P0171 Code (System Too Lean Bank 1): Meaning, Causes and Fix

When you’re driving your car, you need to keep an eye out for any warning lights or indicators that your engine isn’t functioning properly. If you see any warning lights or indicators, then it’s time to check your air/fuel mixture. When insufficient combustion takes place in the engine, the check engine light turns on and throws a trouble code. One of the trouble codes thrown by the engine control module is P0171. In this blog, I’ll explain what P0171 means, why it happens, and how to fix it. 

P0171 code indicates that the combustion mixture is too lean (bank1). If the air-fuel mixture inside the combustion chamber of the engine is not correct, inefficient combustion will take place. As a result, the engine will not produce enough power, and exhaust emissions can also be harmful.

Bonus Read: Check engine light on but car runs fine

P0171 Code Meaning

In the P0171 code, ‘P’ is related to the powertrain of the engine and the first digit i.e. ‘0’ indicates that this trouble code is the same for all engines. A lean air-fuel mixture results in either excess oxygen (air) in the combustion chamber or the fuel is not enough in the combustion chamber to create the right air-fuel ratio. The lean air-fuel mixture can be caused by vacuum leaks or faulty fuel injectors.

There are two sides of the engine i.e. bank 1 and bank 2. P0171 trouble code is connected with bank 1 of the engine. Bank 1 is always on the side of cylinder 1, which is always on the front of the engine. In inline engines, there is only bank 1. While in V-type engines with 8 cylinders, there is Bank 1 and Bank 2.

You can check out the below video to get an idea of bank 1 and bank 2 in the engine.

Why Is Proper Air-fuel Ratio Necessary In Engine?

The air-fuel ratio is the amount of air and fuel (gasoline) that is used to power an engine. A car’s air-fuel ratio is determined by the size of the air intake, the amount of fuel injected, and the number of revolutions per minute (RPM). 

The air-fuel ratio must be kept within a certain range to ensure proper combustion. A low ratio makes it difficult to ignite the mixture and a high ratio makes the engine difficult to control. 

In an engine, the ideal air/fuel ratio is 14.7. An engine never runs on this ideal air/fuel ratio. Instead, it constantly bounces back and forth around the ideal air/fuel ratio. When the air/fuel ratio of the engine becomes greater than 14.7, the mixture becomes lean.

The air/fuel ratio of an engine is maintained by measuring the unburnt oxygen levels in the exhaust gases, which is performed by the O2 sensor upstream of the catalytic converter.

The O2 sensor in the engine lets the computer know if the fuel mix is burning rich (not enough oxygen) or lean (too much oxygen). Similarly, the mass air flow sensor in the air intake manifold calculates the total amount of air entering the engine and sends the signals to the engine computer to adjust the fuel injection accordingly.

Is P0171 Serious?

P0171 code can become a serious issue if you do not locate the source of this trouble code and fix it. When the air/fuel mixture is too lean, the engine will run rough or stutter.

Running lean can damage the engine. Due to the improper air/fuel ratio, the engine will not produce enough power while driving. You must first understand what the P0171 code is telling you before you can figure out the problem. If you have an engine that is running rough or producing low power, the code P0171 is triggered.

P0171 Symptoms 

Here are some most common symptoms of P0171 code.

  • Rough idling
  • Loss of power
  • Misfire
  • Rough acceleration
  • Check engine light on
  • Hard Starting Condition

Causes of P0171 Code

Here are some most common causes of the P0171 code in engines.

1. Bad O2 Sensor

If you have a P0171 code in your engine, it means that the oxygen sensor before the catalytic converter is defective or malfunctioning. The oxygen sensor sends a signal to the ECU, telling the ECU to adjust the fuel injection by measuring the unburnt oxygen in the exhaust gases. If there is a very less amount of unburnt oxygen in the exhaust gases, the O2 sensor will send signals to ECU to inject less fuel into the combustion chamber through the fuel injector.

If the O2 sensor is faulty, the computer will not receive accurate data and will not make the required adjustments. This will result in a lean air/fuel mixture which can damage the engine.

If you have OBD2 scanner, run the scan and check the voltage of O2 sensor with the graph up. It should vary between 0 and .5v at idle. 0v is lean and .5 is rich. When you rev the engine above 2000 rpm, it will go in a closed loop and show a voltage in relation to the oxygen content. You are looking for voltage to go above and below 0.5 volts.

If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your O2 sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45)? If the voltage is near the middle, your engine might not be hot yet as the O2 sensor shows a false reading when the engine is cold. Rev the engine again above 2000 RPM. Now, artificially create a lean condition by opening the vacuum line.

