P0171 Code: Symptoms and Quick Fixes for Your Car

Key Takeaways:

  • The P0171 code indicates the engine is running too lean (too much air, not enough fuel)
  • Common symptoms include rough idle, lack of power, engine misfires
  • Vacuum leaks, faulty fuel pump, clogged injectors can cause the P0171 code
  • Check the vacuum system, fuel pressure, and oxygen sensors to diagnose
  • Repairing vacuum leaks, cleaning injectors, or replacing faulty components fixes code P0171
  • Driving with the check engine light on can lead to further engine damage

The dreaded check engine light appearing on your dashboard is never a welcome sight for drivers. That little glowing icon means something is amiss under the hood. Diagnostic trouble code P0171 is one of the more common reasons for the check engine light.

This code points to an issue with the air-fuel ratio – specifically, too much air and not enough fuel, aka a “lean” mixture. Engineering your engine to run optimally lean can boost fuel efficiency. However, an unintended lean condition can seriously impact drivability and lead to engine overheating or even failure if left unchecked.

Properly diagnosing and repairing the root cause of a P0171 code is crucial.

Symptoms of P0171 Code

The check engine light coming on is the main indicator that your vehicle’s computer has detected an issue triggering code P0171. However, some other symptoms may accompany this code:

Rough Idle

With the air/fuel mixture out of whack, you may notice the engine idling rougher than normal when stopped at a light or pulling into your driveway.

Lack of Power

Trying to accelerate for merging onto the highway or navigating an incline, you may find the engine lacks its normal pep. This lackluster performance occurs because the engine can’t produce optimal combustion.

Engine Misfires

As the spark plug struggles to ignite the lean mixture, you may feel the engine misfiring or sputtering as you drive. This sensation may be barely noticeable or more pronounced depending on the severity of the fuel starvation.

“Coughing” Sounds

In conjunction with rough idling, your engine may produce odd coughing or choking sounds. This noise points to the lean condition disrupting the proper firing of cylinders.

If you don’t notice any drivability issues, it’s possible a sensor fault triggered the code erroneously. However, it’s still vital to diagnose P0171 even without driving symptoms. The oxygen sensors detecting a lean condition likely points to an underlying issue needing resolution.

What Causes Code P0171?

Now let’s explore the most common culprits behind those dreaded P0171 lean codes:

Vacuum Leaks

The engine vacuum system provides suction to properly meter and distribute air. Cracks or damages in these vacuum lines allow additional air to enter, creating an overly lean mixture.

Potential leak points include the PCV valve hoses, brake booster line, evaporative emissions hoses, intake manifold gasket, and any other vacuum lines snaking around the engine. Even a minuscule leak or small hole can be enough to trigger code P0171.

Weak Fuel Pump

To balance the air intake, sufficient fuel must reach the combustion chambers. Issues with the fuel pump or fuel pressure deliver inadequate gasoline, resulting in a lean mix.

As the pump ages and wear increases, the pump mechanism provides less pressure. Contaminated fuel or sediment accumulation can also hinder the pump and fuel injectors from delivering their proper supply.

Clogged Fuel Injectors

Dirty fuel injectors become obstructed and cannot spray the proper amount of gasoline into the engine. Your vehicle may need professional cleaning or replacement of the fuel injectors if they become sufficiently clogged to cause a lean code.

Faulty Fuel Pressure Regulator

This component maintains the proper fuel pressure feeding into the injectors. A failed regulator can leak fuel back into the tank or supply too little pressure, starving the engine of sufficient gasoline.

Damaged Oxygen Sensors

The oxygen sensors monitor the exhaust to provide feedback about the air/fuel ratio. Faulty O2 sensors may incorrectly report the engine is running lean when no underlying issue exists.

Before condemning the sensors, you’ll want to rule out true vacuum leaks or fuel delivery problems first. But degraded oxygen sensors can be the cause of a misleading P0171 code.

Is Code P0171 Serious?

Seeing your check engine light illuminated is always cause for concern and means you should limit driving the vehicle. Continuously operating an engine too lean risks overheating and catastrophic engine failure.

