GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Cavalier (CNG) 🢒 DTC 🢒 DTC P1202

Circuit Description

The gas mass sensor (GMS) and the mixture control valve (MCV) are contained in one non-serviceable assembly. The GMS/MCV is supplied ignition voltage and a ground. The GMS monitors the mass and flow of the gaseous fuel entering the engine and converts this information into a GMS ACTUAL Gas Flow signal circuit frequency . The alternative fuels engine control module (AF ECM) commands fuel flow by supplying the MCV with a frequency signal. The DESIRED Gas Flow signal circuit frequency varies from around 1050 Hz (0.40 gms) at idle, to around 5000 Hz (18 gms) at wide open throttle.

The AF ECM converts the ACTUAL Gas Flow signal circuit frequency into a grams per second value. During low fuel flow rates, such as at engine idle, the GMS sensor Actual Gas circuit will produce a low frequency signal of around 700 Hz (0.21 gms). During high fuel flow rates, such as at wide open throttle-road load, the GMS sensor will produce a high frequency signal of around 2700 Hz (16.65 gms).

This DTC is used to indicate a GMS ACTUAL Gas Flow signal circuit frequency that is less than the expected signal.

Conditions for Running the DTC

Cranking Test

    • The engine is cranking on alternative fuel.
        The engine speed is between 25-500 RPM.
    • The battery voltage is greater than 8 volts.

Running Test

    • The engine is operating on alternative fuel.
    • The engine has been operating for greater than 10 seconds.
    • The engine speed is greater than 500 RPM.
    • The battery voltage is greater than 10 volts.

Conditions for Setting the DTC

Cranking Test

The AF ECM monitors a GMS Actual Gas Flow signal circuit frequency of less than 250 Hz for at least 0.5 seconds continuously.

Running Test

The AF ECM monitors a GMS Actual Gas Flow signal circuit frequency of less than 500 Hz for at least 0.5 seconds continuously.

Action Taken When the DTC Sets

The AF ECM will switch-over to gasoline fuel operation. The AF ECM will not attempt alternative fuel operation until the next ignition key cycle.

    • The Alternative Fuels ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
    • The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The Last Test Failed status will clear when the DTC runs and passes.
    • The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
    • All DTC Information can be cleared with a scan tool.

Diagnostic Aids

Important: The Desired Gas and Actual Gas circuits only output a frequency signal when the engine is cranking or running on CNG. No frequency SIGNAL is present with the ignition ON and the engine OFF or during any time the vehicle is operating in the gasoline mode.

Intermittent Conditions

A low pressure lock-off (LPL) than intermittenly sticks closed may cause this DTC to set. Connect a gage downstream of the LPL and monitor pressure while commanding the LPL ON and OFF several times. Replace an LPL that fails to flow fuel while commanded ON. If the LPL functions normally the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions diagnostic information located in Symptoms-Engine Controls. If any additional DTCs were stored, proceed to the applicable DTC table in the order stated in the Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Always test for fuel flow to the GMS/MCV for any intermittent condition. A low pressure lock-off solenoid that intermittently sticks closed may cause this DTC to set.

    2.

  2. This step determines if the GMS/MCV connector and the MCV motor connector terminals and wiring are OK.

    3.

  3. This step tests for the proper ignition voltage to the GMS/MCV.

    4.

  4. This step tests for the proper ground circuits to the GMS/MCV.

    5.

  5. This step tests the Actual Gas circuit for continuity to the AF ECM. The AF ECM supplies about 5 volts on the Actual Gas circuit.

    6.

  6. This step tests the Desired Gas circuit for continuity to the AF ECM. With the ignition ON and the engine OFF the AF ECM grounds the Desired Gas circuit.

    7.

  7. This step tests the Desired Gas circuit for a short to ground or a defective AF ECM. With the ignition OFF, the AF ECM can no longer ground the Desired Gas circuit and the test lamp should turn OFF.

    8.

  8. This step tests for proper initial system pressures. Any unexpected loss of fuel pressure may cause the GMS/MCV to output a low frequency signal and set this DTC. The AF ECM will open the lock-off solenoids and attempt to start on CNG every engine crank even when this DTC is stored. CNG fuel pressure might only be observed momentarily if this DTC sets during or shortly after engine cranking. An ignition cycle is required to reset the AF ECM.