You can use the power brake vacuum supply hose. If this drives the voltage of the O2 sensor to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you can’t get the sensor voltage to change by artificially creating the lean mixture, the O2 sensor is damaged.

2. Faulty Mass Airflow Sensor

MAF sensor in engine

A faulty mass airflow sensor can also be the reason for the P0171 System Too Lean code in the engine. Mass Airflow Sensor (MAF) is located in the fresh air intake tube between the air filter and throttle body of the engine.

The mass airflow sensor measures the amount of air flowing into the throttle body of the engine. A mass airflow sensor sends out a square wave of a certain frequency that varies depending on the reading. A MAF sensor tells the ECU how much air is coming in, measured in grams per second. MAF sends air mass flow data to the ECU which determines whether it needs to adjust the fuel delivery rate.

A mass airflow sensor basically uses a hot element that’s kept hot by current, and the air cools it down; more current to keep it hot means more air is coming in.

Dirt or dust can also cause the MAF sensor to malfunction. When the MAF sensor becomes bad, it causes rough idle. Try unplugging the MAF and drive around with it unplugged and see if performance improves at all. If the car accelerates smoothly, the MAF sensor has gone bad or it needs to be cleaned.

Another way to diagnose a bad MAF sensor is by plugging in the OBD2 scanner. If your engine is at operating temperature at idle (warm engine) with all loads off (AC off), you should be setting the MAF sensor reading around 3.6-3.8 g/s.

Press the gas pedal to rev the engine. The number should increase dramatically since a lot more air is suddenly rushing into the engine. You can also hold the engine at certain RPMs to check the MAF sensor value there. If the MAF sensor readings do not match this behavior or the line of the graph is flat, the MAF sensor has gone bad. On a healthy MAF sensor, you can see a period of healthy readings where it’s fairly level but jagged.

To clean the MAF sensor, follow these steps:

  • Disconnect the negative terminal of the battery.
    Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
    Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner.
    Reinstall and reconnect the battery terminal.
  • Start the engine and let the ECU relearn idle for 15 minutes.

If still after cleaning the MAF sensor, the graph is not correct, it means you need to replace the MAF sensor. If after replacing the MAF sensor, you are not able to get rid of the P0171 trouble code, it means there is an unmetered air that the MAF sensor is unable to detect. I have explained it in the following sections.

3. Air Leaks After The Mass Airflow (MAF) Sensor

Mass airflow sensor only measures the air flowing through it. If after the MAF sensor there is any vacuum leak, it will not be able to detect that. This air will be considered “unmetered air“.

If there is any loose connection at the hose connected through the throttle body, extra air will enter the engine cylinder, which will not be measured by the MAF sensor. This causes the PCM to not be able to adjust the injector pulse width properly to accommodate that air so the O2 sensors now see a lean condition. If it goes too lean for too long, it will set a code. In my case, there was a crack in the intake hose.

air intake hose crack causes the unmetered air to suck into the engine

4. PCV System Is Leaking or the Valve Stuck Open

The PCV (Positive Crankcase Ventilation) system in an engine routes the gases produced by the oil in the engine crankcase back to the engine cylinder so that they can be safely burned and do not harm the environment.

The PCV hose leads from the PCV valve on the crankcase to the air intake manifold. The PCV valve is a one-valve, meaning that the vapors and gases of the motor oil can only flow from the crankcase to the air intake manifold.

If there is any leak in the PCV hose or the PCV valve is stuck open, it will cause an increased volume of over-regulated air to enter the intake manifold system and cause a lean air mixture. You will usually find the PCV valve on or near the air intake manifold.

PCV valve

PCV valve is stuck open because gases flowing through your PCV valve leave traces of oil that can clog it up.

5. Vaccum Leaks

There are vacuum hoses in the engine that lead from the air-intake manifold. You can use spray brake cleaner for the suspected vacuum leaks in the engine. Use a stethoscope to find the suspected areas of a vacuum leak. Spray a brake cleaner on them. 

Since a spray brake cleaner is flammable, you must be very careful because an arcing plug wire or hot exhaust pipe can turn this into an explosive situation quickly. Brake cleaner also comes in non-flammable form. But the reason to buy a flammable brake cleaner is that when it is sucked by the engine, it will combust and the RPM starts to weird.

There are the following suspected areas of vacuum leaks where you spray the brake cleaner:

  • All vacuum hoses connected to the intake manifold
  • Around brake booster hose
  • Areas around the throttle body for the engine to check whether the gasket between the throttle body and air intake manifold is damaged and has started to leak.

If you have an OBD2 tool, check the short fuel trim during this process. If it has a major jump to a negative number you’ve found your vacuum leak. 

6. Bad Air Filter

Bad air filter can also cause a P0171 code. If you are using an aftermarket oiled air filter like K&N, it can foul your MAF sensor with oil. Moreover, if the air filter is clogged, it will cause turbulence in the coming air.

The MAF sensor will not be able to determine the correct amount of air entering the engine. As a result, the engine control computer will reduce the amount of fuel delivered by the injectors so as to maintain the proper air/fuel ratio despite the limited volume of air provided.

7. Clogged Fuel Filter

The fuel filter is responsible for filtering the gasoline, and this helps to keep the engine running smoothly. If the fuel filter is clogged, enough fuel will not be pumped through the fuel injector. As a result, the mixture will run lean and the ECU will throw the P0171 code.

You can follow the below tutorial to remove the fuel filter from the engine:

To diagnose, whether there is a problem with the fuel filter, you can run a fuel pressure diagnostic test. This will give you real-time engine fuel pressure readings, directly from the fuel pressure sensor.

Connect the diagnostic reader to the car and read the engine parameters. When that is done, select the fuel delivery system option. You will get access to different parameters of the fuel delivery systems such as fuel rail pressure, short-term fuel trim and long-term fuel trim.

The OBD scanner should also show you the fuel rail pressure set point. This is the optimum fuel pressure on the fuel rail when the engine is running, and you can use it to compare to the real-time fuel pressure value. 

Every OBD2 scanner has different settings. You have to select that option that most closely matches the ‘actual (real-time) fuel pressure parameter’ from the menu. 

To check whether this stays constant under load, try revving the engine to 1500 rpm and hold it there for a few seconds, then try 2500-3000 rpm.

If the actual fuel pressure is the same as the fuel pressure set point, this means that there is no issue with the overall pressure in the fuel delivery system.

If there is no such option you can see, then you will have to perform a fuel pressure test using a pressure gauge.

Check out the below video to learn how to perform a fuel pressure test using a gauge.

8. Bad or Contaminated Fuel Injector

A bad fuel injector is one of the most common problems of P0171 System Too Lean code in the engines. In modern engines, direct fuel injection technology is implemented instead of a carburetor that is controlled by ECM. 

The contaminated fuel injectors are usually the result of carbon build-up in the injector nozzle. Over time, this carbon builds up, and the injector is unable to atomize the fuel properly. As a result, the engine runs lean.

I found this great video to test the fuel injectors.

9. Exhaust Leaks In The Exhaust Manifold Gasket Or Mating Gaskets Before Or Near The Heated Oxygen Sensor

Exhaust leaks can cause a P0171 code. One of my friends had a P0171 code detected. He found that it was caused by the exhaust leaks that were found on a loose exhaust pipe. 

It is a very tricky concept to explain how exhaust leaks can cause the system lean as exhaust gases have higher pressure than ambient air and it should not let the ambient air in through the exhaust leak.

Well, in the engine, there is a process called scavenging. I’m a mechanical engineer and I have studied this process a lot in detail. Basically, during the engine combustion cycle, there is a process called valve overlap.

Just before the exhaust valve closes, the intake valve opens to help exhaust gases leave the engine cylinder faster. Since exhaust gases pass at a very high speed. According to the Venturi principle in engineering, the area where gases flow with a speed creates a low-pressure region. 

So, if there is any leak in the exhaust before the O2 sensor, the ambient air will suck in. Since the ambient air will bring extra oxygen levels. The O2 sensor will record higher levels of unburnt oxygen in the exhaust gases. In this way, the O2 sensor will detect that system is lean, due to which the P0171 code is detected.

The symptom of exhaust leaks is that you will hear a whistling noise. To detect exhaust leaks, you can read this guide.

P0171 Code in Nissan, BMW, Ford and Chevy

In Nissan, BMW, Ford and Chevy, the P0171 code is caused by the following reasons:

  • Faulty Mass Air Flow (MAF) sensor
  • Incorrect Positive Crankcase Ventilation (PCV) hose connection
  • Faulty front heated oxygen sensor
  • Exhaust leaks
  • Dirty fuel injectors
  • Clogged fuel filter
  • Fuel pump beginning to wear out
  • Vacuum leaks downstream of the MAF sensor around the throttle body and or intake manifold
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