GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Cavalier (CNG) 🢒 Engine 🢒 Engine Controls - 2.2L (LN2) 🢒 Diagnostic System Check - Alternative Fuels (Engine Controls)

Description

The Diagnostic System Check-Engine Controls is an organized approach to identifying a condition that is created by a malfunction in the engine control system. The Diagnostic System Check must be the starting point for any driveability concern. The Diagnostic System Check directs the service technician to the next logical step in order to diagnose the concern. Understanding and correctly using the diagnostic table reduces diagnostic time and prevents the replacement of good parts.

Diagnostic Aids

Important: System voltage must remain between 9-16 volts for serial data communication.

When performing diagnostic procedures in the J Platform service manual always review the wiring schematics within the Bi-Fuel Supplement in order to determine if an Alternative Fuels circuit or component may be effecting the vehicle.

Always store control module (AF ECU, VCM / PCM or ECM) DTC information into a scan tool before proceeding with diagnosis. The stored control module DTC information may contain data (Freeze Frame or Failure Records) useful for intermittent diagnosis. Clearing DTC information with a scan tool or the loss of control module power or ground will erase the control module DTC information. Some diagnostic procedures call for disconnection of control module connectors and loss of control module stored DTC information may occur.

Test Description

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

  1. This step inspects for proper initial MIL operation.

    2.

  2. If the scan tool fails to turn ON, check scan tool operation on a known good vehicle. The data link connector (DLC) supplies the scan tool with a power circuit (terminal 16) and two ground circuits (terminals 4 and 5). All three circuits are required for proper scan tool operation.

    3.

  3. Both control modules simultaneously test certain DTCs regardless of the fuel system that is in operation.

    4.

  4. Checking for a shorted MIL circuit and proper control module operation of the MIL circuit.

    5.

  5. The fuel indicator lamp (FIL) located within the fuel gauge select switch button will illuminate when the engine is operating on gasoline.

    In addition the NGO Enable Output scan tool parameter will indicate the AF ECU's commanded state of the NGO Enable circuit while the NGO Enable Input parameter will indicate the gasoline control modules monitored state of the NGO Enable circuit. Regardless of the fuel system in operation these two data parameters will always indicate the same data unless an NGO Enable circuit fault is present.

    6.

  6. If actual and typical data values are different and a fault is suspected check the DTC information with a scan tool and verify that the suspected component or system DTCs have ran and passed. If they have passed use the sensor information in Description and Operation to help diagnose suspected sensor faults. If a system fault is suspected (EGR operation, EVAP operation, etc.) refer to the DTC that applies to the system and if possible use the diagnostic table to aid in diagnosis.

    7.

  7. Check for applicable bulletins before proceeding with DTC diagnosis.

    After determining which control module has stored DTCs, diagnose DTCs in the following order:

    8. • PCM MEMORY or INTERNAL FAULT DTCs (PCM Memory, PCM Not Programmed, etc.)
    • SYSTEM and IGNITION VOLTAGE DTCs (System Voltage High or Low, etc.)
    • COMPONENT / CIRCUIT DTCs (TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc.)
    • SYSTEM DTCs (MAP System Performance, TP Sensor Performance, etc.)

Step

Action

Yes

No

1

Important: 

   • Inspect for applicable service bulletins before proceeding with this test.
   • Ensure BOTH fuel tanks are at least ¼ full.
   • Do not clear DTCs unless instructed by a diagnostic procedure.

  1. Turn the ignition OFF.
  2. Connect the scan tool.
  3. Turn the ignition ON with the engine OFF.

Is the MIL on?

Go to Step 2

Go to Malfunction Indicator Lamp (MIL) Inoperative

2

Using a scan tool, attempt to communicate and retrieve data from the gasoline control module (VCM/PCM or ECM).

If the scan tool fails to turn ON, refer to Scan Tool Does Not Power Up in Data Link Communications in the J Platform service manual.

Does the scan tool display gasoline control module data?

