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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 DTC P1020

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 8 PCM injector pulse width modulated (PWM) signals are received by the fuel injector control module (FICM) and duplicate PWM signals 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 PWM input signal. If the control module does not receive a fuel injector PWM signal from the PCM DTC P1020 sets.

Conditions for Running the DTC
    • DTCs P0148, P0611, or P1209 are not set.
    • The ignition voltage is between 6 and 18 volts.
    • The engine is operating on CNG.
    • The PCM is not in fuel shut-off, or decel fuel cut-off mode.

Conditions for Setting the DTC
    • The fuel injector control module (FICM) detects a condition on a fuel injector PWM input signal.
    • 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
    • A stuck AF injector relay may cause this DTC to set.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

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

  1. If DTC P0200 also sets, diagnose P0200 first. A PWM circuit condition between the AF injector relay and the PCM will cause DTCs P0200 and P1020 to set.

    2.

  2. This step determines if the condition is causing an engine misfire. The AF fuel mode relay must be removed in order to prevent a fuel system switch-over.

    3.

  3. This step tests the ignition feed circuit to the AF fuel mode relay. An open ignition feed circuit to the relay will set this DTC.

    4.

  4. This step tests the AF enable circuit to the FICM. An open AF enable circuit between the splice and FICM will set this DTC without an engine misfire.

    5.

  5. This step verifies that the FICM is receiving the PWM input signal from the PCM. The test lamp should blink while the engine is cranking.

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. Turn ON the ignition, with the engine OFF.
  2. Clear the DTC information with a scan tool.
  3. Start and idle the engine.
  4. Observe the DTC information with a scan tool.

Does DTC P1020 set?

Go to Step 3

Go to Step 4

3

Is DTC P0200 also set?

Go to DTC P0200

Go to Step 5

4

  1. Observe the Freeze Frame/Failure Records for this DTC.
  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 5

Go to Diagnostic Aids

5

  1. Turn OFF the ignition.

    2. Important: Removal of the AF fuel mode relay may cause other DTCs to set.

  2. Remove the AF fuel mode relay (KL6).
  3. Turn ON the ignition, with the engine OFF.
  4. Clear the DTCs with a scan tool.
  5. Idle the engine at the normal operating temperature.
  6. Monitor the misfire current counters with a scan tool.

Are any of the misfire current counters incrementing?

Go to Step 9

Go to Step 6

6

Probe the ignition feed circuit of the AF mode relay coil with a test lamp connected to B+.

Does the test lamp illuminate?

Go to Step 7

Go to Step 15

7

  1. Turn OFF the ignition.
  2. Install the AF fuel mode relay.
  3. Disconnect the FICM. Refer to Fuel Injector Control Module Replacement .
  4. Probe the AF enable circuit with a test lamp connected to a good ground.
  5. Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

Go to Step 14

Go to Step 8

8
  1. Turn OFF the ignition.
  2. Probe the AF enable circuit with a test lamp connected to B+.

Does the test lamp illuminate?

Go to Step 12

Go to Step 11

9

  1. Turn OFF the ignition.
  2. Disconnect the FICM. Refer to Fuel Injector Control Module Replacement .
  3. Probe the PWM input signal circuit which displays the highest number of misfire current counters with a test lamp connected to B+.
  4. Crank the engine.

Does the test lamp blink?

Go to Step 14

Go to Step 10

10

Test the PWM input signal circuit for an open or a short to ground between the FICM and the AF injector relay. 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 19

Go to Step 13

11

Test the AF enable circuit for an open between the FICM and the AF fuel mode relay. 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 19

Go to Step 13

12

Test the AF enable circuit for a short to ground between the following:

    • PCM and the splice
    • FICM and the splice
    • AF fuel mode relay and the splice

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 19

Go to Step 18

13

Test for an intermittent and for a poor connection at the AF injector relay. 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 19

Go to Step 16

14

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 19

Go to Step 17

15

Repair the ignition feed circuit to the AF fuel mode relay coil. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 19

--

16

Replace the AF injector relay.

Did you complete the replacement?

Go to Step 19

--

17

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

Did you complete the replacement?

Go to Step 19

--

18

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

Go to Step 19

--

19
  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 20

20

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