscle and automotive history. Chandler’s passion and knowledge of the automotive industry help him deliver high-quality, insightful content to TuningPro readers.
One of the most frustrating parts of car ownership is dealing with engine problems. The engine is the lifeforce of a vehicle, and anytime it’s having issues you need to get them fixed immediately. A common diagnostic trouble code (DTC) that engines can have is the dreaded P0171 code. The P0171 code indicates “System Too Lean (Bank 1),” and is a serious problem for your engine. In this article, we’ll explain everything you need to know about the P0171 code, including what it means, how it hurts your engine, related codes, common symptoms and causes, and how to diagnose and ultimately fix the issue.
*Check out our FAQ at the bottom for quick answers to your questions.
DTC P0171 Code Information
As we stated, the P0171 code stands for “System Too Lean (Bank 1).” This means that your engine is having trouble measuring the correct air-to-fuel ratio. Before we go any further, first let’s explain what your air-to-fuel ratio is and why it’s important for it to be correct.
All engines have an optimal air-to-fuel ratio, also seen as AFR, which is responsible for ensuring they run smoothly. For the vast majority of gasoline powered street cars, the correct AFR while driving around at low engine loads is 14.7:1, meaning 14.7 parts air to 1 part fuel (gas).
At low loads, a 14.7:1 AFR is completely stoichiometric, meaning it creates a perfect mixture of air and fuel for a perfectly complete burn. Anything higher, like 15.7:1 is considered lean, and anything lower is considered rich. Lean fuel mixtures have less fuel, whereas rich fuel mixtures have excess fuel.
Now, there are times when you want a richer fuel mixture, usually when trying to get more performance, and times when you want a leaner mixture, usually for improved fuel economy. But for just average cruising, a 14.7:1 AFR is usually perfect for gas mileage and engine health.
You might be wondering why we use 14.7:1? It’s not just a random number. Our engines use oxygen sensors to measure our AFR to make sure it’s at the right level. When converting those oxygen sensors to the gas scale, a stoichiometric reading of 1 converts to a 14.7:1 ratio on the gas scale. Other fuels have different ratios, but for gasoline it is 14.7:1.
Why does my AFR matter?
Your air-to-fuel ratio is important because it ensures your engine is operating at peak efficiency and peak health. Using fuel mixtures that are too rich can result in poor fuel economy, premature spark plug wear, ringland failure, and decreased performance. Lean fuel mixtures result most often in engine detonation and pre-ignition. Both detonation and pre-ignition can cause catastrophic engine failure, and are serious problems.
Detonation occurs during the combustion process after the spark plug has fired. It is a phenomenon where cylinder pressures spike with excessive heat, potentially causing damage to the piston crown and ringlands. Pre-ignition is similar, but occurs before the spark plug has fired. During pre-ignition, when the piston is moving up during the compression stroke, the mixture spontaneously ignites, again spiking cylinder pressures.
If cylinder pressures get too high catastrophic engine damage can occur. Pre-ignition is much more serious than detonation, but both can cause serious issues. You can check out this blog from Edge Autosport for a more thorough explanation of detonation and pre-ignition.
What does the P0171 code mean?
Okay, so now we know that maintaining the correct air-to-fuel ratio (AFR) is very important. We also now know that lean fuel mixtures can be damaging to the engine, primarily through causing pre-ignition and detonation. So what does that mean for the P0171 code?
The P0171 code indicates that your “System Too Lean (Bank 1),” meaning your engine has detected a lean AFR. Normally, even though your engine is always trying to achieve its optimal AFR, in the real world that doesn’t always happen as drivers are constantly modulating the throttle pedal. So, your engine is constantly adding or subtracting fuel based on your car’s oxygen sensor’s reading to achieve the optimal AFR. This is known as closed loop fueling. The ECU takes the O2 sensor’s reading and changes fueling to always achieve as close to 14.7:1 AFR as possible.
Your engine optimizes AFR through what are known as fuel trims. Positive fuel trims mean fuel is being added, and negative fuel trims mean fuel is being taken away. They are usually expressed in terms of a percentage. For cars running smoothly, you would expect to have fuel trims with 1-3%. On cars with leaks in the intake system, those fuel trims spike to 30-40% or even higher. You want the lowest possible fuel trims to ensure the best fuel economy and engine efficiency.
If the P0171 code shows up, that means that your engine’s AFR is too lean and the engine cannot compensate enough through fuel trims. Usually, this means fuel trims have exceeded the 25-30% range, and your car is really struggling to run efficiently.
What does Bank one mean for the P0171 code?
