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P0171 — Fuel System Too Lean (Bank 1)

P0171 is set when the engine control module detects that long-term fuel trim on Bank 1 has been pushed beyond approximately +25 % to compensate for what it interprets as a lean air-fuel mixture. In other words, the ECM is adding the maximum amount of fuel it is allowed to, and the oxygen sensors still report a lean condition. The cause is almost always either unmetered air entering the engine, a fuel delivery problem, or a sensor reporting bad data.

P0171 means fuel system too lean (bank 1). A vehicle usually stays drivable short-term with this code, but it should be diagnosed promptly. The most common cause is vacuum leak (intake manifold gasket, pcv hose, intake boot, brake booster hose) (typically $80–$600). Causes and cost vary by make and model; confirm the root cause before replacing parts.

Severity: moderate powertrain Safe to drive (short term)

Reviewed by MECH AI Editorial · Last verified

What does P0171 mean?

P0171 is set when the engine control module detects that long-term fuel trim on Bank 1 has been pushed beyond approximately +25 % to compensate for what it interprets as a lean air-fuel mixture. In other words, the ECM is adding the maximum amount of fuel it is allowed to, and the oxygen sensors still report a lean condition. The cause is almost always either unmetered air entering the engine, a fuel delivery problem, or a sensor reporting bad data.

What are the symptoms of P0171?

What causes P0171?

Cause Likelihood Estimated repair (USD)
Vacuum leak (intake manifold gasket, PCV hose, intake boot, brake booster hose) — The single most common P0171 cause across all makes. Most common $80–$600
Dirty or failing mass airflow (MAF) sensor — Cleaning with MAF-safe spray solves a large fraction of cases. Most common $30–$350
Weak fuel pump or clogged fuel filter (low fuel pressure) Common $80–$900
Failing fuel pressure regulator Occasional $100–$400
Clogged or leaking fuel injectors on Bank 1 Occasional $150–$1,200
Faulty oxygen sensor reporting false lean Occasional $150–$450
Exhaust leak upstream of the front O2 sensor Occasional $100–$500

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0171?

In most cases a vehicle stays drivable for short trips with P0171 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0171

  1. Read fuel trims and short-term/long-term values

    Connect a scan tool capable of live data. Compare short-term (STFT) and long-term (LTFT) fuel trims at idle and at 2500 RPM. If LTFT is +15 % or higher at idle but drops near zero at 2500 RPM, suspect a vacuum leak. If LTFT is high at all RPMs, suspect fuel delivery or MAF.

    Tools: Bidirectional scan tool with live PIDs

  2. Smoke-test the intake for vacuum leaks

    Cap the intake and introduce low-pressure smoke into the manifold. Watch for smoke escaping at intake gaskets, the PCV system, the brake booster hose, vacuum tees, and the throttle body base. Cracked plastic intake manifolds are common on many Ford, Chrysler, and Nissan engines.

    Tools: Smoke machine, Inspection mirror, Flashlight

  3. Inspect and clean the MAF sensor

    Remove the MAF sensor and spray the sensing elements with MAF-safe electronics cleaner. Reinstall, clear the code, and drive. If P0171 returns within a few drive cycles with high LTFT at cruise, the MAF may need replacement.

    Tools: MAF-safe cleaner, Trim panel tools

  4. Measure fuel pressure key-on-engine-off and at idle

    Attach a fuel pressure gauge to the fuel rail Schrader port (or use a banjo adapter on returnless systems). Compare to the OEM specification for the vehicle — most port-injected systems run 45–60 psi at idle. Pressure that drops more than 5 psi under load points to a weak pump or regulator.

    Tools: Fuel pressure gauge, Vehicle-specific service data

  5. Compare upstream O2 sensor activity to mode 6 data

    A lazy or biased O2 sensor will set P0171 even when the engine is running correctly. Graph the sensor voltage at 2500 RPM — it should switch rapidly between roughly 0.1 V and 0.9 V. If it sticks low, the sensor (not the fuel system) is the problem.

    Tools: Scan tool with graphing

How do I fix P0171?

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P0171: frequently asked questions

What does diagnostic trouble code P0171 mean?

P0171 is set when the engine control module detects that long-term fuel trim on Bank 1 has been pushed beyond approximately +25 % to compensate for what it interprets as a lean air-fuel mixture. In other words, the ECM is adding the maximum amount of fuel it is allowed to, and the oxygen sensors still report a lean condition. The cause is almost always either unmetered air entering the engine, a fuel delivery problem, or a sensor reporting bad data.

What are the symptoms of P0171?

Check Engine Light is illuminated. Rough or unstable idle, especially when cold. Hesitation, stumble, or stall when accelerating from a stop. Reduced fuel economy. Mild loss of power. Whistling or hissing sound from the intake area (vacuum leak)

What causes P0171?

Vacuum leak (intake manifold gasket, PCV hose, intake boot, brake booster hose) (most-common). Dirty or failing mass airflow (MAF) sensor (most-common). Weak fuel pump or clogged fuel filter (low fuel pressure) (common). Failing fuel pressure regulator (occasional). Clogged or leaking fuel injectors on Bank 1 (occasional). Faulty oxygen sensor reporting false lean (occasional). Exhaust leak upstream of the front O2 sensor (occasional)

Is it safe to drive with P0171?

In most cases a vehicle stays drivable for short trips with P0171 active, but it should be diagnosed and repaired promptly — this is a moderate-severity code. Ignoring it can lead to further damage or a failed emissions test. Specific tolerance varies by make and model.