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P0171 Ford – Meaning, Causes, Symptoms, & Fixes


What Does Code P0171 Mean for Your Ford?

  • P0171 definition: Bank 1 has too much air or not enough fuel (lean bank)
  • Issue Severity: MODERATE– Extended driving with this code can cause internal engine damage.
  • Repair Urgency:  Get this code fixed as soon as possible to avoid damage to spark plugs, pistons, and catalytic converters.
  • Diagnosis: It is important to complete the entire diagnostic process when diagnosing the Ford P0171 code. Many people will replace the air-fuel sensor or oxygen sensor as soon as they get a bad reading, but the root cause can also be a dirty or faulty mass airflow sensor or vacuum leak, thus causing the O2 or MAF sensor to read differently to compensate. Use the FIXD Sensor and app to diagnose and fix P0171 yourself, see the most likely fix and cost for your Ford, access our Mechanic Hotline, and more.
  • One of the top 5 most common causes of a check engine light in 27 out of the 42 Ford models we have data on.

Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. Too much fuel and/or too little air results in the engine running rich setting a P0172 trouble code, while too little fuel and/or too much air results in the engine running lean resulting in a check engine light for the Ford P0171 code. The lean condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the lean condition by injecting more fuel to the mixture in an effort to maintain the proper 14.7:1 ratio.

When this problem arises in bank 1 and becomes so large it cannot be managed by fuel trim, the Ford P0171 code is triggered.

All of the causes you’ll see below have to do with components involved in delivering the proper air to fuel ratio to the cylinders of the engine bank 1. 

Whether more air is entering the bank 1 than should or, less fuel is making it to bank 1 than should, it result in the same problem – too much air in the cylinder and not enough gas when it ignites. 

It may be helpful to think of these as the 2 possible diagnosis categories:

1. Leaks that let in air and

2. A slow or burdened fuel supply 

The extra air could be coming from an exhaust or vacuum leak which can be tricky to find.

A slow or burdened fuel supply could be a caused by a bad pump or just a clogged filter.

There is an exception to these categories of course – If it isn’t the air and it isn’t the fuel, it may just be a broken sensor. Sensors eventually become too worn (after years in use) and may start sending inaccurate data to the cars computer (OBD) after a while. 

Keep in mind that a faulty sensor may only be affecting one bank of your engine today, but, a faulty O2 sensor can also cause problems with your catalytic converter if left for too long. 

Beyond just broken sensors, the other causes of a lean bank 1 on this list can also contribute to damaging other parts of the engine system (including the catalytic converter)

With that out of the way, let’s dive in.

P0171 Causes (Lean Bank 1)

  • Faulty or dirty mass airflow sensor
  • Vacuum leaks – PCV hoses, vacuum hoses, intake manifold gasket
  • EGR or EGR valve
  • Weak fuel pump
  • Clogged or dirty fuel injectors
  • Clogged fuel filter
  • Exhaust leak
  • Faulty oxygen sensor
  • Faulty air-fuel ratio sensor

P0171 Symptoms

  • Check Engine Light is on or flashing
  • Lack of power from the engine
  • Rough idle
  • Engine “coughing” or misfiring
  • Tip of spark plugs are white
  • Commonly associated with error codes: P0174

How Do I Fix a Code P0171 Check Engine Light?

With a Ford P0171 code, the first step is to figure out what is causing engine bank 1 to run lean. Using the live data function of the FIXD sensor and app allows you to read and analyze fuel trims to properly diagnose a P0171 code.

The FIXD sensor reads engine codes that are causing your light and helps you diagnose those codes in plain English, on an easy to use app. Whether it’s P0171 today or P0420 tomorrow, we’ve got you covered.

> Click here to get FIXD for $19.99 for a limited time!

If your Fords’ engine is running lean and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

How Much Does It Cost to Fix Code P0171?

If you take your Ford to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your Ford P0171 code or any other codes your engine my have along with it.

