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Home >> Chevrolet >> 2001 >> Venture Base, Van Passenger >> Repair and Diagnosis >> External Pages >> Different car >> Section 71 (Engine Controls Self-Diagnostics - 2.2L CNG) >> Diagnostic Tests >> DTC P1202: Actual Gas Flow Signal Circuit Frequency Low >> 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. See DIAGNOSTIC TROUBLE CODE DEFINITIONS  . After repairs, go to step  25 .
  4. Establish communication with the AF ECM using a scan tool. Observe the Engine Speed data parameter while cranking the engine. Is engine speed indicated while the engine is cranking? If yes, go to next step. If no, go to step  23 .
  5. Clear the AF ECM DTC Information using the scan tool. Turn ignition off and wait 30 seconds. Start engine. View the captured data Freeze Frame records for this DTC. 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 Setting this DTC and the parameters observed when the DTC set, until the scan tool indicates the diagnostic Ran. See CONDITIONS FOR SETTING DTC  . Does the scan tool indicate the diagnostic Passed? If yes, problem is intermittent. Refer to INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
  6. Turn ignition off. Visually inspect the Gas Mass Sensor/Mixture Control Valve (GMS/MCV) connector and motor connector for proper terminal contact. Test for circuit continuity of the motor power and ground circuits between the two connectors. Repair terminal contact or wiring as necessary. Was a terminal or wire repair necessary? If yes, go to step  25 . If no, go to next step.
  7. Disconnect the GMS/MCV connector. Turn ignition on, with engine off. Connect the J-39200 Digital Multimeter (DMM) test leads between the GMS/MCV ignition circuit terminal and a known-good ground. Measure the voltage on the ignition circuit. Is voltage within 11-13 volts? If yes, go to next step. If no, go to step  15 .
  8. Connect the J-34142-B 12-Volt Unpowered Test Light between battery positive and the 2 GMS/MCV ground circuit terminals one at a time. Does the test light illuminate brightly when connected to each ground circuit terminal? If yes, go to next step. If no, go to step  16 .
  9. Connect DMM test leads between the GMS/MCV ACTUAL GAS circuit terminal and a known-good ground. Measure the voltage on the actual gas circuit. Is the voltage about 5 volts? If yes, go to next step. If no, go to step  17 .
  10. With ignition on, with engine off, connect the J-34142-B between battery positive and the GMS/MCV desired gas circuit terminal. Is the test light illuminated? If yes, go to next step. If no, go to step  18 .
  11. While monitoring the test light, Turn ignition off. Did the test light turn OFF? If yes, go to next step. If no, go to step  19 .
  12. Reconnect all disconnected or removed components, connectors, fuses, etc. Using scan tool, clear all DTC Information from the AF ECM. Turn ignition off. Connect the J-39970-100 Fuel Test Port Adapter to the underhood CNG fuel line test port. Connect the J-39200 to J-39970 Electronic Pressure Gauge. Connect the J-39970 to J-39970-100. Monitor the fuel pressure as you crank and Start engine. Was the observed fuel pressure ever greater than 115? If yes, go to next step. If no, diagnose fuel pressure. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS (CAVALIER CNG) in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article.
  13. Turn ignition off. Remove fuel test port adapter. Remove fuel pump relay from underhood fuse block. Turn ignition on. Using scan tool, clear AF ECM DTCs. Using scan tool, observe NGO ENABLE INPUT parameter and ensure alternative fuel is enabled. An ignition cycle with a 30 second OFF time may be required to enable alternative fuel after clearing DTCs. Using DVOM connected to ground, backprobe GMS connector terminal No. 7 (Dark Blue wire). Set DVOM to AC volts to observe frequency. Crank engine while observing frequency for approximately 5 seconds or until a switchover to gasoline is observed. Engine will not start with fuel pump relay removed. If frequency is more than 750 Hz, go to next step. If frequency is less than 750 Hz, go to step  20 .
  14. Keep the GASOLINE fuel pump relay removed in order to prevent the engine from starting. Backprobe the GMS/MCV connector ACTUAL GAS circuit terminal with the J-39200 DMM. Set DMM to DC Volts in order to observe frequency. While observing the DMM frequency crank the engine for about 5 seconds or until a switchover to gasoline is observed. The engine will not start with the fuel pump relay removed. Is the maximum observed frequency greater than 250 Hz? If yes, go to step  24 . If no, go to step  22 .
  15. Turn ignition off and wait 30 seconds. Observe the Fuel Indicator Light (FIL), located in fuel gauge select switch, as you turn ignition on, with engine off. Did the FIL illuminate for a 2 second bulb check or stay ON? If yes, go to step  21 . If no, diagnose ignition relay. See IGNITION RELAY DIAGNOSIS (CAVALIER - CNG) under IGNITION SYSTEM in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article.
  16. Repair the GMS/MCV ground circuit with high or infinite (open) resistance. After repairs, go to step  25 .
  17. Inspect the actual gas circuit for an open, short to ground or short to voltage. Inspect for proper terminal contact at the AF ECM connector actual gas circuit terminal. Repair terminal contact or wiring as necessary. Was a terminal or wire repair necessary? If yes, go to step  25 . If no, go to step  24 .
  18. Inspect the desired gas circuit for an open, or short to voltage. Inspect for proper terminal contact at the AF ECM connector desired gas circuit terminal. Repair terminal contact or wiring as necessary. Was a terminal or wire repair necessary? If yes, go to step  25 . If no, go to step  24 .
  19. Inspect the desired gas circuit for a short to ground. Repair wiring as necessary. Was a wire repair necessary? If yes, go to step  25 . If no, go to step  24 .
  20. Turn ignition off. Disconnect the AF ECM connectors. Turn ignition on, with engine off. Connect DMM test leads between the desired gas circuit terminal and a known-good ground. Measure the voltage of the desired gas circuit. Is the voltage WITHIN 4-6 volts? If yes, go to step  24 . If no, go to step  22 .
  21. Repair the open ignition circuit between the GMS/MCV connector and ignition circuit splice. After repairs, go to step  25 .
  22. Replace GMS/MCV. After repairs, go to step  25 .
  23. Test the 7X CKP sensor signal circuit for an open or high resistance condition between the AF ECM connector terminal and the circuit splice. Inspect the AF ECM connector terminal for proper terminal contact. Repair the wiring or terminal contact as necessary. Was a wiring repair performed? If yes, go to step  25 . If no, go to next step.
  24. Replace AF ECM. See ALTERNATE FUEL ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CARS article. Program (flash) AF ECM with correct vehicle software and calibration. See ALTERNATE FUEL ENGINE CONTROL MODULE  under PROGRAMMING. After performing repairs and procedures, go to next step.
  25. 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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