  9. This step tests the AF ECM's ability to generate a sufficient Desired Gas frequency signal.

    10.

  10. This step tests the GMS/MCV's ability to generate a sufficient Actual Gas frequency signal.

    11.

  11. This step tests for a defective GMS/MCV or AF ECM. The GMS/MCV supplies about 5 volts on the Desired Gas circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Alternative Fuels Engine Controls Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2
  1. Capture the DTC Information from the AF ECM into the scan tool.
  2. Inspect BOTH the AF ECM and the gasoline control module (VCM/PCM or ECM) for other stored DTCs.

Are other DTCs stored?

--

Go to Step 3

Go to Step 4

3

Perform the other DTC diagnosis before attempting diagnosis of this DTC. Refer to Diagnostic Trouble Code (DTC) List .

Is the other DTC diagnosis and/or repair complete?

--

Go to Step 25

--

4
  1. Establish communication with the AF ECM using a scan tool.
  2. Observe the Engine Speed data parameter while cranking the engine.

Is engine speed indicated while the engine is cranking?

--

Go to Step 5

Go to Step 23

5

  1. Clear the AF ECM DTC Information using the scan tool.
  2. Turn the ignition OFF and wait 30 seconds.
  3. Start the engine.
  4. View the captured data Freeze Frame records for this DTC -- that was stored in step 2.1.

    5. Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.

  5. 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.

Does the scan tool indicate the diagnostic Passed?

--

Go to Intermittent Conditions

Go to Step 6

6

  1. Turn the ignition OFF.
  2. Visually inspect the GMS/MCV connector and the MCV motor connector for proper terminal contact.
  3. Test for circuit continuity of the motor power and ground circuits between the two connectors.
  4. Repair terminal contact or wiring as necessary.

Was a terminal or wire repair necessary?

--

Go to Step 25

Go to Step 7

7

  1. Disconnect the GMS/MCV connector.
  2. Turn the ignition ON leaving the engine OFF.
  3. Connect the J 39200 DMM test leads between the GMS/MCV Ignition circuit terminal and a known good ground.
  4. Measure the voltage on the Ignition circuit.

Is the voltage WITHIN the value range specified?

11-13 V

Go to Step 8

Go to Step 15

8

Connect the J 35616-200 12 -V Unpowered Test Lamp between BATTERY POSITIVE and the two GMS/MCV ground circuit terminals one at a time.

Does the test lamp illuminate brightly when connected to each ground circuit terminal?

--

Go to Step 9

Go to Step 16

9

  1. Connect the J 39200 DMM test leads between the GMS/MCV ACTUAL GAS circuit terminal and a known good ground.
  2. Measure the voltage on the Actual Gas circuit.

Is the voltage near the value specified?

5 V

Go to Step 10

Go to Step 17

10

With the ignition ON and the engine OFF, connect the J 35616-200 between BATTERY POSITIVE and the GMS/MCV DESIRED GAS circuit terminal.

Is the test lamp illuminated?

--

Go to Step 11

Go to Step 18

11

While monitoring the test lamp, turn the ignition OFF.

Did the test lamp turn OFF?

--

Go to Step 12

Go to Step 19

12

  1. Reconnect all disconnected or removed components, connectors, fuses, etc.
  2. Using a scan tool clear all DTC Information from the AF ECM.
  3. Turn the ignition OFF.
  4. Connect the J 39970-100 Fuel Test Port Adapter to the underhood CNG fuel line test port.
  5. Connect the J 39200 to the J 39970 Electronic Pressure Gage.
  6. Connect the J 39970 to the J 39970-100 .
  7. Monitor the fuel pressure as you crank and start the engine.

Was the observed fuel pressure EVER greater than the specified value?

115 psi

Go to Step 13

Go to Fuel System Diagnosis

13

  1. Turn the ignition OFF.
  2. Remove the fuel pressure test port adapter.
  3. Remove the GASOLINE fuel pump relay in order to prevent the engine from starting.
  4. Turn the ignition ON while leaving the engine OFF.
  5. Using a scan tool clear all DTC Information from the AF ECM.
  6. Observe the NGO Enable Input parameter and ensure that Alternative Fuel is enabled. An ignition cycle with a 30 second OFF time may be required to enable Alternative Fuel after clearing DTCs.
  7. Backprobe the GMS/MCV connector DESIRED GAS circuit terminal with the J 39200 DMM.
  8. Set the J 39200 DMM to AC Volts in order to observe frequency.
  9. While observing the DMM frequency crank the engine for about 5 seconds or until a switch-over to gasoline is observed. The engine will not start with the fuel pump relay removed.

Is the maximum observed frequency greater than the value specified?

750 Hz

Go to Step 14

Go to Step 20

14

  1. Keep the GASOLINE fuel pump relay removed in order to prevent the engine from starting.
  2. Backprobe the GMS/MCV connector ACTUAL GAS circuit terminal with the J 39200 DMM.
  3. Set the J 39200 DMM to DC Volts in order to observe frequency.
  4. While observing the DMM frequency crank the engine for about 5 seconds or until a switch-over to gasoline is observed. The engine will not start with the fuel pump relay removed.

Is the maximum observed frequency greater than the value specified?

250 Hz

Go to Step 24

Go to Step 22

15
  1. Turn the ignition OFF and wait 30 seconds.
  2. Observe the fuel indicator lamp (FIL), located in the fuel gauge select switch, as you turn the ignition ON while leaving the engine OFF.

Did the FIL illuminate for a 2 second bulb check or stay ON?

--

Go to Step 21

Go to Ignition Relay Diagnosis

16

Repair the GMS/MCV ground circuit with high or infinite -- open resistance.

Is the repair complete?

--

Go to Step 25

--

17
  1. Inspect the Actual Gas circuit for an open, short to ground or short to voltage.
  2. Inspect for proper terminal contact at the AF ECM connector Actual Gas circuit terminal.
  3. Repair terminal contact or wiring as necessary.

Was a terminal or wire repair necessary?

--

Go to Step 25

Go to Step 24

18
  1. Inspect the Desired Gas circuit for an open, or short to voltage.
  2. Inspect for proper terminal contact at the AF ECM connector Desired Gas circuit terminal.
  3. Repair terminal contact or wiring as necessary.

Was a terminal or wire repair necessary?

--

Go to Step 25

Go to Step 24

19
  1. Inspect the Desired Gas circuit for a short to ground.
  2. Repair wiring as necessary.

Was a wire repair necessary?

--

Go to Step 25

Go to Step 24

20

  1. Turn the ignition OFF.
  2. Disconnect the AF ECM connectors.
  3. Turn the ignition ON leaving the engine OFF.
  4. Connect the J 39200 DMM test leads between the DESIRED GAS circuit terminal and a known good ground.
  5. Measure the voltage of the Desired Gas circuit.

Is the voltage WITHIN the range specified?

4-6 V

Go to Step 24

Go to Step 22

21

Repair the open ignition circuit between the GMS/MCV connector and the ignition circuit splice.

Is the repair complete?

--

Go to Step 25

--

22

Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve Replacement .

Is the replacement complete?

--

Go to Step 25

--

23
  1. Test the 7X CKP sensor signal circuit for an open or high resistance condition between the AF ECM connector terminal and the circuit splice.
  2. Inspect the AF ECM connector terminal for proper terminal contact.
  3. Repair the wiring or terminal contact as necessary.

Was a wiring repair performed?

--

Go to Step 25

Go to Step 24

24
  1. Replace the AF ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  2. Program -- flash-- the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 25

--

25
  1. Reconnect all disconnected or removed components, connectors, fuses, etc.
  2. Using a scan tool clear all DTC Information from BOTH the gasoline control module (VCM/PCM or ECM) and the AF ECM.
  3. Using the scan tool communicate with the BOTH the gasoline control module and the AF ECM in order to reset the Fuel Trim values to zero.
  4. Start the engine.
  5. 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.
  6. Operate the vehicle within the Conditions for Running this 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?

--

System OK

Go to Step 2