WOMS
P code fuel air Medium severity

P0171 System Too Lean (Bank 1)

The PCM has detected that the long-term fuel trim on Bank 1 is adding too much fuel to compensate for an unmetered air leak or fuel delivery problem.

Definition

P0171 indicates that the fuel control system on Bank 1 has reached the upper limit of its ability to add fuel and the mixture is still lean. Either too much air is entering the engine after the MAF sensor (a vacuum leak) or not enough fuel is being delivered.

Common causes

  • Vacuum leak (intake manifold gasket, PCV, brake booster, vacuum lines)
  • Dirty or failing Mass Air Flow (MAF) sensor
  • Weak fuel pump or restricted fuel filter
  • Leaking or clogged fuel injectors
  • Failed fuel pressure regulator
  • Exhaust leak upstream of the front O2 sensor

Common misdiagnoses

  • Replacing the O2 sensor when the actual cause is a vacuum leak.
  • Cleaning the MAF without checking for downstream vacuum leaks.

Troubleshooting steps

  1. 1. Inspect for vacuum leaks

    Smoke-test the intake or use propane to find leaks while watching short-term fuel trim.

  2. 2. Check MAF sensor

    Compare MAF grams/sec at idle and 2500 RPM against the service manual spec. Clean with MAF-safe cleaner if contaminated.

  3. 3. Verify fuel pressure

    Check fuel pressure at idle and under load against spec.

  4. 4. Read fuel trims

    STFT + LTFT should sum to less than ±10% combined. Persistent positive trims point to lean condition.

Related codes