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P0171Cars & SUVs
Moderate

P0171 — System too lean (Bank 1)

The ECM sees an air-fuel ratio that is too lean on Bank 1—too much air or not enough fuel. That can cause misfires, hesitant acceleration, and a check engine light.

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Likely causes

  • Vacuum leak at the intake manifold or cracked PCV hoses
  • Dirty or failing mass air flow (MAF) sensor
  • Weak fuel pump or clogged injectors

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Recommended checks

  1. 1

    Inspect intake hoses and the PCV system for leaks

  2. 2

    Clean or replace the MAF sensor

  3. 3

    Measure fuel pressure and test the fuel pump

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Parts to replace