Home Diagnosis and TroubleshootingOBD Diagnostic Error Codes P0171 – How To Diagnose This Problem?

P0171 – How To Diagnose This Problem?

by Jordan Harris

Have you started to experience misfires lately? Along with poor engine work? If this is the case and you are in this situation, then you are at the right place because there will be a lot to cover on this topic where we will learn everything you need to know when it comes to the error code P0171.

Troubleshooting problems in your car is a real mastery. But you as a beginner should not be afraid of it. Why I’m saying this? Well, I’m saying this because troubleshooting is something that everybody should learn. Everybody should learn how to wrench on their cars and should have a diagnostics tool.

Nowadays, these dealerships are only waiting for people who are not experienced and do not like to wrench on their cars to rip them off. Literally. For a simple situation like the P0171, sometimes you could end up paying thousands at a dealership. And you probably don’t want that. You want your money to be spent on some more fulfilling things and not something like going out and doing more fun activities. And we are going to help you out save some money.

That’s why in this article we will start by covering the basics of car diagnostics and learn what is the OBD2 scanner and codes. Then we will cover what the P0171 error code means and how serious it is. Later on, we will discuss the symptoms of this error code as well as the causes for this message showing up. Lastly, we will cover the diagnostics and how to fix them. So, if you want to learn more, follow along.

What Is An OBD2 Scanner?

Now before we dive into the P0171 code. Let’s familiarize ourselves with car diagnostics. This will be extra useful for beginners who are just starting and do not have experience with diagnostic tools and do not know what these error codes mean. If you think that you are up to the task, you can move to one of the following chapters where we will elaborate more on the symptoms and causes. If not, keep up with us.

As you probably know, modern cars are all computerized. Meaning that they work with the help of something called an ECU.


The ECU is constantly working and evaluating data that is sent to it by a number of different sensors installed in the vehicle. Then the computer adjusts the work of the engine based on this data that it receives. And this is called a map. A map generally is a pre-programmed setting from the factory that tells the ECU what are the best optimal settings for the engine to work on. Then based on this, the ECU determines the limits of the values that it receives from these sensors.

If the values are not up to the spec, meaning that the sensors are starting to produce weird readings, the ECU will detect this and will react accordingly. Especially if it gets a few alerts, it will show the check engine light on the dashboard.

The check engine light means that this problem has to be further diagnosed. And in order to get it diagnosed, you will need a specific tool called an OBD2 scanner tool. Every car has an OBD2 port and this is a standard on each vehicle. That’s why you will need an OBD2 scanner to see if you have a P0171 code.

What Is The P0171 Error Code?

Now as we learned that the P0171 code can only be diagnosed via a tool called an OBD2 scanner. Let’s see what this code really means?

Well, the description of this code is “System Too Lean (Bank 1)”. But what is all this nonsense? In fact, is not nonsense. This means that the fuel to air mixture on bank 1 is running lean.

If you get this code, then you probably running a V6, V8, V10, or a V12. As you probably know, each side of these engines is called a bank. There are bank 1 and bank 2. Bank 1 is the bank where cylinder number 1 is located.

This diagnostic code is a generic code that appears on a variety of different makes and models that have an engine in V configuration. Meaning that if you run an engine that is not V, you will not receive the bank one message but some other code that will indicate that the system is too lean. But what system too lean means?

Well, this message means that the fuel to air mixture in your vehicle is rather messed up. The fuel to air mixture should be 14.7:1. 14.7 grams of oxygen for 1 gram of fuel inside of the combustion. If there is too much air and too little fuel will cause the engine to run lean and show you the check engine and this P0171 code. Something which is not ideal. But also not the end of the world.

What is important is that the air to fuel mixture should always be close to ideal in order to prevent codes like the P0171 to appear.

How Serious Is This Code?

We learned more about car diagnostics and what this code means. Now let’s see how serious is this error and should be your top concern to sort it out?

