P0171 Code: Meaning, Symptoms, Causes, Fixes And Repair Cost

What does the P0171 code mean? What causes a P0171 code and can you drive with it? Here's a short guide and what you can do to fix it or the repair cost.

A P0171 indicated that the engine is running lean on bank 1. In other words, there is too much air or too little fuel in the combustion chamber.

There may be several reasons why this code is shown. In this guide, we will go through the most common ones.

We will also go through the different symptoms that may show and, most importantly, how to get rid of the problem in the most effective way.

Find out everything about OBD code P0171 down below!

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What Does Code P0171 Mean?

The P0171 code means that the engine is running lean on bank 1.

This means that the fuel/air mixture is inadequate as there is too much air or too little fuel in the combustion chamber.

Bank 1 points to the side of the engine that is affected. In this case, it is the side on which the first cylinder is located.

Symptoms Of P0171

Car Inspection Or Search (Symptoms)

Code P0171 causes a variety of symptoms, such as poor performance, rough idle, stalling, difficulty starting, and more.

Sometimes the only symptom that accompanies P0171 is a check engine light, however, the cause should be diagnosed and solved as soon as possible to prevent it from being worse.

Driving with a P0171 can usually be done but limit it and get the car to a mechanic or a place where you can fix the problem as soon as possible.

There might be some instances where it is not safe to drive the car, but these signs are very explicit, such as stalling multiple times, even while driving.

The causes of these symptoms will be covered further down in this guide.

Poor Performance

Poor Performance

Perhaps the most common symptom that accompanies a P0171 (except for a check engine light) is the poor performance of the car.

This may not come as a surprise as the air/fuel mixture is inadequate for effective and ideal combustion, which translates to poor engine performance.

If the car runs too lean, you may also notice the RPMs going up and down when idling. This may also vary depending on the specific cause of the code, which will be covered further down.


Car Stalls While Driving

A more serious and severe symptom is stalling, as the air/fuel mixture is too lean to the extent that no combustion takes place, which will stall the engine.

This symptom only occurs in very severe cases, and chances are you will fix the problem before coming to this stage.

It is also possible that the car has difficulty starting, only to stall later. This is most probably caused by the same problem.


car cylinder head

A symptom that precedes stalling is misfiring cylinders. Causes include low fuel pressure but may also be due to, for example, a bad O2 sensor.

Depending on the cause of the lean mixture, this problem can be isolated to only one cylinder or to multiple cylinders on the same bank (or even Bank 2 if another additional code is shown).

P0171 Causes

There are several causes for the P0171 code. As previously mentioned, they all have in common that the engine is running lean.

This may be due to little fuel in the combustion chamber or too much air.

The causes for PO171 include:

  • Bad Mass Airflow Sensor
  • Vacuum Leak
  • Bad O2 Sensor
  • Bad Fuel Pump/Filter

The cause is almost always one of the four mentioned above or closely related to them. All the abovementioned potential causes will be discussed below.

Bad Mass Airflow Sensor

mass airflow sensor

The job of the mass airflow sensor is self-explanatory – it registers how much air is getting in the engine.

Sometimes, this sensor can get dirty or simply stop working, which calls for a replacement, but sometimes a cleanup will make the MAF work properly again.

If you have a scan tool available, it is possible to read the value of the MAF sensor output. An abnormal reading would suggest that the problem lies with the MAF sensor.

A mass airflow sensor will cost you about $200-$300 to replace, but you should definitely give it a try to clean it first, as there is a great possibility that it will be sufficient to fix the problem.

Vacuum Leak

Car Engine Vacuum Leak

A vacuum leak most commonly refers to a small crack in the intake manifold. This causes air to get sucked into the engine while surpassing all relevant sensors.

Because additional air gets sucked into the combustion chamber, too much air will be present in the cylinder without the ECU knowing beforehand that extra air is on its way.

Finding a vacuum leak can be time-consuming and tough. The best way to find a leak is by using a propane torch to localize the leak.

Fixing a leak in the intake manifold varies greatly depending on your car’s model, but expect to pay a few hundred dollars for a fix, including labor.

Bad O2 Sensor

Oxygen sensor bank 1 bank 2 sensor 1 sensor 2

O2 sensors read the O2 content in the exhaust, which, in turn, is relayed to the ECU to adjust the air/fuel mixture accordingly.

A faulty O2 sensor will give the wrong parameters to the ECU, causing the engine to run too rich or lean.

It is important to note that faulty combustion, due to other reasons, will make the O2 content of the exhaust high, causing the engine to run lean.

This means that another problem may cause the abnormal air/fuel ratio instead of the O2 sensor. It is, therefore, important to go through each potential problem and evaluate which is more likely.

If it turns out that the O2 sensor is the problem, you are looking at a replacement cost of around $100.

Bad Fuel Pump/Filter

Car Fuel Pump

Sometimes the problem of a lean running engine is not too little air but too little fuel due to a variety of reasons.

In short, a lean air/fuel mixture caused by fuel-related problems includes a bad fuel pump, a clogged fuel filter, and clogged fuel injectors.

A clogged fuel filter will partly block the fuel that the fuel pump is sending, which will increase the workload of the fuel pump, making it prone to go bad.

Another way a fuel pump can go bad is by driving the car with low fuel in the tank.

This is a problem as the fuel cools down the pump, and it is generally easier for the pump to send the fuel when there is more fuel in the tank.

Clogged fuel injectors also cause an inadequate amount of fuel to be delivered to the combustion chamber, making the engine run lean.

Changing fuel injectors can be expensive, especially if you drive a premium brand. In some cases, cleaning the fuel injectors at a professional shop will do the trick.

If it turns out that you must replace the fuel injectors, the average cost for one fuel injector is around $200, and every cylinder has one of its own.

It can be expensive to replace them, but it is necessary in order to not cause further damage to the fuel system.


The P0171 code means that there is either too little fuel or too much air in the combustion chamber, making the engine run lean.

There are many potential causes for the engine running lean, either due to extra air coming into the combustion chamber, such as a vacuum leak, or too little fuel being delivered due to a faulty fuel system.

Whatever the case may be, it is important to handle the problem and resolve the issue as soon as possible to protect the engine and all the systems from further damage.

Symptoms that may present together with a P0171 code include misfires, poor engine performances, stalls, and rough idles.

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Bajro Muric

Bajro is a big car enthusiast with a love for writing and teaching. He writes about anything regarding cars - from common problems and fixes to racing.

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