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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 106 (Engine Controls Self-Diagnostics - 2.2L CNG) >> Diagnostic Tests >> DTC P0171: Fuel Trim System Lean >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2002 Chevrolet Cavalier. 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 O2S signal wire intermittently open or shorted to ground.
    • O2S connector water intrusion.
    • Lean engine condition such as incorrect CNG fuel pressure, vacuum leaks, PCV system malfunction or exhaust system leaks between engine and oxygen sensor.

    Repair as necessary. After repairs, go to step  12 . If condition is not found and repaired, 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, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, refer to INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  7. Perform fuel system diagnosis then return to this test step. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS (CAVALIER CNG) in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article. If fuel system problem is found and repaired, go to step  12 . If fuel system problem is not found and repaired, 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 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 this DTC and the parameters observed when the DTC set. See CONDITIONS FOR RUNNING DTC  . Observe the Short Term Fuel Trim values. Does 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 Gas Mass Sensor/Mixture Control Valve (GMS/MCV). When complete, go to next step.
  12. Reconnect all disconnected components and connectors. Using scan tool, clear DTCs for PCM and AF ECM. Start engine and allow to idle until engine reaches normal operating temperature. Select DTC and specific DTC function. Enter DTC number and operate vehicle, within conditions for setting this DTC, until scan tool indicates diagnostic has run. If DTC diagnostic runs and passes, system is okay. If DTC resets, go to step  2 .
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