GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2003 🢒 Express (CNG) 🢒 DTC 🢒 DTC P0148

Circuit Description

The powertrain control module (PCM) grounds the AF Enable circuit when operating on CNG. When the AF Enable circuit is grounded the fuel injector control module (FICM) operates the CNG injectors based upon the PCM fuel injector control pulse width signals. On Bi-fuel (KL6) vehicles the PCM opens the AF Enable circuit when gasoline operation is desired. Switch-over from one fuel to the other is always performed in an orderly, sequential manner. Since some injectors are in the middle of injecting the previous fuel, the FICM will wait until that cylinders fuel delivery is complete and will then switch over in sequential firing order.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

The PCM detects that the commanded state and the actual state of the AF Enable circuit do not match.

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.

Test Description

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

  1. This step determines if the condition is active. The PCM grounds the AF Enable circuit when operating on CNG.

    2.

  2. This step tests for a short to ground on the AF Enable circuit. The test lamp should not illuminate with the ignition OFF.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: 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

Important: Verify that there is adequate fuel in the CNG fuel tank before proceeding.

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the Fuel Mode Command parameter with a scan tool.

Does the scan tool display ALT FUEL?

Go to Step 3

Go to Step 4

3
  1. Observe the Freeze Frame and/or the Failure records data 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 and/or the Failure records data.

Does the DTC fail this ignition cycle?

Go to Step 4

Go to Intermittent Conditions

4

  1. Turn OFF the ignition.
  2. Disconnect the FICM. Refer to Fuel Injector Control Module Replacement .
  3. Probe the AF Enable circuit with a test lamp connected to B+.

Does the test lamp illuminate?

Go to Step 7

Go to Step 5

5
  1. Turn ON the ignition.
  2. Command the AF Enable circuit ON and OFF with a scan tool.
  3. Observe the test lamp.

Does the test lamp respond to the scan tool commands?

Go to Step 9

Go to Step 6

6

Test the AF Enable circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual.

Did you find and correct the condition?

Go to Step 12

Go to Step 8

7

Test the AF Enable circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual.

Did you find and correct the condition?

Go to Step 12

Go to Step 10

8

Test for an intermittent and for a poor connection at the PCM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual.

Did you find and correct the condition?

Go to Step 12

Go to Step 10

9

Test for an intermittent and for a poor connection at the FICM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the G/H Van Service Manual.

Did you find and correct the condition?

Go to Step 12

Go to Step 11

10

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the G/H Van Service Manual.

Did you complete the replacement?

Go to Step 12

--

11

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

Did you complete the replacement?

Go to Step 12

--

12
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30  seconds.
  3. Turn ON the ignition, with the engine OFF.
  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 13

13

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