Buick Regal: DTC B388B or B388C

Diagnostic Instructions

• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC B388B 02

Secondary Left Tail and Brake Lamp Circuit Short to Ground

DTC B388B 04

Secondary Left Tail and Brake Lamp Circuit Open

DTC B388C 02

Secondary Right Tail and Brake Lamp Circuit Short to Ground

DTC B388C 04

Secondary Right Tail and Brake Lamp Circuit Open

Diagnostic Fault Information

Circuit/System Description

When the headlamp switch is placed in either the park or low beam position, ground is applied to the tail lamp signal circuit to the body control module (BCM). The brake pedal position sensor provides an analog voltage signal to the BCM. The BCM sends this messages to the trailer lighting control module.

The trailer lighting control module responds by applying battery voltage to the left and right tail lamp control circuits illuminating the tail lamps.

Conditions for Running the DTC

• Battery voltage must be between 9 - 16 V.
• Brake/tail lamp ON.

Conditions for Setting the DTC

DTC B388B 02

The trailer lighting control module detects a short to ground in the secondary left tail and brake lamp control circuit.

DTC B388B 04

The trailer lighting control module detects an open in the secondary left tail and brake lamp control circuit.

DTC B388C 02

The trailer lighting control module detects a short to ground in the secondary right tail and brake lamp control circuit.

DTC B388C 04

The trailer lighting control module detects an open in the secondary right tail and brake lamp control circuit.

Actions Taken When the DTC Sets

The appropriate brake/tail lamp is inoperative.

Conditions for Clearing the DTC

• The condition responsible for setting the DTC no longer exists and the ignition is cycled.
• A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Reference Information

Schematic Reference

Exterior Lights Wiring Schematics

Connector End View Reference

COMPONENT CONNECTOR END VIEWS - INDEX

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information.

Circuit/System Testing

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the appropriate component listed below. It may take up to 2 min for all vehicle systems to power down.

• E51L Tail Lamp Assembly - Auxiliary Left
• E51R Tail Lamp Assembly - Auxiliary Right

2. Test for less than 5 Ω between the ground circuit terminal D and ground.

• If 5 Ω or greater

1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, repair the open/high resistance in the ground connection.

• Go to next step: If less than 5 Ω

3. Connect a test lamp between the control circuit terminal A and ground, ignition ON

4. Verify the test lamp turns ON and OFF when commanding the appropriate Auxiliary Tail and Brake Lamp test ON and OFF with a scan tool.

• If the test lamp is always OFF

1. Ignition OFF, disconnect the X1 harness connector at the K68 Trailer Lighting Control Module.
2. Test for infinite resistance between the control circuit and ground.
   • If less than infinite resistance, repair the short to ground on the circuit.
   • Go to next step: If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, replace the K68 Trailer Lighting Control Module.

• If the test lamp is always ON

1. Ignition OFF, disconnect the X1 harness connector at the K68 Trailer Lighting Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
   • If 1 V or greater, repair the short to voltage on the circuit.
   • If less than 1 V, replace the K68 Trailer Lighting Control Module.

• Go to next step: If the test lamp turns ON and OFF

5. Test or replace the appropriate E51 Tail Lamp Assembly - Auxiliary.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• Tail Lamp Replacement (5-Door Hatchback 68) Tail Lamp Replacement (5-Door Wagon 35)
• Control Module References for trailer lighting control module replacement, programming, and setup

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