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Home >> GMC >> 2018 >> Terrain Denali, FWD >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 12 (Engine Controls And Fuel - 1.5L (LYX) - Diagnostic Information And Procedures (P0010 - P034B)) >> Diagnostic Information And Procedures >> DTC P0171 Or P0172: Fuel Trim System >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different variant/trim than selected.
  1. Ignition ON.
  2. Verify the manifold absolute pressure sensor is within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure
    • If not within the specified range 

      Refer to DTC P0106 

    • Go to next step: If within the specified range 
  3. Engine running at the operating temperature.
  4. Verify the Long Term Fuel Trim parameter is between -10 % and +10 %.
    • If greater than +10 % 
    1. Ignition OFF.
    2. Inspect for the conditions listed below:
      • Mass air flow (MAF) sensor signal skewed - If the Short Term Fuel Trim parameter changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101 .
      • Fuel contamination - Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
      • Missing, restricted, or leaking exhaust components - Refer to Symptoms - Engine Exhaust .
      • Malfunctioning fuel injectors - Refer to Fuel Injector Diagnosis .
      • Vacuum hoses for splits, kinks, and improper connections
      • Insufficient fuel in the tank
      • Low fuel pressure - Refer to Fuel System Diagnosis .
      • Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings.
      • Leaks in the air induction system and the air intake ducts
      • Leaks in the components of the vacuum operated engine mount system
      • Missing air filter element
      • Cracked EVAP canister
      • Evaporative pipes obstructed or leaking
      • The crankcase ventilation system for leaks - Refer to Crankcase Ventilation System Inspection/Diagnosis .
        • If a condition is found, repair as necessary.
        • If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .
    • If less than -10 % 
    1. Ignition OFF.
    2. Inspect for the conditions listed below:
      • Mass air flow (MAF) sensor signal skewed - If the Short Term Fuel Trim parameters for both banks changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101 .
      • Fuel contamination - Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
      • Missing, restricted, or leaking exhaust components - Refer to Symptoms - Engine Exhaust .
      • Malfunctioning fuel injectors - Refer to Fuel Injector Diagnosis
      • Collapsed or restricted air intake duct
      • Excessive fuel pressure - Refer to Fuel System Diagnosis
      • Dirty or restricted air filter
      • Objects blocking the throttle body
      • Improper operation of the evaporative emissions control system
      • Excessive fuel in the crankcase - Change the engine oil as necessary.
    • If a condition is found, repair as necessary.
    • If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .
    • Go to next step: If between -10 % and +10 % 
  5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed in the Freeze Frame/Failure Records data.
  6. Verify DTC P0171 or P0172 is not set.
    • If any of the DTCs set 

      Refer to conditions listed above.

    • Go to next step: If none of the DTCs set 
  7. All OK.
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