P0171 Error Code: Meaning, Causes, Symptoms, And Fixes

Auto manufacturers design cars to run at designated air-fuel mixture ratios. If there’s too much or too little air or fuel mixture, it’ll disrupt the optimal engine operation.

When the powertrain control module (PCM) notices an increased airflow into the engine, it’ll store the P0171 engine code on the memory and trigger a check engine light on the dashboard.

Several parameters like faulty fuel pumps, vacuum leaks, and lousy car sensors can cause increased airflow into the engine. If you ignore tracking and fix the root cause, it may escalate to a major engine problem.

Rx Mechanic team of experts has explained the causes, symptoms, diagnosing, and described the fixing procedure of the P0171 error code that even non-technical individuals to understand easily. You’ll also learn the severity of the problem and common diagnosing mistakes. But, before we go any further, let’s see the meaning of P0171 on Toyota and other vehicle models.

p0171 code

What Does Error Code P0171 Mean?

Diagnostic trouble code P0171 means System Too Lean (bank 1). That means the car computer noticed the engine is running lean. A lean system means the air-fuel ratio entering the combustion chamber is not what the engine wants. It means there is too much air or too little gas to balance the specified mixture ratio.

When a car computer registers P0171, it means there is a vacuum leak on the lines that allow unmeasured air to mix with the air-fuel mixture, or there’s a problem with the fuel system that is restricting adequate fuel input to the engine.

The latter probable cause possibly means you have a defective fuel pump, clogged fuel filter, or lousy fuel injectors. It’s essential to note that both P0171 and P0174 diagnostic trouble codes mean there is too much air in the system.

The only difference is that the P0171 shows that bank 1 is running lean while P0174 shows bank 2 is running lean. Find out bank 1 and bank 2 locations.

Read Also: P0172 Diagnostics Trouble Code: Meaning, Causes, Symptoms, and Fixes

What Is The Symptom Of Error Code P0171?

Several error codes like P0454, P0171 P0457, P0174, P0450, etcetera project some symptoms to notify the driver of imminent issues in a vehicle. Of course, P0171 is no exception. So here are the symptoms of P0171.

Check engine light

Your car computer triggers the engine warning light whenever it detects system issues on electrical and mechanical components. Therefore, this light can pop up for a variety of reasons.

If the ECM notices a change in the air-fuel mixture and displays the engine warning light on the dashboard, you may not notice a change in the vehicle operation. But before thinking of a lean air mixture in bank 1, diagnose the vehicle and pull the registered codes.

Loss of power

Since a registered P0171 means there’s a compromised air-fuel mixture, it’ll affect the optimal engine operation. Therefore, P0171 causes a loss of engine power, depending on the severity of the problem.

Rough idle

When the car computer detects too much oxygen in the combustion chamber, the combustion process will be compromised. When you turn on your vehicle and allow it to idle, you will notice the engine is running rough. You may also hear a coughing sound.

Stumble and hesitation from the engine

When you eventually hit the road, it may feel like your engine hesitates when driving uphill or accelerating on level pavement. It may feel like the engine is stumbling or misfiring.

Hard starting

Another symptom of P0171 on Ford and other vehicle models is difficulty starting the vehicle. Again, though, it’s worth noting that several factors can cause a hard start.

Damaged catalyst converter

If you ignore the P0171 on Chevy or any vehicle model for an extended period, it may damage the catalyst converter.

p0171 code toyota

What Causes Error Code p0171?

Lean air mixture is a common issue with some vehicle models regardless of the production year. Therefore, compact cars, Sudan, SUVs, Coupes, and trucks can have a peak share on the P0171 error code. However, different factors can trigger this diagnostic trouble code.

Many vehicles experience poor fuel mixture several times before the end of their lifespan. Regardless of the root cause, once your car computer registers P0171, Track the culprit and fix it as soon as possible.

However, if you are after the most common cause of P0171 fault codes;

Vacuum leaks

The vacuum leaks are the first things to check when your car registers the P0171 fault code. That is a crack on the positive crankcase ventilation (PCV) hose, intake manifold systems, or vacuum hoses.

If all the vacuum lines are in good condition, one of the following will be the culprit.

  • Weak or bad fuel pump
  • Clogged fuel filters
  • Exhaust leaks
  • Defective injectors
  • Mass airflow sensor (MAF)
  • Bad Oxygen sensor
  • Lousy fuel pressure regulator
  • A faulty powertrain control module. (not likely the cause, though, possible).

How Serious Error Code P0171 Is?

If you’re asking, Is P0171 serious? Whenever the ECM logs the P0171, do not drive the vehicle for an extended period. The severity is moderate. Ignoring it for an extended period can cause an engine breakdown.

When driving with lean air, the engine receives a compromised air-fuel ratio, and the fuel ratio will be below the required amount for optimal operation. Therefore, the vehicle may not run efficiently to climb a hill or meet traffic speed.

How to diagnose

If you want to save some cash and diagnose the fault at home, read to the end. In any case, note that P0171 is an intermediate repair task. It is not a beginner DIY skill. So again, you’ll have to invest your time and rapt attention.

Tools and Items needed

Step 1: Scan the vehicle

Locate the OBD port on your vehicle and plug in the scan tool. Ensure the car computer did not register any other code. If the scan tool pulls out other codes, fix them before heading to the next step.

