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P1171

Fuel System Lean During Acceleration.

  • GMC
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
1
AreaFuel and air metering
71
FaultThe specific fault in that area

What it means

P1171 on a GMC means the engine control module detected a lean air-fuel mixture while the engine was accelerating under load. The fuel trims could not add enough fuel to correct the mixture, or the oxygen sensors reported a lean condition beyond the allowed limit during acceleration. It usually points to a fuel delivery shortfall or unmetered air entering the engine.

Symptoms

  • Hesitation, stumble or flat spot when accelerating
  • Lack of power under load or when climbing hills
  • Engine misfire or surging on acceleration
  • Check engine light on, possibly with other lean or misfire codes
  • Rough idle or poor fuel economy
  • Hard starting in some cases

Possible causes

  • Low fuel pressure from a weak fuel pump, clogged fuel filter or failing pressure regulatorlikely
  • Vacuum leak at an intake hose, intake manifold gasket or brake servo connectionlikely
  • Dirty or faulty mass airflow sensor under-reading airflowlikely
  • Partially blocked or faulty fuel injector or injectorspossible
  • Exhaust leak upstream of the oxygen sensor letting in false airpossible
  • Faulty or slow oxygen sensor giving a false lean readingpossible
  • Fuel contaminated or of poor quality, or restricted fuel line or tank pickuprare

How to diagnose

  1. Read all stored codes and freeze frame data, noting engine load, speed, coolant temperature and fuel trim values when the code set
  2. Visually inspect the intake tract, vacuum hoses, air filter housing and wiring connectors for splits, loose clamps and damage
  3. Check live short and long term fuel trims at idle and at around 2500 rpm; a trim that improves at higher revs suggests a vacuum leak, while one that worsens under load suggests a fuel supply issue
  4. Compare the mass airflow sensor reading against expected values at idle and under load, and inspect the sensing element for contamination
  5. Test fuel pressure at idle and under load against the specification for the specific engine, and check the fuel filter if one is serviceable
  6. Smoke test the intake system to locate small air leaks
  7. Inspect the exhaust upstream of the oxygen sensors for leaks, and check oxygen sensor response on a live data graph
  8. Check injector operation and balance using the procedure given for the specific engine

How to fix

  • Replace the fuel pump, filter or pressure regulator if fuel pressure is below specification, following the procedure for the specific model
  • Repair vacuum leaks by replacing split hoses, loose clamps or leaking manifold gaskets
  • Clean or replace the mass airflow sensor as the testing indicates
  • Clean or replace faulty fuel injectors
  • Repair exhaust leaks found upstream of the oxygen sensors
  • Replace any oxygen sensor proven to be faulty, then clear the code and road test to confirm the fault does not return

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but a lean mixture can cause misfires, higher combustion temperatures and possible engine or catalyst damage. Book diagnosis soon and avoid heavy loads or hard acceleration until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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