Go to Step 3

Go to Scan Tool Does Not Communicate with Class 2 Device in Data Link Communications in the J Platform service manual

3

Using a scan tool , attempt to communicate and retrieve data from the alternative fuels engine control unit (AF ECU).

Does the scan tool display alternative fuels data?

Go to Step 4

Go to Ignition Relay Diagnosis

4

  1. Inspect BOTH control modules for stored DTCs.
  2. Record and store the DTC Freeze Frame and Failure Records information using the scan tool if any of the above are set.

Were any DTCs stored?

Go to Step 10

Go to Step 5

5

Command the MIL OFF utilizing the scan tool Engine Output (Gasoline) for the MIL.

Does the MIL turn OFF?

Go to Step 6

Go to Malfunction Indicator Lamp (MIL) Always On

6

Crank the engine for at least 10 seconds.

Does the engine start?

Go to Step 7

Go to Step 9

7

Determine which fuel system the engine is operating on by observing the fuel indicator lamp and the scan tool NGO Enable Output and Input parameters.

    • If the vehicle is operating on GASOLINE and the FIL is NOT illuminated, refer to Fuel Indicator Lamp diagnosis in the Body and Accessories section of this supplement.
    • If the NGO Enable Input and Output parameters do not indicate the same data, refer to Fuel System Unwanted Switch to Gasoline .

Is the engine operating on Alternative Fuel?

Go to Step 8

Go to Fuel System Unwanted Switch to Gasoline

8

  1. Operate the vehicle and observe/record the engine data parameters.
  2. Compare the engine data on the scan tool with the Engine Scan Tool Data List.

Are the values normal or within typical range?

Go to Step 11

Go to applicable DTC diagnosis

9

  1. Remove the CNG 20 amp fuse.
  2. Crank the engine for at least 10 seconds.
  3. Install the CNG 20 amp fuse prior to proceeding to other diagnostic information.

Does the engine start and continue to run with the CNG 20 amp fuse removed?

Go to Ignition Relay Diagnosis

Go to Diagnostic System Check - Engine Controls

10

  1. Compare the stored gasoline control module DTCs to the stored AF ECU DTCs.
  2. Diagnose DTCs in the following order:
    3. • If a DTC is stored ONLY in the gasoline control module, refer to the gasoline Service Manuals applicable DTC diagnostic information.
    • If a DTC is stored in BOTH modules or only in the AF ECU, refer to the applicable Alternative Fuels DTC diagnostic information.
    • If DIFFERENT multiple DTCs are set , perform diagnosis on the gasoline control module DTCs first.
  3. Diagnose DTCs.

Have all DTCs been diagnosed?

Go to Step 15

Go to applicable DTC diagnosis

11

Is there a driveability symptom?

Go to Step 12

Go to Step 13

12

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

Did the fuel system diagnosis locate and repair a concern?

Go to Step 15

Go to Symptoms - Engine Controls

13

Is there a fuel gauge, fuel gauge selector switch or fuel indicator lamp operation concern?

Go to Fuel Gauge Relay Diagnosis - Alternative Fuels , Fuel Indicator Lamp Always On - Alternative Fuels or Fuel Indicator Lamp Inoperative - Alternative Fuels in Body and Electrical section of this supplement

Go To Step 14

14

  1. With the ignition OFF, remove the CNG 20A fuse in order to force a switch-over to gasoline operation.
  2. Crank the engine for at least 10 seconds.

Does the engine start and run on gasoline?

Go to Step 15

Go to Alternative Fuel (AF) Fuel Pump Relay Diagnosis

15
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool clear DTC information from the AF ECU and the gasoline control module (VCM/PCM or ECM).
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle. If a DTC was set ensure that the operation of the vehicle allows the DTC to test. Refer to the Conditions For Running the DTC.
  6. Observe the fuel indicator lamp and the MIL.
  7. Observe vehicle performance and driveability.
  8. After operating the vehicle inspect BOTH control modules for stored DTCs.

Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs?

System OK

Go to Step 1