GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 DTS 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 DTC P0171, P0172, P0174, or P0175

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0171: Fuel Trim System Lean Bank 1

DTC P0172: Fuel Trim System Rich Bank 1

DTC P0174: Fuel Trim System Lean Bank 2

DTC P0175: Fuel Trim System Rich Bank 2

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 or P0174 sets. If the ECM detects an excessively rich condition, DTC P0172 or P0175 sets.

Conditions for Running the DTC
    • DTCs P0050, P0056, P0059, P0060, P0068, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0120, 0122, P0123, P0128, P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0201-P0208, P0220, P0222, P0223, P0300, P0301-P0308, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P1153, P1516, P2101, P2119, P2120, P2125, P2135, P2138, P2A03, P2A04 are not set.
    • The engine is in CL status.
    • 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 +150°C (-36.4 and +302°F).
    • The manifold absolute pressure (MAP) is between 5-255 kPa (0.7-36.9 psi).
    • The vehicle speed is less than 134 km/h (83 mph).
    • The engine speed is between 400-6,000 RPM.
    • The mass air flow (MAF) is between 0.5-510 g/s.
    • The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
    • The fuel level is more than 10 percent.
    • DTC P0174 or P0175 runs continuously when the above conditions have been met.

Conditions for Setting the DTC
    • The long term FT weighted average value is more or less than a calibrated value.
    • The above condition is present for approximately 3 minutes after the Conditions for Running the DTC have been met.

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

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

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.

  1. Verify that other DTCs are not set.
    2. ⇒If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter with a scan tool. The HO2S value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.
    3. ⇒If the value does not vary from approximately 40 mV to approximately 900 mV, refer to DTC P0131, P0132, P0137, or P0138 , DTC P0133, P0134, P0140, P1133, P2270, P2271, P2A00, or P2A01 , or DTC P0153, P0154, P1153, or P2A03 .

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

  3. The normal Short Term FT parameter should be between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.
  4. The Long Term FT parameter should be between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.
  5. Engine running, observe the DTC information with a scan tool. DTCs P0171, P0172, P0174 or P0175 should not set.
  6. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame Records data.

Circuit/System Testing

P0171 or P0174

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 less than approximately 24 percent with the engine running at operating temperature.

⇒If the value is more than 24 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 and DTC P0107 or P0108 .
    • Vacuum hoses for splits, kinks, and improper connections--Refer to Evaporative Emissions Hose Routing Diagram .
    • Insufficient fuel in the tank
    • A malfunctioning MAF--Refer to DTC P0068 or P0121 or DTC P0101 or P1101 .
    • Low fuel pressure--Refer to Fuel System Diagnosis .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Malfunctioning fuel injectors--Refer to Fuel Injector Diagnosis .
    • Missing, loose, or leaking exhaust components from the HO2S forward--Refer to Symptoms - Engine Exhaust .
    • Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings
    • The air induction system and the air intake ducts for leaks or for a missing air filter element
    • A cracked or damaged EVAP canister
    • Evaporative canister purge lines plugged or obstructed
    • The crankcase ventilation system for leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • The HO2S for improper 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--Refer to Symptoms - Engine Mechanical .

P0172 or P0175

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 approximately -23 percent with the engine running at operating temperature.

⇒If the value is less than -23 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 and DTC P0107 or P0108 .
    • Vacuum hoses for splits, kinks, and improper connections--Refer to Evaporative Emissions Hose Routing Diagram .
    • 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--Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • The evaporative emissions control system for improper operation
    • Excessive fuel pressure--Refer to Fuel System Diagnosis .
    • Improperly functioning fuel injectors--Refer to Fuel Injector Diagnosis .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • 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 Element Replacement
    •  Evaporative Emission Canister Replacement
    •  Evaporative Emission Canister Purge Solenoid Valve Replacement
    •  Evaporative Emission Hoses/Pipes Replacement - Engine/Chassis
    •  Fuel Injector Replacement
    •  Fuel Injection Fuel Rail Assembly Replacement
    •  Fuel Level Sensor Replacement
    •  Fuel System Description
    •  Fuel Tank Draining
    •  Heated Oxygen Sensor Replacement - Bank 1 Sensor 1
    •  Heated Oxygen Sensor Replacement - Bank 1 Sensor 2
    •  Manifold Absolute Pressure 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 or P0430 and verify that DTC P0420 or P0430 runs and passes.

  1. Install any components or connectors 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.