This gets a bit confusing, but most modern engines have a minimum of two cylinder banks. On a V-style engine, bank 1 refers to the side of the block that has the number 1 cylinder. On an inline-style engine it’s a little different. Since there is no left and right side to an inline engine, bank 1 refers to half the cylinders (on a 2 bank engine). Usually, bank 1 is the first half of the cylinders (i.e. 1, 2, 3) and bank 2 is the second half (i.e. 4, 5, 6). Some 4-cylinder engines only have a bank 1.
The purpose of having two banks is for more precise fueling, and it also helps you narrow down where the potential problem is. Most car owner’s manuals have a diagram of the configuration of the engine explaining which side is bank 1, making it easy to look up.
P0171 most common causes
The most common causes of the P0171 code are:
- Aftermarket intakes
- Clogged/faulty fuel injector
- Faulty/weak fuel pump or fuel pump sensor
- Faulty/weak fuel pump regulator
- Vacuum leak in intake system
- Faulty or dirty oxygen sensor
- Faulty or dirty mass airflow sensor (MAF)
- Exhaust leak
There are many possible causes to the P0171 engine fault code. One possible cause of the P0171 code are problems with the fueling system. If your injectors, fuel pump, or fuel pressure regulator are clogged, defective, or failing due to wear and mileage, they can struggle to supply the amount of fuel that is needed. Vacuum leaks can also cause lean AFRs. Vacuum leaks are leaks in the intake tract where air can escape or enter. With most vacuum leaks new air is entering the engine that is unaccounted for, causing a lean condition.
Other common causes include faulty oxygen sensors or faulty or dirty MAF sensors. If the oxygen sensor is old or defective it can give incorrect readings to the engine control module (ECU), resulting in a P0171. In addition, if your MAF is damaged, dirty, or defective, it can struggle to accurately measure how much air is entering through the intake. This can result in the ECU not adding enough fuel thinking there will be less air, creating a lean mixture.
Finally, a common cause of the P0171 DTC is an exhaust leak. Exhaust leaks can make your oxygen sensor give false readings which cannot be compensated for and can throw the P0171 code.
The P0171 code and aftermarket intakes
One of the more common things that causes the P0171 code are the use of aftermarket intakes, like cold air intakes. While these are great power adders for some cars and we have covered them extensively, they often need to have supplemental ECU tuning most of the time to make them run correctly. Slapping on an aftermarket intake on a MAF based system without tuning is a recipe for disaster.
MAFs, or mass airflow meters, job is to measure the amount of air passing through the intake and into manifold and engine. It does so based on calculations using the stock intake’s measurements and dimensions. Since most aftermarket intakes are larger than stock and have different dimensions, the MAF calibration is skewed when you put them on. ECU tuning allows a tuner to change your MAF scaling to compensate properly. Without tuning, you will often see a lean code, because unmetered air is entering the engine that your fuel trims cannot account for.
Another problem with aftermarket intakes are those that used oiled filters. Excessive oil will often leak out of the filter and into the MAF, which causes false readings. These can eventually trigger a P0171 code if enough oil leaks in and throws off readings too much.
P0171 Code Common Symptoms
These are the common symptoms of the P0171 code:
- Check Engine Light illuminated or flashing
- Rough idle
- Loss of power
- Car thrown in limp mode
- Misfiring or fouled spark plugs
- Engine won’t start
- Detonation and Pre-ignition
- Catalytic converter failure
If you have a problem with the P0171 code, chances are it will be very clear that something is wrong with your engine. As your engine is unable to properly inject enough fuel, the engine will struggle to operate and idle. The lack of fuel will cause a rough idle, loss of power, and poor overall drivability. If your car experiences trouble driving for an extended period of time, it may throw itself in limp mode. This effectively neuters the car and severely reduces power and limits speed.
Your check engine light will likely be illuminated with the P0171, and might even be flashing repeatedly. As we mentioned before, lean fuel mixtures often cause detonation and pre-ignition, which can lead to catastrophic engine failure. Repeated pre-ignition can also wind up fouling spark plugs prematurely or cause them to have white deposits on them.
If the condition goes on long enough, you might even experience catalytic converter failure. Usually, your engine will go before the cat, but it has still happened to some people first.
Related P0171 Codes
- P0170 Fuel Trim Malfunction (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)
There are several codes related to the P0171 DTC, and many of them will pop up at the same time. One of the most common is the P0170 “Fuel Trim Malfunction (Bank 1).” This refers to the fuel trims we were talking about, and means they are exceeding the 30% threshold in bank 1.