And the answer to this question is yes, it should be your top concern to sort this problem out. Running lean air to fuel mixture will affect a lot of things in your car. You might think, hey maybe I will save some money on fuel. But this isn’t the case in reality.

You will spend more on fuel in this case since the engine will constantly think that you have a lean mixture and will try to make up for this by dumping more fuel into the cylinders and worsening your fuel economy.

Not to mention that there will be a problem with your spark plugs that will foul, as well as the O2 sensor and catalytic converter that will be quite affected by this work from the engine. So, if you want to avoid any of this stuff. It is worth learning how to diagnose this problem. But first, let’s elaborate more on the problem and learn the symptoms that will often appear when you are faced with a P0171 code.

Symptoms Of P0171 Code

Now as we covered the P0171 code and learned what it represents. Let’s discuss more the symptoms that you will experience when this code appears.

You will not be able to tell if these symptoms are because of the code P0171 if you don’t diagnose the problem. But still, there are a few indications that point to bad air to fuel mixture in your car. And even without diagnostics, an experienced mechanic will be able to tell if the car is running rich or lean. So, what are these symptoms? Let’s elaborate more on them next.

1. Engine Down On Power

The first symptom that you will probably face will be the engine down on power significantly. That’s right when there is a problem with the internal combustion and the fuel mixture is messed up the engine will not work smoothly as it did before.

You will probably lose a third of the horsepower if one of the banks is affected. Meaning that all of the cylinders on this bank are affected. This is not a small number of cylinders and it could give you a good hit to your power ratings.

The car will feel sluggish and the throttle response will be absolutely miserable if the car is having a code P0171. That’s why you need what could be the cause for this error and more on that later in this article where we will elaborate on the causes.

2. Engine Misfires

Engine misfires will also be present when you have a P0171 code. Why is this the case? Well, this is the case because the fuel to air mixture is wrong.

And if there is too much air into the cylinders, engine misfires will happen. But what are misfires? Well, misfires are detonations that do not happen when they have to. Meaning that the ignition of the fuel is not properly timed.

These misfires will result in pops from the exhaust. You will notice how the gas flow is not smooth but sometimes pops are heard and a bigger amount of gases escape through the exhaust pipe. Ticks from the top of the engine will also be present. Especially in bank 1 as in our example with the error code P0171. Now let’s move to the next symptom.

3. Rough Idle

Rough engine idle is another symptom of the error code P0171. The engine as we said will not be happy with this type of work.

So, whenever there is a problem with misfires and poor engine work, the idle of the engine will also be affected. The needle of the tachometer will shake and the RPM will increase and decrease. Sometimes also causing the engine to completely stall in the process. And you want to avoid this at any cost. That’s why you will probably press on the gas a bit more to keep the engine alive.

So, whenever this is present, you will need to learn the possible causes for the P0171 code happening. And more on that we are going to discuss later in the article where we will cover that in-depth.

Causes Of P0171 Code

Since we are familiar with the symptoms of the code P0171 and as we can recall, these include the engine down on power, misfires, and rough idle. Now let’s pay attention to the causes for the P0171 code. What could possibly cause an error code like this to happen? In the next chapters, we will see exactly what could go wrong and make this problem occur.

1. Bad MAF Or MAP Sensor

A bad MAF or MAP sensors are probably one of the first probabilities for this problem happening. So, why is this the case?

Well, this is the case because the MAF and MAP sensors are measuring the gases that go into the intake manifold. These sensors are really crucial when it comes to determining how much gas has flown inside of the cylinders.

Some cars have both of these sensors, especially turbocharged vehicles. While most modern cars are only running MAF sensors. So, you need to determine what sensor you have, older cars have MAP sensors. While modern vehicles only run MAF sensors.

Cleaning these sensors is one way to revive them. But the best would be to replace them completely with new units. Also, check the air filter and replace it as well. Since these sensors develop errors like the P0171 if the air filter is clogged.