Step 2: Check for vacuum leaks

Examine the vacuum hoses and lines for leaks. If there’s a vacuum leak, you’ll hear a hissing sound. However, you need to pay rapt attention before you can hear this sound as the engine is running.

Step 3: Examine the MAF sensor and Exhaust system

Remove the mass air flow (MAF) sensor and clean it with a contact cleaner or a mass air flow cleaner.

After that, check the exhaust necks, manifolds, air-fuel ratio sensors, oxygen sensors, and pipes for leaks. If you notice any leaks on any of these components, fix them before proceeding to the next step.

Step 4: Examine the fuel pressure systems

A common cause of P0171 on Chevy Cruze and other cars is limited fuel supply. As reiterated, fuel supply issues are from the lousy fuel pump, clogged fuel filter, bad fuel regulator, and fuel injectors.

Check these components simultaneously and ensure they are working correctly.

Step 5: Replace oxygen sensors/air-fuel ratio sensor

The code should have disappeared by now. However, if the code persists, change the oxygen sensor or the air-fuel ratio sensor.

Step 6: Replace or clean the spark plugs

After tracking and fixing the root cause of P0171, unscrew the spark plugs and inspect them. If you see white crust on the tip of the plug, replace it. However, if you see black carbon, clean the plugs and reinstall them.

Common P0171 Diagnosis Mistakes

The common P0171 diagnosis mistake among car owners and mechanics is not getting the specific repair bulletin for their car model. In any case, some error codes do not have a service bulletin. So always check, because it can save you time and money.

How Do You Fix P0171?

If your vehicle registers P0171, the first approach to fixing it is to diagnose the fault and track the culprit. You can diagnose the fault with a scan tool and by examining the related components as explained above.

The error code P0171 requires an intermediate repair approach. This means it is not a beginner repair job. Therefore, if you don’t have the confidence to handle the repair task, consult your dealership or an experienced service technician.

Some repair shops will offer you a repair warranty and fair pricing. However, you can consider letting an experienced technician do the job for you instead of wasting your time and spending money purchasing the wrong parts.

Approx. Repair Cost

The cost of repairing a P0171 fault depends on several factors like the hourly rate of the auto repair shop, your location, and the root cause of the problem. Most shops will charge you 1hr for diagnoses. Diagnoses will help the service technician to track the culprit.

The hourly rate range is $75 to $150. And, of course, you’ll have to pay for damaged parts and extra hours.

The diagnosis may track one or more of the following as the culprit and replacing them will rectify the P0171 error code.

  • Fuel pressure regulator $150 to $400
  • Mass airflow sensor $100 to $200
  • Cleaning MAF sensor $75
  • Fuel pump $500 to $1000
  • Exhaust repair is $75 to $200 (if it requires welding)
  • Vacuum leak $75 to $200.


Q: What is the system too Lean Bank 1 mean?

The diagnostic trouble code P0171 means the air-fuel mixture ratio is too lean, or there is too little fuel or too much air on the bank 1 cylinders. If you continue driving with the P0171 error code for an extended period, it will cause engine breakdown because when the engine is running lean, the temperature goes up.

Q: Can a dirty air filter cause a P0171 code?

The air filter is a crucial component that filters the airflow before it gets to the engine. If the air filter is extremely dirty or clogged, the engine pressure may cause it to collapse and suck unmeasured air into the engine.

Normally, an extremely dirty or clogged air filter should restrict airflow and cause richer fuel. Therefore, unless there is a crack or hole on the filter housing or the filter collapse due to high pressure from the engine, it will not cause a P0171 error code.

Q: Can a bad O2 sensor cause a P0171 code?

When the O2 sensor fails, it will send false readings to the car computer, and the computer will act accordingly. For instance, the oxygen sensor can fail and signal the ECM that there is too little fuel or too much air inside the combustion chamber.

Then the ECM will log the P0171 fault code and project the check engine light on the instrument cluster.

Q: Can an exhaust leak cause a P0171 code?

Exhaust leaks can throw off the o2 sensor and cause it to send false signals to the ECM. For example, it can say the engine is running lean or rich. However, exhaust leaks are more likely to cause richer fuel than lean air.

Q: Can spark plugs cause a lean condition?

Yes, spark plugs will cause a lean condition. The spark plug has no business with the air-fuel mixture, but if it fails, it’ll leave unburnt fuel in the combustion chamber, not because of too much fuel but because it lacks the strength to burn it properly.

But once the oxygen sensor notices the dumped fuel in the combustion chamber, it’ll misunderstand it and signal the car computer that there is too much fuel in the chamber. So the computer will try to compensate by reducing the air ratio, leading to a lean condition.

Q: Can a bad PCV valve cause code P0171?

Vacuum leaks or intake leaks, lousy mass airflow (MAF) sensor, low fuel pressure, and bad positive crankcase ventilation (PCV) valve are the primary causes of P0171. Of course, some other parameters can cause the powertrain control module to log P0171 on Honda and any vehicle model.

Final Words

Several issues can trigger the engine control unit (ECU) to register diagnostics trouble code P0171 and illuminate the check engine light on the dashboard.

If your vehicle logs a P0171 fault code, always check the vacuum lines and see if the air filter is collapsed before consulting your mechanic. We’ve also provided a walk-through guide above to help you fix the problem.

However, if you think the diagnoses and repair are tedious for you, drive to a nearby garage and let them fix the bank 1 lean conditions.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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