GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2006 🢒 Terraza - 4WD 🢒 DTC 🢒 DTC P0171 or P0172

DTC Descriptors

DTC P0171: Fuel Trim System Lean Bank 1

DTC P0172: Fuel Trim System Rich Bank 1

Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit/System Description

The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open Loop and Closed Loop (CL). During Open Loop, the ECM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During CL, the ECM adds HO2S inputs and level of purge to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain an optimum air/fuel ratio. A block of memory cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of cells currently being used. The ECM selects the cells based on the engine speed and load. The FT diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition.

If the ECM detects an excessively lean condition, DTC P0171 sets. If the ECM detects an excessively rich condition, DTC P0172 sets.

Conditions for Running the DTC
    • DTCs P0030, P0036, P0053, P0054, P0068, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0120, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201-P0206, P0220, P0222, P0223, P0300, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0462, P0463, P0461, P0464, P0496, P0506, P0507, P1133, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2172, P2176, P2A00, P2A01 are not set.
    • The Loop Status parameter indicates Closed.
    • The Fuel Trim Learn is enabled.
    • The engine coolant temperature (ECT) is between -38 and +150°C (-36.4 and +302°F).
    • The intake air temperature (IAT) is between -38 and +158°C (-36.4 and +302°F).
    • The manifold absolute pressure (MAP) is between 10-255 kPa (1.5-36.9 psi).
    • The vehicle speed is less than 300 km/h (186 mph).
    • The engine speed is between 400-7,000 RPM.
    • The mass air flow (MAF) is between 1-510 g/s.
    • The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
    • The fuel level is more than 10 percent.
    • DTC P0171 or P0172 runs continuously when the above conditions have been met.

Conditions for Setting the DTC
    • The average long term FT value is more or less than a calibrated value.
    • The above condition is present for more than 3 minutes, after the conditions for running the DTC have been met.
    • The diagnostic runs continuously.

Action Taken When the DTC Sets

The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second ignition cycle the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:

    • The control module detects the same fuel trim failure during 2 consecutive trips.
    • The control module detects any fuel trim failure during any subsequent trip if the conditions at the time of failure meet the following criteria:
       - The engine load is within 20 percent of the previous test that failed.
       - The engine speed is within 375 RPM of the previous test that failed.
       - The engine coolant temperature is in the same range of the previous test that failed.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) at the beginning of the fourth ignition cycle, after 3 ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC and related Freeze Frame data clears after 40 warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Reference Information

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Emission System Information Reference

    •  Evaporative Emissions Hose Routing Diagram
    •  Crankcase Ventilation System Inspection/Diagnosis

Engine Mechanical Information Reference

    •  Symptoms - Engine Mechanical
    •  Symptoms - Engine Exhaust

Fuel System Information Reference

    •  Fuel System Diagnosis
    •  Fuel Injector Diagnosis
    •  Fuel Pump Electrical Circuit Diagnosis
    •  Alcohol/Contaminants-in-Fuel Diagnosis

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Output Controls

Circuit/System Verification

Important: 

   • Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators.
   • If any DTCs other than DTC P0171 or P0172 are also set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

    • Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter with a scan tool. The HO2S 1 value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.
       ⇒If the value does not vary from approximately 40 mV to approximately 900 mV, refer to DTC P0131, P0132, or P0134 , DTC P0133, P1133, or P2A00 , DTC P0030, P0053, or P0135 , DTC P0137, P0138, or P0140 , DTC P2A01 , or DTC P0036, P0054, or P0141 .

       Important: EVAP purge enablement may cause the FT to be momentarily outside the normal range.

    • The normal Short Term FT parameter should be moving rapidly between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.
    • The Long Term FT parameter should be moving slowly between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.