2. Bad O2 Sensor

A bad O2 sensor is also one of the causes for error code P0171. This sensor is located before and after the catalytic converter. Meaning that there are two of these sensors on your car on each bank.

Sometimes one of them can fail and will send the right measurements to the ECU and the computer will think that there is something wrong and will not be able to adjust the air to fuel mixture accordingly.

These sensors measure the flow of gases and are notorious for breaking down and often require replacement. Replacing them is a challenge because you need to get under the car. The procedure for replacement is pretty simple though. Just unbolt the old one and bolt the new one. Then hopefully there will be no more P0171 code.

3. Vacuum Leak Or Bad EVAP Purge Valve

As you know, there are a ton of vacuum lines inside of the engine bay. These vacuum lines are rubber hoses that often tend to break down and cause leaks. Whenever they leak, there is an obstruction in the flow of gases in the intake manifold.

Also, there is a system called EVAP. This system with the help of vacuum lines brings the harmful fumes from the fuel tank into the intake. This system is controlled with the help of something called a purge valve. This purge valve tends to fail and the flow of gases will be unobstructed. Resulting in unmetered air entering the intake manifold that will result in the P0171 error code.

4. Bad PCV Valve

A bad PCV valve can also be a cause for the code P0171. So, why is this happening, what has the PCV valve has to with this problem with the lean mixture?

Well, it has a lot to do with because these gases from the engine need somewhere to escape. And PCV valve is designed precisely to perform that work. It takes the gases from the engine and redirects them to the intake manifold. Sometimes this PCV valve can fail and can cause unmetered air to enter the combustion process. Resulting in lean air to fuel mixture and error code P0171.

5. Exhaust Leak In The Exhaust Manifold

An exhaust leak can also cause a P0171 error code. You are probably asking yourself, why is this the case?

Well, this is the case because whenever there is a leak in the exhaust manifold, there will be problems with the readings of the O2 sensor that is located after the exhaust manifold.

The O2 sensor will register poor readings and the computer will try to adjust the fuel to air mixture accordingly. Which in this case, will be lean. Resulting in the P0171 code.

That’s why it is often advisable for you as a car owner to see if there is a leak from the exhaust. On some cars, exhaust bolts often snap and cause the gases to leak. This is notable on LS engines and also on HEMIs. Sometimes even the manifolds break because of a bad casting process. Anything is possible. So, if you note a hissing sound from the engine and the engine. It means that there is either a vacuum leak or an exhaust leak. Both of these leaks can cause the P0171 code.

6. Clogged Fuel Filter

And the last thing that we are going to cover when it comes to the P0171 code is the clogged fuel filter. You might be asking why is this the case?

Well, this is the case because the fuel filter is an extremely crucial component that makes sure that the fuel that enters the combustion is nice and clean.

Sometimes if the fuel filter is not replaced on the service interval and the car was driven like that, it might clog up completely. Resulting in P0171. This will be caused by the lack of fuel that enters the combustion process. Meaning that there will be more parts air and fewer parts fuel. So, beware of this situation.

Always replace this component on time if you want not to have this issue on your car. But how you can diagnose a P0171 code? How do you find a solution to this problem? Well, that’s what we are going to cover next.

How To Diagnose What Is Causing The P0171 Code? And How To Fix It?

Now let’s see how you can diagnose what is causing the P0171 code. For starters, you are going to need an OBD2 scanner tool for this purpose. Without it, you can never know if you have a P0171 code. If you do have one, then excellent. Scan the car for codes and see what is appearing.

If the error P0171 appears, then you know that you have a problem with bank 1 from your engine and this bank is running lean. Then you should also see and address other codes. See if there are also codes on the specific sensors that we have discussed above. There sure will be a lot more codes if the sensors are also broken.

Then based on your findings if there are errors on specific sensors, you should turn your attention towards them and bench test them. Or troubleshoot them. Troubleshooting is really easy when you have a V configuration engine. Just swap out the sensors and see if the codes still persist. If they don’t you find your problem. If they do persist then you should look for your problem elsewhere.

More notably on the vacuum lines, EVAP system, as well as PCV valve. Not to forget the fuel filter that can also cause this problem with the P0171 code to happen. Overall, that’s everything you need to know when it comes to troubleshooting. It is not easy but you will need to dive deeper if you want to sort the problem out.

Cost To Fix This P0171 Code

The cost to fix the P0171 code really depends on the problem itself. Meaning that the root of the problem determines the price for this work.

Let’s say you need to diagnose the car, diagnostics will cost you from $100 to $200 if you go to a shop. We talk about computer diagnostics here. If you do it by yourself, you only need an OBD2 scanner tool which can be found for a good deal.


If the problem is a sensor, then you will highly likely be needing to pay about $150 to $250 for the part alone. PCV or a purge valve only costs between $10 to $35. Also, the vacuum lines are cheap. The fuel filter will cost you about $40.

The most expensive thing that could happen to you is an exhaust leak. These leaks are really expensive to fix. You will easily pay from $300 and up to $1,000 to get this problem sorted out. Overall, these are the costs involved in solving the P0171 code. Now let’s conclude the article.

Facts about P0171 Code and How to Fix it:

  1. A P0171 code indicates a lean condition or low fuel trim code, which means either too much oxygen or too little fuel in the exhaust.
  2. Some possible causes of a P0171 engine code are a vacuum leak, weak fuel pump, broken fuel pressure regulator, blocked fuel filter, faulty powertrain control module, defective injectors, faulty oxygen sensors, clogged catalytic converter, and problems with the mass airflow sensor.
  3. It’s not okay to drive with a P0171 code as it creates performance issues that might result in costly damage, ranging from low fuel economy to poor engine performance to internal engine damage.
  4. Symptoms of code P0171 include a check engine light on or flashing, loss of power, rough at idle, misfiring or “coughing” engine, and white spark plug tips.
  5. The cost to fix a P0171 issue varies, but most mechanics will begin with an hour of labor diagnosing your specific issue, costing between $80 and $150. Repairs may include vacuum leak fix, MAF replacement, fuel pump replacement, fuel pressure regulator replacement, exhaust repair, and oxygen sensor replacement.
  6. To diagnose the P0171 code, a mechanic can use a vacuum gauge to examine the engine for suction leaks, check the fuel pressure sensor and the MAF sensor, and test the mass airflow and oxygen sensors.
  7. A common mistake in P0171 code diagnosis is failing to look at any technical service bulletins for your specific car model that may be relevant to this issue.
  8. It’s unlikely for bad spark plugs to cause a P0171 code. A vacuum leakage downstream from the MAF sensor or a defective MAF sensor is the most common cause of a P0171 code.
  9. A bad PCV valve, low fuel pressure, a vacuum or intake leak, or a faulty MAF sensor are the most typical reasons for the P0171 code.
  10. Lean conditions in a Chevy can be caused by a vacuum leak or a poor fuel system, while a dirty or defective mass airflow (MAF) sensor or a vacuum leak downstream of the MAF sensor could cause a P0171 code in a Toyota. A dirty MAF sensor is a common cause of P0171 and P0174 lean codes in a Ford.

Conclusion To P0171

In this article, we have covered quite a bit when it comes to the P0171 code. We learned what is computer diagnostics and how it is performed with the help of the OBD2 port that each car has. Then we learned more about this code and we acknowledged that this code refers to lean air to fuel mixture on bank 1 from your engine.

Then we covered the causes for this problem as well as the symptoms that often occur. As we learned there are engine misfires, rough idle work of the engine, and overall loss of performance.

After we discussed how you can diagnose the problem with some troubleshooting ingenuity overcome this issue quickly and at what cost.

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