GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Malfunction Indicator Lamp (MIL) Always On

Circuit Description

The malfunction indicator lamp (MIL) is used to indicate to the driver of the vehicle that an engine control system condition has occurred that may cause an increase in emissions and/or cause a driveability concern.

When the ignition is first turned ON, the gasoline control module (VCM/PCM or ECM) will illuminate the MIL for a bulb and system check. When the engine is started and operating the gasoline control module will turn the MIL OFF if no DTCs are set.

The Alternative Fuels ECM connects into the normal MIL Control circuit path and does not affect the gasoline control module operation of the MIL. The Alternative Fuels ECM is connected into the MIL Control circuit path in order to command the MIL ON when a CNG DTC sets. Both the Alternative Fuels ECM and the gasoline control module simultaneously test certain DTCs regardless of the fuel system that is in operation and may illuminate the MIL.

The instrument panel gauge fuse supplies the positive voltage source for the MIL. The gasoline control module and/or the Alternative Fuels ECM can supply the ground path.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the symptom is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

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

  1. The System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.

    2.

  2. With the ignition ON and the engine OFF, the gasoline control module (VCM/PCM or ECM) should continuously illuminate the MIL.

    3.

  3. The scan tool will command BOTH the gasoline control module and the AF ECU to turn the MIL OFF.

    4.

  4. This step tests if the MIL Input circuit is requesting MIL illumination. There should not be a MIL Input while the MIL is commanded OFF.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Were you sent here from another diagnostic procedure?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

Turn the ignition ON, with the engine OFF.

Is the MIL continuously illuminated?

Go to Step 3

Go to Malfunction Indicator Lamp (MIL) Inoperative

3

While communicating with the Alternative Fuels ECM, command the MIL OFF.

Is the MIL illuminated?

Go to Step 5

Go to Step 4

4

The fault is not present. Refer to Diagnostic Aids.

Is an operational concern still present?

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

Go to Intermittent Conditions

5

Observe the MIL Input (PCM) data parameter in the Alt. Fuels Data List while the MIL is commanded OFF.

Is a gasoline MIL Input request present?

Go to Step 7

Go to Step 6

6
  1. Test the MIL Control circuit between the Alternative Fuels ECM connector terminal and the IP for a short to ground.
  2. Test the IP for an internally shorted condition.
  3. Repair the circuit or IP as necessary.

Was a repair necessary?

Go to Step 9

Go to Step 8

7
  1. Test the MIL INPUT circuit between the Alternative Fuels ECM connector terminal and the gasoline control module connector terminal for a short to ground.
  2. Repair the circuit as necessary.

Was a circuit repair necessary?

Go to Step 9

Go to Step 8

8
  1. Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  2. Program the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 9

--

9
  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 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.
  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 Diagnostic System Check - Alternative Fuels (Engine Controls)