At minimum, the lean mixture lowers combustion efficiency resulting in sluggish performance and wasted fuel. But sustained high engine temperatures can crack heads and piston tops or cause melted catalytic converters.

Diagnosing and repairing the root cause of P0171 codes promptly reduces the chances of incurring major engine damage down the road. While the code alone may not produce immediate drivability issues, ignoring the problem can lead to a breakdown, so repairs should not be delayed.

How to Fix Code P0171:

Now let’s review the steps for properly diagnosing and repairing code P0171:

Step 1 – Scan for Codes

Connect an OBD2 scanner to pull any diagnostic trouble codes (DTCs). Make sure to clear any other pending codes unrelated to system running lean. The other faults may be contributing to the P0171 or masking the true root cause.

Step 2 – Check for Vacuum Leaks

Visually inspect all vacuum hoses, lines, the intake manifold gasket, and other potential leak points around the engine. Repair any cracked, loose, or detached hoses. Spraying carburetor cleaner near suspected leak areas and listening for changes in engine speed can help identify smaller leaks.

Step 3 – Check Fuel Pressure

Using a fuel pressure gauge is the best way to test that fuel delivery falls within the specified range. Consult your vehicle repair manual for the required pressure specifications.

Abnormally low pressure indicates worn fuel pump components or leaky injectors unable to maintain pressure. No fluctuation points to a problematic fuel pressure regulator.

Step 4 – Check Fuel Components

Remove and inspect the fuel filter for any excessive debris or sediment buildup hindering fuel flow. The fuel injectors may also need professional cleaning if they’ve become sufficiently clogged.

While inspecting fuel system components, check for any damaged fuel line connections that could introduce air leaks.

Step 5 – Monitor Oxygen Sensor Readings

Use a live data scanner to view the oxygen sensor voltage outputs while idling and at various engine speeds and loads. The switching between lean and rich should occur in a regular pattern several times per second.

Erratic data fluctuations or a sensor stuck reading lean likely indicates a bad oxygen sensor requiring replacement.

Step 6 – Repair Vacuum Leak

If a vacuum leak is found, properly reattach any loose hoses or replace cracked, brittle lines. Replace the intake manifold gasket or PCV valve if they are the source of vacuum loss.

Step 7 – Replace Faulty Components

Should diagnostic testing reveal issues with the fuel pump, injectors, fuel pressure regulator or oxygen sensors, replace the malfunctioning part(s).

Use OEM or high quality aftermarket components, as low quality parts often fail prematurely or introduce new problems. Proper repairs should resolve the P0171 lean code.


A P0171 lean code appearing alongside your check engine light should not be ignored. Driving with this code risks degraded performance, poor fuel economy, and potential engine damage over time if left unaddressed.

Following the diagnostic steps outlined and repairing any underlying issues provides the best assurance of resolving the problem for good. Paying attention to this warning light and code now prevents more headaches and repair bills down the road.

Frequently Asked Questions:

What if I reset the code P0171 and the check engine light comes back on?

If the light reappears, this typically means the underlying problem was not properly fixed. You’ll need to double check for vacuum leaks, fuel pressure, dirty injectors, etc per the troubleshooting steps. Don’t just reset the code and hope the issue resolves itself.

Are there any quick fixes to try for P0171?

Unfortunately no – a proper diagnosis is required to pinpoint if it is a leak, fuel delivery problem, sensor fault, etc. Replacing a sensor without confirming it’s bad is just guessing and probably won’t actually fix the root issue.

How much does it cost to fix code P0171?

Cost depends entirely on the cause. Resealing a small vacuum leak could be a cheap fix. But replacing a fuel pump or multiple clogged injectors can run $500+. Diagnostic testing at a shop usually runs $100-150 to determine the root cause.

Can I drive with the P0171 code?

You should limit driving as much as possible. Technically you can still drive short distances and your car will not immediately break down. But this risks further engine damage from sustained overheating, so repairs should be done promptly.

Is it safe to use engine cleaners or fuel additives to fix P0171?

Fuel additives likely won’t resolve a true lean air/fuel ratio condition. Diagnosing the true problem is the only sure fix. Some cleaners do help provide temporary benefits but no substitute for repairing underlying issues.

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