GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Savana (CNG) 🢒 DTC 🢒 DTC P2668

Circuit Description

The fuel indicator lamp (FIL) is located within the headlamp switch assembly. The FIL indicates which fuel system is utilized to operate the vehicle. The FIL illuminates for a few seconds at start up for a bulb check. The FIL remains illuminated while the vehicle is operating on gasoline. The powertrain control module (PCM) controls the FIL by grounding the control 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 P2668 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P2668 Fuel Mode Indicator Lamp Control Circuit

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC
    • The control module 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 5 seconds.

Action Taken When the DTC Sets
    • The powertrain control module (PCM) stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records.

Conditions for Clearing the DTC
    • A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Use a scan tool in order to clear the DTC.

Test Description

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

  1. This step tests if the concern is active. The FIL should turn ON and OFF with the scan tool commands. Repeat the commands as necessary.

    2.

  2. This step verifies that the control module is providing ground to the FIL.

    3.

  3. This step tests if a ground is constantly being applied to the FIL.

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. Command the fuel indicator lamp (FIL) ON and OFF with a scan tool.

Does the FIL respond to the scan tool commands?

Go to Step 3

Go to Step 4

3
  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 4

Go to Intermittent Conditions

4
  1. Turn OFF the ignition.
  2. Remove the headlamp switch assembly. Refer to Headlamp Switch Replacement in Lighting Systems in the G/H Van Service Manual.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the ignition voltage circuit of the FIL with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 5

Go to Step 11

5

  1. Connect a test lamp between the control circuit of the FIL and the ignition voltage circuit of the FIL.
  2. Command the FIL ON and OFF with a scan tool.

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

Go to Step 9

Go to Step 6

6

Does the test lamp remain illuminated with each command?

Go to Step 8

Go to Step 7

7

Test the control circuit of the FIL for an open or for a short to voltage. 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 14

Go to Step 10

8

Test the control circuit of the FIL 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 14

Go to Step 13

9

Inspect for poor connections at the headlamp switch. 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 14

Go to Step 12

10

Inspect for poor connections at the harness connector of the control module. 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 14

Go to Step 13

11

Repair the open or short to ground in the ignition voltage circuit. Refer to Wiring Repairs in Wiring Systems in the G/H Van Service Manual.

Did you complete the repair?

Go to Step 14

--

12

Replace the headlamp switch assembly. Refer to Headlamp Switch Replacement in Lighting Systems in the G/H Van Service Manual.

Did you complete the replacement?

Go to Step 14

--

13

Replace the powertrain control module (PCM). Refer to Powertrain Control Module Replacement in the G/H Van Service Manual.

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