Possible Repair Costs for Ford P0171 Code

When it comes to making repairs associated with the Ford P0171 code, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

  • Vacuum leak $100-$200
  • Clean MAF $100
  • Replace MAF $300
  • Fuel Pump $1300-$1700
  • Fuel pressure regulator $200-$400
  • Exhaust repair $100-$200 (if welded to repair)
  • Air fuel sensor or oxygen sensor $200-$300
  • EGR valve replacement $251-$413

DIY Steps to Diagnose Code P0171

If you’d like to try to fix code Ford P0171 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners. Diagnosis requires more specialized equipment beyond what the FIXD Sensor can provide and it can be a time and labor-intensive process for inexperienced DIYers.

DIY difficulty level: Intermediate

This repair requires mechanical knowledge and is not recommended for beginners.

Tools/parts needed (our top picks from Amazon):


Use FIXD to scan your vehicle to verify the Ford P0171 code is the only DTC present. 

If other codes are present, they must be addressed first. Having other codes may indicate potential diagnosis beyond what we have outlined in the scope of this article. 

If the P0171 code is one of a few codes, P0171 may be a symptom and not the root cause of the codes you are seeing. If you feel confident in your diagnosis skills, take a look at the codes yourself and see if you can unravel the clues. 

If analyzing a few codes and the comparing potential causes doesn’t seem like your cup of tea, this might be the time to let a mechanic take the reigns on the project.


Inspect all vacuum lines and hoses for leaks, and make sure they are properly connected. If a leak is present, you will hear a hissing sound, though it may be difficult to hear with the engine running. If you suspect a vacuum leak but are having trouble finding it, check out this detailed guide to finding vacuum leaks (Applies to all vehicles, not just Ford)


Remove the mass airflow sensor and clean the sensor using mass air flow cleaner or contact cleaner.

> How to clean your MAF sensor (Applies to all vehicles, not just Ford)


Check the exhaust system components for leaks before the Air Fuel Ratio Sensors or Oxygen sensors; if you find any leaks have them repaired. Exhaust leaks can usually be spotted visually by a buildup of soot in the general vicinity of the leak.

STEP 5: CHECK FUEL SYSTEM (Fuel Pressure, Fuel Injectors, Fuel Filter, & Fuel Pump)

Low fuel pressure or blocked fuel delivery can throw off the air-fuel mixture ratio, and this could be caused by anything from the fuel injectors and fuel pressure regulator to the fuel pump or even a clogged fuel filter (This and the following links apply to all vehicles, not just Ford). Check out the links to some our other posts on the subject highlighted in green below.

> How to check fuel pressure 

  • Check fuel injectors: Make sure the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also, check that the fuel injector wiring is not damaged and is connected properly.
  • The fuel pump could also be broken, make sure to check if it’s running when triggered. If the fuel pump is running, make sure you check the fuel filter and other components.

> How to check fuel injectors by ear 

> How to check fuel injectors with a digital multimeter 


At this point if the code still persists you may want to consider replacing your A/F sensor or O2 sensors, they may just sensing too much air as a result of being too worn or otherwise broken.


After you repair the cause of your P0171 code, inspect the spark plugs. If there is a white haze or crust on the tip of the spark plugs (near the diode that screws into the engine bank 1), then you should replace all of the spark plugs (Applies to all vehicles, not just Ford).

Common P0171 Diagnosis Mistakes

It is important to complete the entire diagnostic process when diagnosing Ford P0171 codes. Many people will replace the air-fuel sensor or O2 sensor as soon as they get a bad reading, but the root cause is often a dirty or faulty mass airflow sensor or vacuum leak, thus causing the O2 or A/F sensor to read differently to compensate. Reading and analyzing fuel trims and the freeze frame data is the key to properly diagnosing P0171.

Still Need Help Fixing Code P0171?

If you’ve followed the steps above and are still experiencing a lean engine condition and Ford P0171 code or any other codes you need help with, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price. Don’t have a FIXD Sensor yet? Click here to get yours for just $19.99, including a free 14-day trial of FIXD Premium!


Popular Engine Codes P0420 P0172 P00171 P0128 P0300 P0456 P0430 P0442 P0455 P011

Fixed It But The Check Engine Light Is Still On?

Check engine light sometimes need to be reset manually, check out our article:

How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)

Jeffrey N. Ross

Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals

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About the Author

Jeffrey N. Ross

Jeffrey N. Ross

Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a '91 GMC Syclone. #GirlDad #SaveTheManuals

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