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OBD-II FAULT CODE

P0171: System too lean (Bank 1)

Fuel and airGeneric powertrain

The controller has reached a lean-correction limit on bank 1. Unmetered air, fuel delivery and measurement faults can all produce this condition.

Possible causes

Examples to investigate; this list is not a probability ranking or a diagnosis.

  • Intake or vacuum leaks
  • Insufficient fuel delivery
  • Incorrect air-flow measurement

Symptoms you might notice

  • Check-engine light
  • Rough idle, hesitation or reduced power

Diagnostic approach

  1. Save freeze-frame data and compare fuel trims at idle and higher engine speed.
  2. Inspect intake hoses and test for leaks using the specified method.
  3. Check fuel pressure and air-metering data against vehicle-specific specifications.

Save codes and freeze-frame information before clearing anything. A cleared warning light does not prove that a repair worked; the applicable monitor must run again.

Possible repairs — after testing

  • Repair a verified intake leak.
  • Restore fuel delivery or repair air-metering faults supported by test results.

Related codes

This is general diagnostic orientation, not a manufacturer-specific repair procedure. A qualified technician can confirm test conditions and safe working methods.