GM Service Manual Online
For 1990-2009 cars only

Circuit Description

When the turn signal switch is placed in the left turn position, the body control module (BCM) cycles positive voltage ON and OFF to the LF turn signal lamp through the left front turn signal lamp supply voltage circuit. The front turn signal lamps illuminate steady for daytime running lamp (DRL) operation.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B3948 Left Front Turn Signal Circuit

Conditions for Running the DTC

    • The battery voltage must be between 9-16 volts.
    • Ignition switch in the RUN position.
    • Turn signal switch placed in the left turn position.

Conditions for Setting the DTC

    • An open in the left front turn signal lamp supply voltage circuit is present for longer than 0.1 second.
    • An open LF turn signal bulb
    • An open in the ground circuit of the lamp
    • An open or short to ground in the battery positive voltage circuit from the STOP/TURN 20 amp fuse in the instrument panel (I/P) fuse block

Action Taken When the DTC Sets

The left front turn signal lamp and DRL does not work.

Conditions for Clearing the DTC

    • The DTC will clear on current status after the condition for setting the fault is corrected.
    • A history code will clear after 100 consecutive ignition cycles without a fault present.
    • History and current DTCs can be cleared using a scan tool.

Diagnostic Aids

    • An open or short to ground in the battery positive voltage circuit of the STOP/TURN 20  amp fuse will cause all 4 turn signal DTCs.
    • Check the LF turn lamp bulb. If the bulb is open, DTC B3948 will be set.
    • If the DTC is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections .
    • An intermittent short to ground or an open in the following locations may cause an intermittent malfunction to occur:
       - The BCM
       - The underhood electrical center
       - The left front turn signal lamp supply voltage circuit

Step

Action

Yes

No

Schematic Reference: Exterior Lights Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Turn ON the ignition, with the engine OFF.
  2. Place the turn signal switch in the left turn position.

Does the LF turn signal lamp flash ON and OFF?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Remove the LF turn signal bulb. Refer to Park/Turn Signal Lamp Bulb Replacement .
  2. Inspect the bulb filament.

Is the bulb OK?

Go to Step 4

Go to Step 8

4

Test the ground circuit of the inoperative lamp for high resistance or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 5

5

Test the left front turn signal supply voltage circuit for the following:

    • A short to ground
    • A high resistance
    • An open
        

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 7

6

Test the battery positive voltage circuit from the STOP/TURN fuse for a short to ground or an open.

Did you find and correct the condition?

Go to Step 10

Go to Step 7

7

Inspect for a high resistance or an open at the harness connector of the body control module (BCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 9

8

Replace the bulb. Refer to Park/Turn Signal Lamp Bulb Replacement .

Did you complete the replacement?

Go to Step 10

--

9

Replace the BCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 10

--

10

  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 3

System OK