GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 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.

Always store control module - Alternative Fuels ECM, 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 numbers below refer to the step numbers 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 2 ground circuits - terminals 4 and 5. All 3 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 gage select switch button will illuminate when the engine is operating on gasoline.

    The AFO Signal Command scan tool parameter will indicate the Alternative Fuels ECM's commanded state of the Alternative Fuel Operation circuit. The AFO Signal parameter will indicate the gasoline control modules monitored state of the Alternative Fuel Operation circuit. Regardless of the fuel system in operation these 2 data parameters will always indicate the same data unless an Alternative Fuel Operation circuit fault is present or the gasoline control module is programmed with standard gasoline software.

    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. DTC P1665 may set if there is an AFO Signal circuit fault or if the PCM is programmed with standard gasoline software.

    8.

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

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

    9. 1. Control Module MEMORY or INTERNAL FAULT DTCs - PCM Memory, PCM Not Programmed, etc.
    2. SYSTEM and IGNITION VOLTAGE DTCs - System Voltage High or Low, etc.
    3. COMPONENT / CIRCUIT DTCs - TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc.
    4. 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 continuously?

Go to Step 2

Go to Malfunction Indicator Lamp (MIL) Inoperative

2

  1. Using a scan tool, attempt to communicate and retrieve data from the gasoline control module (VCM / PCM or ECM).
  2. If the scan tool fails to turn ON, refer to Scan Tool Does Not Power Up in Data Link Communications in the G Van 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 G Van Service Manual

3

Using a scan tool attempt to communicate and retrieve data from the Alternative Fuels ECM.

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 ten 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 AFO Signal and AFO Signal Command parameters.

    • If the vehicle is operating on GASOLINE and the FIL is NOT illuminated refer to Fuel Indicator Lamp Inoperative - Alternative Fuels Instrument Panel, Gages, and Console.
    • If the AFO Signal and the AFO Signal Command 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 13

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 Fuel Pump Electrical Circuit Diagnosis

10

Does the PCM have a DTC P1665 stored?

Go to Step 11

Go to Step 12

11

Inspect the PCM for the correct BI-FUEL software and calibration.

Is the PCM programmed with BI-FUEL software and calibration?

Go to Fuel System Unwanted Switch to Gasoline

Go to Powertrain Control Module Programming

12

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

Have all DTCs been diagnosed?

Go to Step 17

Go to applicable DTC diagnosis

13

Is there a driveability symptom?

Go to Step 14

Go to Step 15

14

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 17

Go to Symptoms - Engine Controls

15

Is there a fuel gage, fuel gage 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

Go to Step 16

16

  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 17

Go to Fuel Pump Electrical Circuit Diagnosis

17
  1. Remove all test equipment except the scan tool.
  2. Connect any dis-connected components, fuses, etc.
  3. Using a scan tool clear DTC information from the Alternative Fuels ECM 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