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Fix P0171 OBD II Error Code: Meaning, Causes, Symptoms & Solutions


Getting the error code P0171 is not a pretty sight. It's an OBD II error related to the air-fuel mixture stating that something is either wrong with the engine or fuel supply. And if not fixed, it may cause further damage to the engine. 

Fortunately, the P0171 is a common error and has multiple known fixes. Below in this post, we have discussed P0171 fixes with symptoms and causes.  

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What does the Error Code P0171 mean?

The P0171-System too Lean (Bank 1) is a trouble code shown when the air-fuel mixture inside the engine is too lean. Meaning either the fuel supplied is too less or the air is too much. This causes less efficient combustion, resulting in low power production and less fuel efficiency. 

Why is the error P0171 raised?

A modern-day engine uses 14.7 parts of air to 1 part of fuel, which is an optimum ratio of 14.7:1. To maintain the ratio, the PCM (Power Control Module) continuously adjusts the air and fuel supply guided by the input from the oxygen sensor. 

The oxygen sensor measures the difference between the amount of exhaust in the exhaust smoke and the atmosphere. Depending on the unburnt oxygen level in the exhaust smoke the PCM determines whether the air-fuel mixture is too rich (more fuel, less air) or too lean(less fuel, more air). This helps the PCM to adjust the fuel supply, maintaining it at a ratio of 14.7:1.  

When the PCM fails to adjust the air/fuel ratio running too lean, the system generates error code p0171-system too lean (Bank 1), turning the check engine light on. If you have a 6-cylinder engine or more, you might also get error code p0174-system too lean (Bank 2). 

The only difference between both is the cylinder bank affected.  

What is Bank 1 vs Bank 2?

Cars with 6 or 8 cylinders have two groups of cylinders on each side. One is called Bank 1 and another Bank 2. With smaller engines (3 or 4 cylinders), there is only one group, hence they only have Bank 1. 

Look into your User manual to find which side is Bank 1 or Bank 2. 

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Symptoms of P0171 error code: 

The error code P0171 has obvious symptoms because of the lean air/fuel mixture. Some of which are: 

Check engine light ON:

The engine light is the first thing you will see when the PCM detects a P0171 fault in the engine.  

Low power production:

Since the air-fuel mixture is running lean, the combustion gets fairly weaker creating less power than normal. This lack of power can be felt entirely or in-between acceleration. Engine knocking is also common with the P0171 error code.  

Engine overheat:

As the engine is producing less power, it has to work harder to make the same level of power as required. Hence, it gets overheated faster.

Trouble starting:

If the P0171 error persists, the engine would have trouble starting eventually and might eventually die. This can be caused by many reasons mentioned below. 

Engine making Hissing noise: 

If there is a leak between the engine and the mass airflow, the extra air sucking in would make hissing noises. Despite a good fuel supply, the extra air would make the mixture too lean. Rough idling and stalling are caused by the Vacuum leak.  


What are different causes of Error Code P0171?

Again, the error P0171 is because of too much air or too less fuel. Meaning the problem is either related to the air intake or the fuel supply. 

Too much air is caused when their is:

  • Vacuum leak: A vacuum leak is caused when there is a leak between mass airflow and the engine. This leak can be due to loose-fitting or worn-out gaskets or vacuum hoses. 

  • Faulty Mass airflow sensor: In electronic fuel injection, the mass airflow sensor measures the air supply to the engine. It can get contaminated over time resulting in faulty readings. This reading would cause the PCM to misjudge the fuel supply resulting in a lean air/fuel mixture.

  • Faulty Oxygen Sensor: A faulty oxygen sensor would send inaccurate data to the PCM. If the sensor detects less unburnt oxygen than there is, the PCM would reduce the fuel supply, causing it to run too lean. 

  • Clogged Catalytic Converter: A faulty oxygen sensor reading can be caused due to a clogged catalytic converter. Which will have the same aftermath as having a faulty oxygen sensor. 

Too less fuel is caused when there is:

  • Faulty fuel pressure regulator: The fuel pressure regulator regulates the pressure of fuel going to the injector. A faulty unit sending in low-pressure fuel would result in a lean condition. 

  • Clogged/faulty fuel pump: The fuel pump pumps the fuel from the tank to the engine. If it gets faulty or clogged, it will supply less fuel causing a P0171 error. 

  • Bad injectors: Fuel injectors inject fuel at high pressure through a very fine nozzle. The nozzle eventually gets blocked/clogged due to contamination, meaning less fuel is supplied in the combustion chamber. Hence, System is too lean.

  • Faulty PCM module: While not common, faulty PCM modules can easily cause a P0171 error code. If the PCM fails to calculate the required air/fuel supply for a proper air-fuel mixture, too lean conditions can easily be generated.  


How to diagnose P0171 codes?

Diagnosing the P0171 error code would require you to have a scan tool. However before that, first check the TSB or Technical Service Bulletin of your manufacturer. 

Check TSB (Technical Service Bulletin):

A technical service bulletin is basically a document with causes and repair procedures of regular occurring problems. Each manufacturer has its own TSB with the list of most occurring problems in their vehicle. If the P0171 is a regularly occurring problem with the model you own, the TSB would have both the cause and the solution. 

Like in BMWs, as per their TSB, a cracked intake boot is the most common reason for P0171 in their vehicle.     

Check under Hood:

Run a quick check under the hood to spot any leaks or ruptures. Things you need to check around are the fitting of the air filter box, vacuum lines and hoses. Look for cracks or leaks in the intake boot.

With a scan tool:

Using a scan tool, run checks on Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). At idle, the reading for both stays around zero. However, if the reading goes higher than 15%-20%  for STFT at idle and reduces at high rpm, you might have a vacuum leak. 

If the reading goes to 15%-20% at high rpm while staying at zero at low, the issue is related to mass airflow, fuel pump or catalytic converter. 

You can diagnose both the fuel pressure sensor and mass airflow sensor using a fuel pressure gauge. Match the readings with the manufacturer’s specification and you will have your culprit. 

What are the possible fixes for Error Code P0171? 

The error code P0171-System too Lean (Bank 1) can be fixed once the source of the fault is diagnosed. However, it is seen in most cases the error arises due to something minor as in clogged/dirty fuel pumps or bad mass airflow sensors. Vacuum leaks are also one of the most common reasons behind the p0171 fault code. 

Other possible fixes for the error code P0171 include replacing the oxygen sensor, cleaning/replacing the catalytic converter, replacing the fuel pump, cleaning the clogged fuel filter, cleaning/replacing fuel injectors and replacing the power control module.


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