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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 141 (Engine Controls Self-Diagnostics - 5.7L - Bi-Fuel) >> Diagnostic Tests >> DTC P0171: Fuel Trim System Lean >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, and 2002 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
  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. Capture the DTC Information from the AF ECM into the scan tool. Inspect BOTH the AF ECM and the Powertrain Control Module (PCM) for other stored DTCs. Are other DTCs stored? If yes, go to next step. If no, go to step  4 .
  3. Perform the other DTC diagnosis before attempting diagnosis of this DTC. After repairs, go to step  12 .
  4. DTC may have been set by one of the following conditions:
    • An HO2S signal wire intermittently open or shorted to ground.
    • HO2S connector water intrusion.
    • Lean engine condition such as incorrect CNG fuel pressure, vacuum leaks, an inaccurate MAF sensor, an intake air duct leak between the MAF sensor and the throttle body, PCV system malfunction or exhaust system leaks between engine and oxygen sensor.

    Was a condition found and repaired? If yes, go to step  12 . If no, go to next step.

  5. Reconnect all disconnected or removed components. Using scan tool, clear DTCs from PCM and AF ECM. Communicate with AF ECM and reset fuel trim values. Turn ignition off for 30 seconds. Start engine and view captured data FREEZE FRAME records on scan tool. Operate vehicle within conditions to run DTC. Monitor short term fuel trim values. Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set? If yes, go to step  7 . If no, go to next step.
  6. The fault is not present. See DIAGNOSTIC AIDS  . Are any DTCs stored that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  7. Perform fuel system diagnosis. See FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Was a CNG fuel system problem found and repaired? If yes, go to step  12 . If no, go to next step.
  8. Inspect the oxygen sensor signal and low circuit for continuity between the AF ECM connector terminals and the oxygen sensor connector terminals. Inspect these circuits for shorts to other circuits or to ground. Ensure the oxygen sensor, PCM and the AF ECM connectors are disconnected before testing circuits. Repair wiring as necessary. Was a wire repair made? If yes, go to step  12 . If no, go to next step.
  9. Reconnect all disconnected or removed components. Using scan tool, clear all DTC Information from BOTH the PCM and AF ECM. Using scan tool, communicate with the PCM and reset the Fuel Trim values to zero. Turn ignition off. Remove the 20-amp CNG fuse in order to force a switchover to gasoline operation. The FIL will may not illuminate due to the removal of the CNG 20-amp fuse. Start engine. View the captured data Freeze Frame records for this DTC that was stored in step  2 . Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set. Operate the vehicle within the Conditions For Running DTC and the parameters observed when the DTC set. Observe the Short Term Fuel Trim values. Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set? If yes, go to next step. If no, go to step  11 .
  10. Repair cause for lean condition as outlined in step  4 . After repairs, go to step  12 .
  11. Replace the fuel mixer. See FUEL MIXER under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CHEVY EXPRESS & SAVANA - BI-FUEL. When complete, go to next step.
  12. Reconnect all disconnected components and connectors. Using scan tool, clear DTCs for PCM and AF ECM. Using the scan tool, communicate with the both the PCM and the AF ECM in order to reset the Fuel Trim values to zero. Start the engine. View the captured data Freeze Frame Records for this DTC that was stored in step  2 . Operate the vehicle within the Conditions For Running DTC and the parameters observed when the DTC set until the scan tool indicates the diagnostic ran. Does the scan tool indicate the DTC passed? If yes, system is okay. If no, go to step  2 .
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