GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC C Sierra - 2WD (CNG) 🢒 DTC 🢒 DTC P2146

Circuit Description

The powertrain control module (PCM) opens the AF enable circuit in order to operate on compressed natural gas (CNG). When the AF enable circuit is open the AF fuel mode relay switch is open. The PCM fuel injector pulse width modulated (PWM) signals are routed through the AF injector relays to the fuel injector control module (FICM). The FICM will operate the CNG injectors based upon the PCM pulse width signals. When gasoline operation is desired, the PCM grounds the AF enable circuit. When the AF enable circuit is grounded the AF fuel mode relay closes. The AF fuel mode relay provides a ground path for all of the AF injector relays. The PCM fuel injector PWM signals are routed through the AF injector relays to the gasoline fuel injectors. The PCM controls the AF enable circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the controlled component. The PCM monitors the status of the driver. If the PCM detects an incorrect voltage for the commanded state of the driver, DTC P2146 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P2146 Injector Positive Voltage Relay Control Circuit

Conditions for Running the DTC
    • The ignition voltage is between 6-18 volts.
    • The engine speed is more than 400 RPM.

Conditions for Setting the DTC
    • The PCM detects that the commanded state of the driver and the actual state of the control circuit do not match.
    • The above condition exists for at least 1 second.

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 tests if the concern is active.

    2.

  2. This step tests if the concern is active only while operating on gasoline. Disconnecting the high pressure lock-off (HPL) solenoid connector will cause a loss of CNG fuel pressure and force a switchover to gasoline. An HPL DTC will set. Reconnect the HPL connector and clear DTC information after all diagnosis and repairs are complete.

    3.

  3. This step determines if the PCM is receiving 12 volts from the AF fuel mode relay.

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

  1. Turn ON the ignition, with the engine OFF.
  2. Clear the DTC information with a scan tool.
  3. Turn OFF the ignition and wait 30 seconds.
  4. Attempt to start and idle the engine.
  5. Observe the DTC information with a scan tool.

Does DTC P2146 set?

Go to Step 3

Go to Step 4

3

Is DTC P0148 also set?

Go to DTC P0148

Go to Step 5

4

  1. Start and idle the engine on compressed natural gas (CNG).
  2. Disconnect the high pressure lock-off (HPL) solenoid connector.
  3. Wait for the engine to stall or switchover to gasoline. Attempt to start and idle a stalled engine.
  4. Observe the DTC information with a scan tool.

Did the DTC fail this ignition?

Go to Step 5

Go to Intermittent Conditions

5

  1. Disconnect the powertrain control module (PCM). Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

    2. Important: Other DTCs will set with the PCM disconnected.

  2. Turn ON the ignition, with the engine OFF.

    3. Important: If the jumper fuse opens, repair the AF enable circuit for a short to voltage condition.

  3. Probe the AF Enable Control circuit with a fused jumper that is connected to a good ground.
  4. Listen for operation of the AF underhood relays as the circuit is grounded.

Do the relays operate when the circuit is grounded?

Go to Step 6

Go to Step 7

6
  1. Turn the ignition OFF.
  2. Disconnect the fuel injector control module (FICM) connector. Refer to Fuel Injector Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the AF Enable Signal circuit at the FICM connector with a fused jumper that is connected to a good ground.
  5. Listen for operation of the AF underhood relays as the circuit is grounded.

Do the relays operate when the circuit is grounded?

Go to Step 9

Go to Step 10

7
  1. Remove the AF Fuel Mode Relay from the AF Left Relay Block.
  2. Probe the ignition feed circuit of the AF mode relay coil with a test lamp connected to a good ground.

Does the test lamp illuminate?

Go to Step 8

Go to Step 11

8

Test the AF Enable Control circuit for an open or shorted to ground condition. 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 14

Go to Step 13

9

Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 14

Go to Step 12

10

Repair the open in the AF enable circuit between the FICM and the PCM. 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 14

--

11

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 14

--

12

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 14

--

13

Replace the AF Fuel Mode Relay.

Did you complete the replacement?

Go to Step 14

--

14
  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 15

15

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