P0172 is very similar to the P0171, but means the engine detects a rich fuel mixture it cannot compensate for instead of a lean one. P0173, P0174, and P0175 are all identical codes but for the other cylinder bank. As we mentioned earlier, this helps you to narrow down where the potential problem is by isolating half the engine’s cylinders.
P0171 Code Severity Level
Level: Very High – Get this fixed immediately
Due to the risk of pre-ignition and detonation, it is very important that you get this code fixed immediately. Do not drive aggressively or spirited with this code, as that can create serious engine problems.
While your car is at low loads it operates under closed loop fueling, which we explained. But while operating under high loads, like when you go wide open throttle, most cars use open loop fueling. Your car cannot compensate for lean conditions while in open loop fueling. This makes detonation and pre-ignition highly likely during spirited driving with the P0171 code.
The higher the engine load the more damage pre-ignition and detonation can cause. Detonation and pre-ignition under high loads often results in catastrophic engine failure.
Fixing the P0171 Code
Confirm the P0171 Code
So how do you go about fixing the P0171 code? Since there are a lot of different possibilities this fix is somewhat of a trial by error. First, make sure you confirm that you are dealing specifically with the P0171 code before making any repairs.
Take off your aftermarket intake and check for leaks
After confirming the correct code, if you have an aftermarket intake that is not tuned for you should remove that immediately and reset the codes. If the code does not return the problem was related to your aftermarket intake, and you will need custom tuning to fix it.
If you have the stock intake, a good place to start looking is testing the intake tract for vacuum leaks. This is most easily accomplished through a smoke test. See here for more information on how to smoke test your car. If you do find a vacuum leak, repair the leak and reset the codes. Most likely, this will be the cause of the issue if you do indeed have a leak. Also check your exhaust header connections to see if a leak is present there.
Clean and test your sensors
Assuming the code comes back again, the next step is to test your MAF and fuel pressure sensors. If necessary, replace the faulty one and clear the codes. If they still come back, the problem may be with your oxygen or O2 sensor. Replace the sensor and clear the codes.
Check your fueling system
If the codes still come back and you have ruled out the MAF, fuel pressure sensor, vacuum and exhaust leaks, and an aftermarket intake, the problem is most likely with your fueling system. At this point, you will need to test your injectors on bank 1 to see if any are clogged, same with your fuel pressure regulator if you have one. If these are fine the problem may be with your fuel pump. You can test the voltage on your fuel pump to make sure it is operating correctly, and if not replace it.
After going through this step by step guide, you should have been able to solve your P0171 DTC code. Keep in mind, these are only the most common fixes. Your car likely has several other smaller sensors that could also be the problem. If you are unable to diagnose through the steps above, you may need to have a professional mechanic look it over.
P0171 Diagnosis and Repair Costs
This repair is serious but also relatively inexpensive. Depending on the repair, if you do it yourself you will probably be looking at $0 – $150 to repair. Most MAF, fuel pressure, and other sensors can be had for less than $100. Keep in mind, OEM replacements are more expensive than generic, and we generally recommend OEM.
If you have a shop repair the problem, you will need to add 1-2 hours of labor. It usually is around $125/hr depending on your location.
P0171 Code FAQ
What does the P0171 DTC mean?
The P0171 DTC code indicates that “System Too Lean (Bank 1).” It means that your engine has detected the air-to-fuel ratio is in a lean condition that it cannot compensate for. Lean air to fuel ratios can be very dangerous for your engine at higher engine loads.
What damage can the P0171 DTC cause?
A lean air-to-fuel ratio can cause pre-ignition and detonation, both of which can lead to catastrophic engine failure if they are severe enough. You can also blow out your spark plugs or cause them to have white deposits on them.
What is the most common cause of the P0171 DTC?
The most common cause of the P0171 DTC is having an aftermarket intake that is not tuned for. Other common causes include vacuum leaks, exhaust leaks, a clogged fuel system (injector, pump, regulator), or faulty MAF, O2, or fuel pressure sensors. Most often it is an intake or vacuum leak causing the code.
How expensive is the P0171DTC to fix?
Depending on what the issue is and how long it takes to find, you will likely be out $0 – $150, most of it going to the cost of the replacement part. If you need to have a professional diagnose and fix the problem, expect to add $125 – $250 in labor charges.
Can I still drive with the P0171 DTC present?
While you can still drive you absolutely should not until you can get it adequately repaired. Lean fuel mixtures are disasters for engines, and can easily cause catastrophic damage. Under no circumstances should you attempt to operate under higher loads, like going wide open throttle, as you can cause serious damage or cause your car to be thrown in limp mode.