GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 DTC P1021-P1028

Circuit Description

The powertrain control module (PCM) controls fuel delivery and determines fuel system operation on Bi-fuel (KL6) vehicles. The PCM is not capable of operating the high current CNG fuel injectors. The PCM injector pulse width (PWM) signals are received by the fuel injector control module (FICM) and duplicate PWM signal are generated by the FICM in order to operate the CNG injectors. Ignition voltage is supplied to the fuel injectors. The FICM controls each CNG fuel injector by grounding the control circuit via a solid state device called a driver. The FICM monitors the status of each driver. If the control module detects an incorrect voltage for the commanded state of the driver, a fuel injector control DTC P1021-P1028 sets.

Conditions for Running the DTC
    • The ignition voltage is between 6 and 18 volts.
    • The engine is operating on CNG.

Conditions for Setting the DTC
    • The FICM detects an incorrect voltage on a CNG fuel injector control circuit.
    • The condition exists for 5 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module switches over to gasoline fuel operation.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • Performing the Fuel Injector Coil Test may help to isolate an intermittent condition. Refer to Fuel Injector Solenoid Coil Test .
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

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

  1. The engine must crank or run on CNG in order for this DTC to test.

    2.

  2. This step verifies that the FICM is able to control the fuel injector.

    Bi-fuel (KL6) equipped vehicles must be operating on CNG prior to pulsing the CNG fuel injector.

    3.

  3. This step tests if a ground is constantly being applied to the fuel injector.

    4.

  4. A short to ground of the CNG injector B+ circuit will open the inline fuse and cause an entire cylinder bank to set DTC's. Test the CNG injector B+ circuits, bank injector monitor circuit and the FICM for a shorted condition whenever the inline fuse has opened.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Fuel Injector Control Module Connector End Views

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

Go to Step 2

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

2
  1. Clear the diagnostic trouble code information with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Crank and attempt to start the engine.
  4. Observe the DTC Information with a scan tool.

Do any of the DTCs P1021-P1028 fail this ignition?

Go to Step 3

Go to Diagnostic Aids

3

  1. Turn OFF the ignition.
  2. Disconnect the appropriate CNG injector.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the B+ voltage circuit of the CNG fuel injector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 4

Go to Step 10

4
  1. Probe the control circuit of the CNG fuel injector with a test lamp connected to B+.
  2. Turn the ignition OFF for 30 seconds.

    3. Important: To ensure the fuel mode command remains ALT fuel, do not crank or start the engine on Bi-fuel (KL6) vehicles.

  3. Turn ON the ignition, with the engine OFF.
  4. Perform the fuel injector balance test on the appropriate CNG fuel injector with a scan tool. Refer to Fuel Injector Balance Test with Tech 2 .
  5. Observe the test lamp while pulsing the appropriate CNG fuel injector.

Does the test lamp blink while the injector is pulsed?

Go to Step 8

Go to Step 5

5

Does the test lamp remain continuously illuminated?

Go to Step 7

Go to Step 6

6

Test the CNG fuel injector control circuit for an open or a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

7

Test the CNG fuel injector control circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

8

Test for an intermittent and for a poor connection at the CNG fuel injector. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

9

Test for an intermittent and for a poor connection at the FICM. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Repair the open or short to ground on the B+ voltage circuit of the fuel injector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 13

--

11

Replace the CNG fuel injector. Refer to Fuel Injector Replacement .

Did you complete the replacement?

Go to Step 13

--

12

Replace the FICM. Refer to Fuel Injector Control Module Replacement .

Did you complete the replacement?

Go to Step 13

--

13
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 14

14

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK