In this article, you will find everything you need to know about the P0171 code and you will learn how to repair and diagnose it easily and quickly at home.
There is a lot of false information about the P0171 code and many dubious tips on how to fix it. I work as a diagnostic technician, and my job is to solve problems like this.
In this article, I will tell you my secrets and I will tell you how I diagnose this code to find solutions as soon as possible. If you are interested in the diagnostic process I am performing, scroll down to the diagnostic section in this article.
What does the P0171 code mean?
P0171 is triggered when the front O2 sensors detect a lean mixture. It can be a short-term lean mixture as well as a long-term lean mixture. If it is a short-term lean mixture, the fuel mixture is often very lean. If it is a long-term lean mixture, it can register the code based on a small indication of a lean mixture over time.
Code P0171 means that the front O2 sensors have detected a lean mixture. The front O2 sensors adjust the fuel mixture leaving the engine. If an O2 sensor detects a slightly lean or rich mixture, it sends a signal to the engine management system to adjust the mixture in the next combustion cycle to obtain a perfect fuel mixture for the best fuel economy.
The O2 sensors usually have a range of +/- 15% for adjusting the fuel mixture. If the fuel mixture is outside of this range, the O2 sensor cannot adjust the mixture, and the engine control unit triggers an error code and stores it in the memory. If the mixture is lean, code P0171 is triggered.
If the mixture is rich instead, you get code P0172, and if you have a V-engine or two O2 sensors, you can also get code P0174, which means that there is a lean mixture, but on bank 2. If you have the P0174 code and want to know which bank to look at and how to find it, read our article Bank 1 vs Bank 2.
If the fuel mixture is slightly lean, you will often get no other symptoms from the P0171 code other than the check engine light on your dashboard. If the fuel mixture is very lean, you may get a lot of different symptoms, which I will list below.
- Check Engine Light On
- Rough Idle or Acceleration
- Loss of Power
- Low/High/Jumping Idle
- Hard Starting Condition
- The engine may die when driving
There are a lot of different sensors or parts that could cause the P0171 code and a lean mixture. I will write down the list below and start with the most common parts to check when you have a lean fuel mixture. It is important to look for error codes on sensors other than the P0171; this may give you an indication of where to start looking for the problem.
You can check the trouble codes and the parameters at home with an OBD2 scanner from Amazon.
- Air intake/vacuum/boost pipe leak (Most common)
- Faulty PCV Valve (Common on VAG Cars like Audi, VW, Seat, Skoda)
- Low fuel pressure (Caused by a bad fuel pump, filter or fuel pressure regulator)
- Faulty EVAP valve
- Faulty O2 sensors
- Faulty EGR valve
- Faulty MAP/MAF sensors
- Exhaust leak (Before Front O2 sensors)
- Coolant temperature sensor
- Faulty sensor wirings
- Faulty ECM/PCM (Rare)
P0171 Possible Solutions
There are many different solutions for the P0171 code. I will write down the most common fixes from the top of the list and go down to the rarer ones at the bottom of the list. There are a lot of diagnostic tools that will be very helpful in troubleshooting if you continue down the article to the diagnostics section.
- Replace faulty vacuum hoses or gaskets around the intake
- Repair other intake leaks
- Replace PCV Valve
- Replace the fuel pump/fuel filter/fuel pressure regulator or repair wirings
- Replace EVAP valve
- Replace O2 sensor(s)
- Replace EGR valve
- Replace MAP/MAF sensor
- Repair exhaust leak
- Replace coolant temperature sensor
- Repair faulty wirings
- Replace ECM/PCM (rare)
|P0171 Trouble Code stored in ECM/PCM||Check Engine Light
Rough Idle or Acceleration
Loss of Power
Hard Starting Condition
The engine may die when driving
|Air intake/Vacuum/boost pipe Leak (Most common)
Faulty PCV Valve(Common on VAG Cars like Audi, VW, Seat, Skoda)
Low fuel pressure (Caused by bad fuel pump, filter or fuel pressure regulator)
Faulty EVAP valve
Faulty EGR Valve
Faulty O2 sensors
Faulty MAP/MAF Sensors
Exhaust leak (Before Front O2 sensors)
Coolant Temperature Sensor
Faulty sensor wirings
Faulty ECM/PCM (Rare)
|Replace faulty vacuum hoses or gaskets around the intake
Repair other intake leaks
Replace PCV Valve
Replace EGR Valve
Replace fuel pump/fuel filter/fuel pressure regulator or repair wirings
Replace EVAP valve
Replace O2 sensor/s
Replace MAP/MAF Sensor
Repair Exhaust leak
Replace Coolant temperature sensor
Repair Faulty Wirings
Replace ECM/PCM (Rare)
How to Diagnose the P0171 Code?
Because the trouble code P0171 means that there is a lean mixture in the car, there could be a lot of different sensors or faulty parts that could cause the trouble code. This is a guide on how to fix the P0171 code the fastest way and how an experienced technician would do to find the problem. This guide may need some required tools to speed up and make the troubleshooting a lot more effective, but you can do it without it also, try to follow the steps.
Since error code P0171 means that there is a lean mixture in the vehicle, there could be many different sensors or faulty parts that could cause the error code. This is a guide to the fastest way to correct error code P0171 and how an experienced technician would find the problem. This guide may need certain tools to make troubleshooting faster and more effective, but you can do it without them as well.
1. Connect a car battery charger to your vehicle
The first step you always want to take when diagnosing a car is to connect a car battery charger. When troubleshooting, you often ignite the ignition and do not want to get low voltage. A low voltage can trigger other, unrelated fault codes and confuse your troubleshooting by giving you incorrect values. Low voltage can in rare cases even damage electrical parts, so I always use a charger when troubleshooting.
If you do not have a car battery charger at home, you can find one at Amazon here: NOCO Genius G3500 6V/12V 3.5A UltraSafe Smart Battery Charger
2. Check all sensor parameters with an OBD2 scanner
If you have an OBD2 scanner at home, this can be very helpful by checking all sensor values in the parameter option in the scanner. Most OBD2 scanners have this feature. Simply check the values of the MAF/MAP, coolant temperature, boost pressure, and intake temperature sensors, making sure the values are appropriate. Many scanners have some basic values, which the value should show at the specific RPM and temperature. Also, check the values of the O2 sensors and make sure they are correct. Replace all defective sensors, delete the codes, and try again.
You can also do it the old-fashioned way and measure all sensors with a multimeter to see if you get the right values. This is very time consuming because you also have to find out what the ideal values are for each sensor. If possible, I would always recommend you check the values with an OBD2 scanner instead. If you don’t have an OBD2 scanner but want to get one, I can recommend the Foxwell OBD2 scanner. You can find it here at Amazon at a reasonable price: FOXWELL NT301 Car Obd2 Code Scanner
When you have checked all sensor values and are satisfied that they appear to be correct, check for other stored and related error codes in the engine control unit. They may give you hints on where to start troubleshooting. Many sensor problems are intermittent and you will get the correct value when you check, but while driving, they are sometimes incorrect and trigger code P0171.
The engine control units are intelligent, and if they detect a faulty value for only one second, this triggers an error code. It is these error codes that you should look for, and if you get another error code from any sensor, you should first start checking that part. This can save you a lot of time when troubleshooting.
4. Check for any intake leaks
Intake leaks are a common problem when it comes to code P0171. Leaks in intake/vacuum/boost pipes can trick the MAF sensor and cause a lean mixture. The MAF sensor measures all air entering the engine and tells the engine control unit what the air volume is. The engine control units then inject fuel into the engine depending on the amount of air entering the engine. A leak can trick this value, resulting in a lean mixture.
The easiest way to find a leak in the suction or pressure pipe is to use an EVAP smoke machine. Unfortunately, these tools are often too expensive for the average driver. A little tip from me: Take a flammable spray, like brake cleaner, and spray around the intake when the engine is idling. If the engine speed increases when spraying, this means that there is a leak in the affected area. You can also use soap and water, and spray around it to see if you can see bubbles from anywhere.
The leak can often also be caused by a defective PCV valve, especially in VAG vehicles such as Audi, Volkswagen, Seat, and Skoda. If you have one of these cars, check the PCV valve under the intake manifold or on top of the engine on newer 2.0 engines.
You should also check the EVAP valve that controls the fuel gases. A leaking or faulty EVAP valve can cause a lean mixture. You can often test this valve by blowing through it to see if it is closed when it should be closed.
Video of how to find intake leaks at home:
5. Check the fuel pressure
Low fuel pressure is a common cause of the P0171 code. It can be quite difficult to check because often you only check the fuel pressure at idle, while the low fuel pressure can occur in many other situations. This is a problem normally solved with a fuel pressure gauge that you can connect while driving. Many cars often have a fuel pressure sensor that you can read the pressure from while driving. However, the problem is that you may also have a problem with the fuel pressure sensor, and in most cases, the engine ECU will enter a low fuel pressure error code.
But it’s always good to check the fuel pressure at idle anyway. This could indicate a defective fuel pump or fuel filter. To check the fuel pressure, you need a Fuel Pressure Tester which is connected to the fuel pressure rail or pressure hose. You must find out what pressure your car should have and remove the vacuum line between the intake and fuel pressure regulators to obtain the correct pressure. You can have a friend rev up the engine to see if the fuel pressure drops.
6. Check EGR Valve
If an EGR valve is open when it shouldn’t be, it can trick the MAF sensor and cause a lean mixture. Checking the EGR valve without disassembly can often be a bit tricky. Usually, you need an EVAP smoke machine to check it properly, or you have to remove it for inspection.
Many OBD2 scanners have the function of performing an electronic EGR test by checking the air entering the engine when it is opened and closed. This can often help you find a faulty EGR valve, as the test will give a positive or negative result. The engine control unit often detects faulty EGR valves and leaves an error code that is stored in the engine control unit, but not in all cases, so it is good to check it twice. Faulty EGR valves that cause P0171 are fairly standard on Opel engines. Clogged EGR lines on Opel engines can also be the cause.
Here is a video of how to check both old vacuum EGR valves or new electronic EGR valves at home:
7. Check exhaust leaks
Exhaust gas leakage before the O2 sensors can trick them and cause code P0171. To check for possible intake leaks, the easiest way is to start the car and go underneath it, paying close attention to any leaks from the exhaust. You can have someone block the rear exhaust pipe to create pressure in the exhaust pipe and make it even easier to find any leaks. If you have access to an EVAP smoke machine, it is easy to check for exhaust leaks. Simply connect it to the rear exhaust pipe and check if you can see smoke coming out of potential leak areas.
8. Clean the MAF/MAP sensor
A little tip from me is to clean the MAF or MAP sensor to solve the P0171 code. This can be done with an electrical cleaner by spraying it onto the sensor string inside the MAF or MAP sensor. Small dust can pass through the air filter and accumulate on the sensor, and this can cause the MAF sensor to obtain erroneous readings of the air entering the engine. This is often remedied by cleaning, and this is a fairly common problem and a cheap method to fix it.
9. In-depth diagnostics
If you have carried out all the above steps but still receive error code P0171, you may need help with a slightly more in-depth diagnosis of the sensors or even damaged parts inside the engine. To get help with a more in-depth diagnosis, you can either seek advice from an experienced diagnostic technician or ask a question on our website and we will see if we can help you. Diagnosing these problems can often be difficult over the internet and it is better to have a diagnostic technician take a look, but we will answer all questions one way or another.
- Intake leaks most often cause code P0171, and these should be checked first. The easiest way to check for suction leaks is with an EVAP smoke machine, but there are several other ways to check for suction leaks.
- Low fuel pressure also often causes the P0171 code and should be checked with a manual fuel pressure tester.
- A faulty PCV/EVAP is common on VAG cars like Audi, VW/Volkswagen, Skoda, and Seat and should be checked if you have one of these vehicles.
- Faulty EGR valves are common on Opel engines but affect a lot of other car manufacturers and if you have an Opel you should check the EGR valve first.
If you have any other questions about the P0171 code or want to tell us about how you fixed it, you can comment down below, and we will answer your questions as fast as possible. If you have any other car questions, you are welcome to ask them on our homepage.
Hello I’m Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I’m specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I’m here to give you answers to all your automotive questions and I hope that you enjoy our content.