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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 P1201: Actual Gas Flow Rate Does Not Match Desired Gas Flow Rate >> 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. Using scan tool, capture AF ECM DTC information. Check PCM and AF ECM for additional DTCs. Are other DTCs stored? If yes, go to next step. If no, go to step  4 .
  3. Diagnose these DTCs first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS  . After repairs, go to step  12 .
  4. Check vehicle for the following malfunction conditions:
    • An open or shorted O2S signal wire.
    • O2S connector water intrusion.
    • Leaking exhaust system.
    • Poor Gas Mass Sensor/Mixture Control Valve (MGS/MCV) connector terminal contact.
    • Inadequate ignition voltage or ground circuits to GMS/MCV.
    • Misrouted engine harness causing Electromagnetic Interference (EMI) on actual gas circuit.
    • Rich or lean engine condition such as EVAP canister purge system malfunction, gasoline contaminated engine oil, incorrect CNG fuel pressure, leaking gasoline fuel pressure regulator, restricted air filter or intake air duct, engine vacuum leak, or an improperly operating PCV system.

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

  5. 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. See CONDITIONS FOR SETTING DTC  . Does the scan tool indicate the DTC passed? If yes, go to next step. If no, go to step  7 .
  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. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS (CAVALIER CNG) in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM 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 for continuity to the AF ECM connector terminal. Inspect both these circuits for shorts to other circuits and the signal circuit for a short 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 components and connectors. Ensure ignition is off. Using scan tool, clear DTCs for PCM and AF ECM and reset Fuel Trim values to zero. Remove CNG fuse (20-amp) from underhood fuse block. This will cause a forced switch to gasoline operation. Start engine and view captured data freeze frame records for this DTC. Observe data parameters that indicate how vehicle was being operated when DTC set. Operate vehicle within conditions for running this DTC. See CONDITIONS FOR RUNNING DTC  . Observe short term fuel trim value. If fuel trim value indicates a large amount of fuel trim adjustment is required or if fuel trim DTCs set, go to next step. If fuel trim value does not indicate a large amount of fuel trim adjustment is required or if no fuel trim DTCs set, go to step  11 .
  10. Repair possible causes of excessive rich or lean engine operation. Refer to items in step  4 . After repairs, go to step  12 .
  11. Replace Gas Mass Sensor and Mixture Control Valve (GMS/MCV). After repairs, 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. See CONDITIONS FOR SETTING DTC  . If DTC diagnostic runs and passes, system is okay. If DTC resets, go to step  2 .
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