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Home › Buick › 2003 › Century Base › Repair and Diagnosis › Engine Performance › System › Engine Controls Self-Diagnostics - 3.1L Engine › Diagnostic Tests › DTC P0171: Fuel Trim System Lean › Diagnostic Procedure

Diagnostic Procedure

  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
    2. NOTE: If any DTCs are set, except P0171, refer to those DTCs before proceeding with this test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  2. Connect scan tool. Start and idle the engine at the normal operating temperature in Closed Loop. Record the long term fuel trim data. Turn ignition switch to OFF position. Turn ignition switch to RUN position, engine OFF. Review the Freeze Frame/Failure Records and record the displayed data for this DTC. Does the scan tool indicate that the long term fuel trim is greater than 23 percent? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
  3. Operate the engine at idle. Using scan tool, observe the HO2S parameters. Does the scan tool indicate that the values are within 200-800 mV and fluctuating? If yes, go to next step. If no, go to step  5.
  4. Turn engine off. Visually and physically inspect the following:
    • Vacuum hoses for splits, kinks, and proper connections.
    • Fuel pressure. If fuel pressure is too low this DTC may set. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 3.1L CENTURY, GRAND PRIX & MALIBU article.
    • Check for fuel contamination.

    Did you find and correct the condition? If yes, go to step  7. If no, go to step  6.

  5. Turn ignition switch to OFF position. Check for the following conditions:
    • Check the HO2S for proper installation.
    • Ensure that the electrical connectors and the wires are secure and not contacting the exhaust system.
    • Test for continuity between the signal circuit and the low reference circuit.

    Did you find and correct the condition? If yes, go to step  7. If no, go to FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 3.1L CENTURY, GRAND PRIX & MALIBU article.

  6. Operate the engine at idle. Check for the following conditions:
    • Check for missing, loose, or leaking exhaust components forward of the HO2S.
    • Check for vacuum leaks at the intake manifold, the throttle body and the injector "O" rings.
    • Check the air induction system and the air intake ducts for leaks.
    • Check the Secondary Air Injection (AIR) system for leaks, improper air delivery, and for the shut-off valve not closing.
    • Check the crankcase ventilation system for leaks.
    Did you find and correct the condition? If yes, go to next step. If no, go to appropriate article in ENGINES.
    7. NOTE: After repairs, use scan tool Fuel Trim Reset function to reset the Long Term Fuel Trim.
  7. Using the scan tool, clear the DTCs. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running Test. See CONDITIONS FOR RUNNING TEST . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  8. Using the scan tool, observe the stored information in CAPTURE INFO. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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