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Generic gasoline powertrain DTC / Fuel control

P0171: System too lean (Bank 1)

The controller reported a lean fuel-trim condition for Bank 1 under its monitoring conditions.

Generic definition
P0172
P0174
P0101
P0300

Likely cause paths · unconfirmed

A possible unmetered-air or intake leak

Air entering outside the expected path can contribute to a lean correction, but the code does not locate a leak.

  • Note whether symptoms are stronger at idle or under load.
  • Record recent intake work and any companion codes.

A possible fuel-delivery or feedback condition

Fuel supply, an exhaust leak, or a biased measurement may contribute. A trim code alone does not prove a pump, injector, or sensor failure.

  • Save freeze-frame and trim values with units and operating conditions.
  • Avoid hard acceleration or unsafe road tests.

Likely causes and repair directions are general, conditional orientation. The SAE citation supports the generic DTC definition only; it does not validate these causes, tests, repairs, or fitment to a particular vehicle.

Conditional likely repairs

A code describes a monitored condition; it does not prove a failed part. Use these directions only after confirming the cause.

If confirmed: A possible unmetered-air or intake leak

If a leak test confirms unmetered air, repair the verified leak and repeat the prescribed fuel-trim check.

Evidence to confirm

  • A smoke, leak, or other OEM-approved test locates the leak.
  • Fuel correction returns to the vehicle-specific expected range after repair.

If confirmed: A possible fuel-delivery or feedback condition

If testing confirms a fuel-delivery, exhaust, or feedback fault, repair only the verified cause; do not select parts from a trim code alone.

Evidence to confirm

  • Pressure/flow, exhaust, or signal testing identifies the cause.
  • The code and related data are reassessed after the repair.

Guided evidence checks

  1. 01

    Record what happened

    Note when the warning appeared, what the vehicle was doing, and any recent service or change. Do not deliberately recreate an unsafe symptom.

  2. 02

    Make only a safe visual check

    With the engine off and cool, look only for an obvious disconnected intake hose; do not loosen fuel lines or open the fuel system.

  3. 03

    Preserve scan-tool evidence

    Qualified technician

    Save short/long fuel-trim data, code status, and freeze-frame if available; do not apply generic thresholds.

  4. 04

    Confirm the system path

    Qualified technician

    A qualified technician should compare fuel correction by bank and operating condition, then test for air leaks, fuel delivery, and exhaust/sensor bias.

Code-specific symptom note

  • Code-specific symptoms are not verified by the cited definition. Record the driver’s actual observations separately.

This generic code reference is not an OEM procedure, does not confirm that a DTC applies to a particular make/model/engine, and is not a repair diagnosis. Follow vehicle-specific service information or consult a qualified technician.