P1202

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 DTC/MIL

- 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.

Intermittant 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, See Testing for Intermittent and Poor Conditions.

Test Description

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

5. 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.
6. This step determines if the GMS/MCV connector and the MCV motor connector terminals and wiring are OK.
7. This step tests for the proper ignition voltage to the GMS/MCV.
8. This step tests for the proper ground circuits to the GMS/MCV.
9. 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.
10. 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.
11. 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.
12. 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.
13. This step tests the AF ECM's ability to generate a sufficient Desired Gas frequency signal.
14. This step tests the GMS/MCV's ability to generate a sufficient Actual Gas frequency signal.
20. This step tests for a defective GMS/MCV or AF ECM. The GMS/MCV supplies about 5 volts on the Desired Gas circuit.