GM Service Manual Online
For 1990-2009 cars only

Circuit Description

When the turn signal/multifunction switch is placed in the left turn position, ground is applied to the body control module (BCM) through the left turn signal switch circuit. The BCM then applies a pulsating voltage to the LF and LR turn signal lamps through the LF and LR turn signal voltage supply circuits.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B3950 Left Rear Turn Signal Circuit

DTC Symptom

DTC Symptom Descriptor

02

Short to Ground

05

Open or Short to Voltage

Conditions for Running the DTC

Battery voltage must be between 9-16 volts.

Conditions for Setting the DTC

    • DTC B3950 02 will set when the BCM detects a short to ground in the LR turn signal voltage supply circuit.
    • DTC B3950 05 will set when the BCM detects an open or short to voltage in the LR turn signal voltage supply circuit.

Action Taken When the DTC Sets

    • When DTC B3950 02 is present, the LR turn signal lamp will not operate.
    • When DTC B3950 05 is present, the LR turn signal lamp will be ON at all times for a short to voltage condition. For an open condition, the LR turn signal lamp will be inoperative.

Conditions for Clearing the DTC

    • The condition responsible for setting the DTC no longer exists.
    • You issue a scan tool CLEAR DTCs command.
    • A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Diagnostic Aids

Check the LR turn signal bulb. If the bulb is open, DTC B3950 05 will set as current only.

Thoroughly inspect the wiring and connectors. An incomplete inspection of the wiring and connectors may result in misdiagnosis causing part replacement with the reappearance of the malfunction. If an intermittent malfunction exists, refer to Testing for Intermittent Conditions and Poor Connections .

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. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.

Is DTC B3950 02 present?

Go to Step 3

Go to Step 4

3

  1. Turn OFF the ignition.
  2. Disconnect the body control module (BCM) C3 harness connector.
  3. Disconnect the LR tail lamp assembly. Refer to Tail Lamp, Turn Signal Lamp, High Mount Stop Lamp, License Lamp, Backup Lamp Replacement - Rear .
  4. Connect a test lamp between battery voltage and the LR turn signal voltage supply circuit at the BCM C3 harness connector.

Does the test lamp illuminate?

Go to Step 8

Go to Step 10

4

Is the LR turn signal lamp ON at all times?

Go to Step 5

Go to Step 6

5

  1. Turn OFF the ignition.
  2. Disconnect the BCM C3 harness connector.
  3. Turn ON the ignition, with the engine OFF.

Does the LR turn signal lamp turn OFF?

Go to Step 10

Go to Step 7

6

  1. Turn OFF the ignition.
  2. Disconnect the BCM C3 harness connector.
  3. Connect a test lamp between battery voltage and the LR turn signal voltage supply circuit at the BCM C3 harness connector.

Does the test lamp illuminate?

Go to Step 10

Go to Step 9

7

Locate and repair a short to battery voltage in the LR turn signal lamp voltage supply circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 12

--

8

Locate and repair a short to ground in the LR turn signal lamp voltage supply circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 12

--

9

Locate and repair an open or high resistance in the LR turn signal lamp voltage supply circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 12

--

10

Inspect for poor connections at the C3 harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 11

11

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

Did you complete the replacement?

Go to Step 12

--

12

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

Does the DTC reset?

Go to Step 2

System OK