GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2007 🢒 G3 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P0171 or P0172

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 provide an overview of each diagnostic category.

DTC Descriptors

DTC P0171:  Fuel Trim System Lean

DTC P0172:  Fuel Trim System Rich

Circuit Description

The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load.

Conditions for Running the DTC
    • DTCs P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0335, P0336, P0337, P0341, P0342, P0444, and P0445 are not set.
    • The fuel system is in closed loop.
    • The engine is operating.
    • The engine coolant temperature is greater than 20°C (68°F).
    • The DTCs run continuously once the above conditions are met.

Conditions for Setting the DTC

P0171

The average of the short term fuel trim values plus adaptive fuel trim is greater than 28 percent during 60 seconds of the 300 second test period.

P0172

The average of the short term fuel trim values plus adaptive fuel trim is less than -25 percent during 90 seconds of the 300 second test period.

Action Taken When the DTC Sets

DTC P0171 and P0172 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTC P0171 and P0172 are Type B DTCs.

Diagnostic Aids

P0171

    • The system will become lean if a fuel injector is not supplying enough fuel.
    • A lean condition could be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe.

P0172

    • The system will become rich if a fuel injector is supplying too much fuel.
    • Review the Freeze Frame/Failure Records with a scan tool.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

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

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification
  1. If there are other DTCs set besides DTC P0171 or P0172, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnosis the applicable DTC.
  2. Ignition ON, observe the manifold absolute pressure (MAP) sensor pressure parameter and compare the reading on the scan tool to the barometric pressure table. Refer to Altitude Versus Barometric Pressure .
    3. ⇒If the MAP sensor pressure is not within the range specified for your altitude, refer to DTC P0107 or P0108 .
  3. Engine running, observe the TP sensor percentage parameter. The reading should be less than 10 percent.
    4. ⇒If greater than 10 percent, refer to DTC P0122, P0123, P0222, or P0223 .
  4. Engine running, observe the HO2S sensor voltage parameter. The reading should fluctuate within 200-800 mV.
    5. ⇒If out of the specified range, refer to DTC P0131 or P0137 and DTC P0132 or P0138 .
  5. With the engine at normal operating temperature, observe the Long Term FT and Short Term FT parameters. The accumulated value of the FT parameters should be between -25 to +25 percent.
  6. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

Circuit/System Testing
  1. Verify the following conditions are not present:
    2. • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Splits, kinks, or improper connections at the vacuum hoses
    • The fuel system is operating lean or rich--Refer to Fuel System Diagnosis .
    • The fuel injectors are operating lean or rich--Refer to Fuel Injector Solenoid Coil Test .
    • Missing, restricted, or leaking exhaust components--Refer to Symptoms - Engine Exhaust .

    P0171

       • Vacuum leaks at the intake manifold, throttle body, and injector O-rings
       • Leaking crankcase ventilation system

    P0172

       • The air intake duct for being collapsed or restricted possibly by a restricted air filter
       • Leaking fuel pressure regulator--Inspect for evidence of fuel in the vacuum line--Refer to Fuel System Diagnosis
       • Excessive fuel in the crankcase
       • The incorrect operation of the evaporative emissions control system
  2. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Fuel Pump Replacement
    •  Fuel Injection Fuel Rail Assembly Replacement