Circuit/System Testing

P0171

    • Allow the engine to reach operating temperature. With the engine running, observe the Long Term FT parameter with a scan tool. The value should be less than approximately 19.5 percent with the engine running at operating temperature.
    • If the value is not less than 19.5 percent, inspect for the following:
       - With the ignition ON and the engine OFF, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor pressure should be within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure .
     ⇒If the MAP sensor does not indicate the correct barometric pressure, refer to DTC P0106, P0107, or P0108 .
       - With the engine idling, observe the mass air flow (MAF) sensor parameter. The MAF sensor parameter should be within 3-10 g/s at idle.
     ⇒If the MAF sensor parameter is not within 3-10 g/s at idle, refer to DTC P0101 or P1101 or DTC P0102 or P0103 .
       - Vacuum hoses for splits, kinks, and proper connections
       - Sufficient fuel in the tank
       - Low fuel pressure
       - Fuel contamination
       - Properly functioning fuel injectors
       - Missing, loose, or leaking exhaust components from the HO2S forward
       - Vacuum leaks at the intake manifold, the throttle body, brake booster hose, and the injector O-rings
       - The air induction system and the air intake ducts for leaks or for a missing air filter element
       - Evaporative canister purge lines plugged or obstructed
       - The crankcase ventilation system for leaks
       - The HO2S for proper installation and for electrical wires or connectors that may have contacted the exhaust system
       - The HO2S signal circuit open, shorted to ground, or shorted to the low reference circuit
       - The HO2S sensor 1 low signal circuit for an open circuit or high resistance
       - Malfunctioning engine components

P0172

    • Allow the engine to reach operating temperature. With the engine running, observe the affected Long Term FT parameter with a scan tool. The value should be more than -25 percent with the engine running at operating temperature.
    • If the value is not more than -25 percent, inspect for the following:
       - With the engine idling and the transmission in the Park or Neutral position, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor parameter should be between 19-42 kPa.
     ⇒If the MAP sensor parameter is not between 19-42 kPa, refer to DTC P0106, P0107, or P0108 .
       - With the engine idling, observe the mass air flow (MAF) sensor parameter. The MAF sensor parameter should be within 3-10 g/s at idle.
     ⇒If the MAF sensor parameter is not within 3-10 g/s at idle, refer to DTC P0101 or P1101 or DTC P0102 or P0103 .
       - Vacuum hoses for splits, kinks, and proper connections
       - The inlet screen of the MAF sensor for blockage
       - A malfunctioning MAF--Refer to DTC P0068 or P0121 or DTC P0101 or P1101 .
       - The air duct for being collapsed or restricted
       - The air filter for being dirty or restricted
       - Objects blocking the throttle body
       - Excessive fuel in the crankcase
       - A saturated evaporative canister, a stuck purge valve, or the fuel cap not sealing
       - The evaporative emissions control system for proper operation
       - Excessive fuel pressure
       - Properly functioning fuel injectors
       - Fuel contamination
       - The HO2S for proper installation and for electrical wires or connectors that may have contacted the exhaust system
       - The HO2S signal circuit shorted to voltage

Repair Instructions
    •  Air Cleaner Assembly Replacement
    •  Air Cleaner Inlet Duct Replacement
    •  Evaporative Emission Canister Replacement
    •  Evaporative Emission Canister Purge Solenoid Valve Replacement
    •  Evaporative Emission Canister Vent Solenoid Valve Replacement
    •  Evaporative Emission Hoses/Pipes Replacement - Chassis/Canister
    •  Evaporative Emission Hoses/Pipes Replacement - Chassis/Canister
    •  Fuel Hose/Pipes Replacement - Chassis
    •  Fuel Injector and Fuel Rail Replacement
    •  Fuel Level Sensor Replacement
    •  Fuel System Cleaning
    •  Fuel Tank Draining
    •  Heated Oxygen Sensor 1 Replacement and Heated Oxygen Sensor 2 Replacement
    •  Manifold Absolute Pressure Sensor Replacement
    •  Mass Airflow Sensor/Intake Air Temperature Sensor Replacement
    •  Throttle Body Assembly Replacement

Repair Verification

    Important: 

       • After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim values.
       • In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the Conditions for Running and Setting DTC P0420, and verify that DTC P0420 runs and passes. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .

  1. Install any components that have been removed or replaced during diagnosis.
  2. Perform any adjustments, programming, or setup procedures that are required when a component or module is removed or replaced.
  3. Turn ON the ignition, with the engine OFF.

    4. Important: DO NOT clear codes with the engine running. DTCs may reset in the same ignition cycle.

  4. Clear the DTCs.
  5. Turn OFF the ignition for 60 seconds.
  6. Start the engine.
  7. Duplicate the Conditions for Running the DTC and use Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to the Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.