GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 DTC P1204

Circuit Description

The powertrain control module (PCM) controls fuel delivery and for KL6 vehicles, determines which fuel system the engine is operating on. The PCM controls the low pressure lock-off (LPL) solenoid, and the high pressure lock-off (HPL) solenoid. The PCM commands the HPL solenoid open for 1 second at every ignition ON, in order to prime the CNG fuel system. The PCM commands the HPL, and the LPL open, when the engine is cranking or running on CNG. This DTC sets if the vehicle does not start on CNG within 8 seconds.

Conditions for Running the DTC
    • DTCs P0005, P0148, P0191, P0192, P0193, P0611, P1020, P1021-P1028, P1209, P2146, or P2665 are not set.
    • The ignition voltage is between 6 and 18 volts.
    • CNG operation is not inhibited.

Conditions for Setting the DTC

The cranking time to start in CNG mode is more than 8 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.

Test Description

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

  1. This step determines if a CNG related DTC is set.

    2.

  2. This step determines if the condition is present.

    3.

  3. This step determines if the CNG fuel system is the cause of the concern.

Step

Action

Value(s)

Yes

No

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

Observe the DTC information with a scan tool.

Are there any DTCs set?

--

Go to Step 3

Go to Step 5

3

With a scan tool, select Captured Info in order to store the powertrain DTC information.

Did you complete the action?

--

Go to Step 4

--

4

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the DTC information with a scan tool.

Does the scan tool display DTC P0005, P0191, P0611, P2146, or P2665?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 5

5

Attempt to start the engine.

Does the engine start and idle?

--

Go to Step 6

Go to Step 7

6
  1. Observe the Freeze Frame and/or the Failure records data for this DTC.
  2. Turn the ignition OFF 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 7

Go to Intermittent Conditions

7

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the fuel rail pressure (FRP) sensor parameter with a scan tool.

Does the FRP display remain steady near the specified value?

310 kPa (45 psi)

Go to Engine Cranks but Does Not Run in Engine Controls in the C/K Pickup Service Manual

Go to Fuel System Diagnosis