GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2000 🢒 Cavalier (CNG) 🢒 DTC 🢒 DTC P1201 Gas Mass Sensor Performance

Object Number: 488521 Size: SF
Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

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 DTC's are set.
    • Engine speed is greater than 500 RPM.
    • Battery voltage is greater than 10 volts.
    • The Fuel Pressure Sensor indicates the CNG fuel level is above one-quarter full.
    • 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 if a rich or lean condition is occuring.

    4. 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.

Calc. MAF (g/sec)

GMS Ideal (g/sec)¹

High Limit (g/sec)¹

Low Limit (g/sec)¹

10

0.59

0.44

0.74

20

1.18

0.89

1.48

30

1.78

1.34

2.23

40

2.38

1.79

2.98

50

2.92

2.19

3.65

75

4.48

3.36

5.60

100

6.00

4.50

7.50

150

8.88

6.66

11.40

200

11.62

8.72

14.53

¹Actual gas circuit output.

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 Symptom diagnosis for additional information. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in the OBD System Check.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. 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.

    2.

  2. This step determines if the fault is still present.

    3.

  3. This step tests the CNG system for proper fuel pressures.

    4.

  4. 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.

Step

Action

Value(s)

Yes

No

1

Did you perform the AF Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

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

Are other DTC's 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 12

--

4

  1. Inspect the vehicle for any of the following conditions:
    2. • An open or shorted O2S signal wire
    • O2S connector water intrusion
    • a leaking exhaust system
    • poor GMS/MCV connector terminal contact
    • inadequate ignition voltage or ground circuits to the GMS/MCV
    • a misrouted engine harness causing EMI interference on the Actual gas circuit
    • A rich or lean engine condition such as the following:
       - EVAP canister purge system malfunction
       - Gasoline contaminated engine oil
       - Incorrect CNG fuel pressure
       - A leaking gasoline fuel pressure regulator
       - a restricted air filter or Intake Air duct
       - an engine vacuum leak
       - an improperly operating PCV system
  2. Repair any of the above or similar engine conditions as necessary.

Was a condition found and repaired?

--

Go to Step 12

Go to Step 5

5

  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 ECU.
  3. Using the scan tool communicate with the BOTH the gasoline control module and the AF ECU 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?

--

Go to Step 6

Go to Step 7

6

The fault is not present. Refer to Diagnostic Aids.

Are any DTC's stored that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

7

Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis .

Was a CNG fuel system problem found and repaired?

--

Go to Step 12

Go to Step 8

8
  1. Inspect the oxygen sensor signal for continuity to the AF ECU connector terminal.
  2. Inspect both these circuits for shorts to other circuits and the signal circuit for a short to ground.

    3. Ensure the oxygen sensor, gasoline control module and the AF ECU connectors are disconnected before testing circuits.

  3. Repair wiring as necessary.

Was a wire repair made?

--

Go to Step 12

Go to Step 9

9

  1. Reconnect all disconnected or removed components.
  2. Using a scan tool clear all DTC Information from BOTH the gasoline control module (VCM/PCM or ECM) and the AF ECU.
  3. Using the scan tool communicate with the gasoline control module and reset the Fuel Trim values to zero.
  4. Turn the ignition OFF.
  5. Remove the 20 amp CNG fuse in order to force a switch-over to gasoline operation.

    6. The FIL will NOT illuminate due to the removal of the CNG 20 amp fuse.

  6. Start the engine.
  7. 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.
  8. Operate the vehicle within the Conditions for Running this DTC and the parameters observed when the DTC set.
  9. Observe the Short Term Fuel Trim value.

Does the fuel trim value indicate a large amount of fuel trim adjustment is required or does a fuel trim DTC set?

--

Go to Step 10

Go to Step 11

10

Repair the possible causes of excessive rich or lean engine operation. Refer to the items in step 4.

Is the repair complete?

--

Go to Step 12

--

11

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

Is the replacement complete?

--

Go to Step 12

--

12
  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 ECU.
  3. Using the scan tool communicate with the BOTH the gasoline control module and the AF ECU 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