GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2003 🢒 Cavalier (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 AF relay is energized. The AF relay supplies power to all of the AF injector relays. The switched AF relay voltage also signals the fuel injector control module (FICM) to operate the CNG injectors based upon the PCM fuel injector control pulse width signals. 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 voltage is between 6 and 18 volts

Conditions for Setting the DTC
    • The PCM detects that the commanded state and the actual state of the AF Enable output circuit do not match.
        OR
    • The FICM detects that the commanded state and the actual state of the AF Enable signal circuit do not match.
    • The condition exists for at least 3 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the 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.

    2.

  2. This step determines if the PCM can control the AF relay. Command the relay ON and OFF several times in order to verify AF relay operation.

    3.

  3. This step tests the voltage circuit to the AF relay coil. If the test lamp illuminates the circuit is OK.

    4.

  4. This step tests the AF enable output circuit from the PCM. The test lamp should turn ON and OFF with each command.

    5.

  5. This step determines if a ground is constantly applied to the AF enable output circuit.

    6.

  6. This step tests the voltage circuit to the AF relay switch. If the test lamp illuminates the circuit is OK. If the test lamp does not illuminate, a short to ground may be the cause of the condition. If the AF relay fuse is open, test all of the related switch circuits.

    7.

  7. This step tests the AF enable signal circuit from the AF relay to the FICM. The test lamp should turn ON and OFF with the scan tool commands.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls 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

Command the AF relay ON and OFF with a scan tool.

Does the AF relay respond to the scan tool commands?

Go to Step 10

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the AF relay.
  3. Probe the ignition feed circuit of the AF relay coil with a test lamp connected to a good ground.

Does the test lamp illuminate?

Go to Step 6

Go to Step 16

6
  1. Probe the AF enable output circuit with a test lamp connected to B+.
  2. Turn ON the ignition, with the engine OFF.
  3. Command the AF relay ON and OFF with a scan tool.

Does the test lamp turn ON and OFF with each command?

Go to Step 13

Go to Step 7

7

Does the test lamp remain illuminated with each command?

Go to Step 9

Go to Step 8

8

Test the AF enable output circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 14

9

Test the AF enable output circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 18

10
  1. Turn OFF the ignition.
  2. Disconnect the AF relay.
  3. Probe the ignition voltage circuit of the AF relay with a test lamp connected to a good ground.

Does the test lamp illuminate?

Go to Step 11

Go to Step 16

11
  1. Turn OFF the ignition.
  2. Install the AF relay.
  3. Disconnect the FICM. Refer to Fuel Injector Control Module Replacement .
  4. Turn ON the ignition, with the engine OFF.
  5. Probe the AF Enable signal circuit with a test lamp connected to a good ground.
  6. Command the AF relay ON and OFF with a scan tool.

Does the test lamp turn ON and OFF with each command?

Go to Step 15

Go to Step 12

12

Test the AF Enable signal for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 13

13

Test for an intermittent and for a poor connection at the AF relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 17

14

Test for an intermittent and for a poor connection at the PCM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 18

15

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 J Platform Service Manual.

Did you find and correct the condition?

Go to Step 20

Go to Step 19

16

Important: The AF relay supplies power to the CNG fuel injectors, and the AF injector relays. If the AF relay fuse is open test all circuits for a short to ground.

Repair the ignition voltage circuit of the AF relay for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual.

Did you complete the repair?

Go to Step 20

--

17

Replace the AF relay.

Did you complete the replacement?

Go to Step 20

--

18

Replace the PCM. Refer to Powertrain Control Module Replacement in the J Platform Service Manual.

Did you complete the replacement?

Go to Step 20

--

19

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

Did you complete the replacement?

Go to Step 20

--

20
  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 21

21

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