P1201
Schematic:
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 AF ECU 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 ECU 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).
The AF ECU monitors and compares the Actual Gas flow rate to a calculated gas flow rate. The calculated gas flow rate is based upon a calculated Mass Air Flow rate.
This DTC is used to indicate that the Actual Gas flow rate does not match a calculated gas flow rate.
CONDITIONS FOR RUNNING THE DTC
^ The engine is operating on alternative fuel.
^ No AF ECU ECT, O2S or GMS DTCs are set.
^ The engine speed is greater than 500 RPM.
^ The battery voltage is greater than 10 volts.
^ The Fuel Pressure Sensor indicates the CNG fuel level is above one-quarter full.
^ The engine coolant temperature is between 80°C (176°F) and 105°C (221°F).
^ The AF ECU is not operating in a fuel cut-off mode (DFCO, speed limiting, etc.).
CONDITIONS FOR SETTING THE DTC
The difference between the Desired Gas Flow and the Actual Gas Flow is greater than 45 percent for 5 seconds.
ACTION TAKEN WHEN THE DTC SETS
The AF ECU will switch-over to gasoline fuel operation.
The AF ECU will not attempt alternative fuel operation until the next ignition key cycle.
^ The AF ECU will illuminate the MIL and store the DTC as History during ONE drive trip in which the diagnostic runs and fails.
^ The AF ECU 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 AF ECU 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 using a scan tool.
^ Interruption of Control Module (AF ECU/VCM/PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed.
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.
1. Use the following table to aid intermittent diagnosis.
2. Connect the scan tool and perform a snapshot of the AF ECU data parameters when this DTC sets.
3. Compare the Actual Gas circuit gas flow to the Mass Air Flow in order to determine it a rich or lean condition is occurring.
If the Actual Gas circuit flow is greater than the High Limit then the GMS/MCV is adding fuel to compensate for a lean engine condition.
Fault Not Present
Fault Not Present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions diagnostic information located in Symptoms-Computers and Control Systems. If any additional DTCs were stored, proceed to the applicable DTC table in the order stated in the Diagnostic System Check - Alternative Fuels (Computers and Control Systems). Intermittent Conditions
TEST DESCRIPTION
Steps 1-3:
Steps 4-8:
Steps 9-12:
The numbers below refer to the step numbers on the diagnostic table.
4. This step inspects the vehicle for conditions that would cause the Actual Gas circuit frequency to operate outside the expected range for a given engine air flow.
5. This step determines if the fault is still present.
7. This step tests the CNG system for proper fuel pressures.
9. This step determines if a base engine condition exists that would also require the gasoline engine control module (VCM/PCM or ECM) to make a large fuel trim adjustment.