Engine Code p0171 Meaning, Causes, Fixes (4 Steps)

Did you just get an error code p0171 on your vehicle? 

Don't worry, you're in the right place!

In this PopularMechanic article you will learn:

  • What Does The p0171 Code Mean?
  • What Caused The p0171 code (Possible Symptoms)?
  • How To Diagnose An p0171 Code?

And much more!

Engine Code p0171 Meaning, Causes, Fixes (4 Steps)

Are you currently faced with a vehicle that exhibits an aggravating check engine light, and a p0171 fault code that has left you in a state of confusion?

The appearance of a check engine light on your vehicle’s dash can certainly be troublesome.

Such a light leaves you questioning your vehicle’s reliability, and leads to thoughts of costly repairs, and vehicle downtime.

Fortunately, the diagnosis and repair of many of the most common causes of a p0171 fault code are relatively simple and can be handled by most avid DIY at home mechanics.

What Does The p0171 Code Mean?

Fault code p0171 means “system too lean bank 1.” This means that the exhaust gases that are monitored by the first downstream O2 sensor on a given engine bank, are high in oxygen content. 

Each of an engine's two halves is designated with a bank number, when speaking of v-configuration engines.

This allows you to narrow down the portion of the engine where your problem has arisen, based on a given code.

Therefore, in the case of trouble code p0171, bank 1 is displaying the characteristics of running too lean.

This is indicative of a larger root problem that is leading to an incorrect air/fuel ratio, which in turn leads to less than optimum combustion and performance.

Error codes P0171-P0175 are the most common error codes that happen on all vehicles. Faulty oxygen sensors are much more common than you think!

Read More >> What Does The Service Engine Soon Light Mean?

What Caused The p0171 code (Possible Symptoms)?

Several factors can contribute to a p0171 trouble code being set. Any issue, mechanically or electronically, that can cause stifled fuel delivery or unmetered air intake can lead to a lean condition.

Some of the most common mechanical causes of a p0171 error code are dirty or clogged fuel injectors, restricted fuel delivery supply or low fuel pressure, leaking vacuum lines or PCV hoses, cracked intake ductwork, and leaking intake gaskets.

The most common of all p0171 electrical-based faults are dirty or damaged MAF (mass airflow sensors).

Such an issue can cause false readings and unreliable inputs to factory ECM software, in turn causing the ECM (engine control module) to call for incorrect adjustments in fuel and air delivery.

Many “lean” conditions do not lead to any observable drivability complaints. However, your vehicle may experience a lack of performance, rough idle, and unresponsive acceleration.

Check out this article for more information about a p0171 OBD II  code and how to fix it.

Read More >> What Does Error Code P0420 Mean & How Do You Fix It?

How To Diagnose A p0171 Code?

Since there are a number of possible root causes related to the onset of a p0171 fault code, it is important to accurately diagnose the problem at hand before moving forward with any repairs, The following process will assist you in quickly and accurately identifying the source of your trouble.

  1. Check For Additional Codes
  2. Check For Vacuum Leaks
  3. Check Fuel Pressure
  4. Check For Pre-O2 SensorExhaust Leaks
  5. Check Sensor Values

Step 1: Check For Additional Codes

Prior to performing additional checks, it is important to use a scan tool to check for the presence of any additional codes. Additional codes outside of the predetermined p0171 code can point you in one specific direction over another.

Step 2: Check For Vacuum Leaks

If no other codes are found, the first order of business is to check for vacuum leaks. This can be done in one of two ways. You can either use a vacuum gauge to check for an inability to hold vacuum, or you can use a smoke machine to watch for any damaged intake manifold gasket, lines, or hoses.

Step 3: Check Fuel Pressure

If you are yet to discover an obvious problem, you will need to check your vehicle’s fuel pressure. This will be done with a mechanical fuel pressure gauge that is hooked into the fuel system at its designated fuel rail port.

Check pressures against factory specifications for your model. These specifications can be found in service literature for your vehicle. If you have access to a bi-directional scan tool, now is also a good time to run injector functionality tests.

Step 4: Check For Pre-O2 Sensor Exhaust Leaks

Check for the presence of any exhaust leaks upstream of your vehicle’s first O2 sensor on the affected bank. This test can be done with the assistance of a smoke machine, similar to how it is conducted during an intake leak test.

Step 5: Check Sensor Values

If your vehicle has passed every test up to this point, a faulty sensor is likely to blame. Check your MAF sensor and fuel pressure sensors in the manner described by factory service literature. You can also use a scan tool to monitor the data from these sensors, checking for out of range values.

Read More >> What Does Error Code P0011 Mean & How Do You Fix It?

How Do I Fix An p0171 Code?

  1. Fix Any Vacuum/Intake Leaks
  2. Fix Fuel Delivery Issues
  3. Fix Upstream Exhaust Leaks
  4. Replace Failing Sensors

Step 1: Fix Any Vacuum/Intake Leaks

One of the most common sources of a p0171 diagnostic trouble code is the introduction of unmetered air into an engine. The introduction of this unmetered air typically stems from a damaged vacuum hose or leaking intake gasket. Therefore, it is important to repair any such leaks that are discovered.

Step 2: Fix Fuel Delivery Issues

If an engine cannot receive the desired amount of fuel, it will never obtain the proper air-to-fuel ratio. If poor fuel pressure is discovered, seek out the underlying issue, and conduct repairs accordingly. This often includes a faulty fuel pump or a clogged fuel filter. If an injector functionality test is failed, clean or replace the affected injector.

Step 3: Fix Upstream Exhaust Leaks

Exhaust leaks that are upstream of a vehicle’s O2 sensor can provide that sensor with false readings. If such a leak is discovered, make any necessary repairs.

Step 4: Replace Failing Sensors

If a failing MAF sensor or fuel pressure sensor is noted during diagnosis, replace the affected components. A failed sensor can trick a vehicle’s ECM into falsely adjusting fuel trims.

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Taking Matters Into Your Own Hands

Although the appearance of a check engine light is indeed aggravating, a few hours of work and a methodical diagnostic approach is often all that is needed to fix what ails your vehicle. This certainly holds true when speaking of a p0171 trouble code. 

By taking your time, and carefully determining the root cause of such an issue, you will save money and have your car, truck, or SUV back on the road in no time.

About Your Mechanic

About Josh Boyd

I’ve been an ASE Certified automotive/heavy equipment technician since 2010. I began my career performing basic automotive maintenance at a service center before working as a Toyota technician at my local dealership.

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