Diagnostic Instructions

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

DTC Descriptor

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	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Secondary Left Tail and Brake Lamp	B388B 02	B388B 04	--	--
Secondary Right Tail and Brake Lamp	B388C 02	B388C 04	--	--

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 converter. The trailer lighting converter responds by applying battery voltage to the left tail lamp and to the right tail lamp control circuits. This energizes the tail lamps.

Conditions for Running the DTC

B388B 02, B388B 04, B388C 02 and B388C 04

The appropriate brake lamp/tail lamp signal is active

Conditions for Setting the DTC

B388B 02 or B388C 02

The control module detects a short to ground on the control circuit.

B388B 04 or B388C 04

The control module detects an open control circuit.

Conditions for Clearing the DTC

The condition for setting the DTC is not longer present.

Action Taken When the DTC Sets

The appropriate brake lamp/tail lamp is inoperative.

Reference Information

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON, perform the appropriate Auxiliary Tail and Brake Lamp test. The appropriate tail and brake lamps should be ON/OFF.

Circuit/System Testing

1. Ignition OFF, disconnect the appropriate harness connector at the tail lamp listed below:

•	E42L Tail lamp left

•	E42R Tail lamp right

2. Test for less than 5 Ω between the appropriate ground circuit harness connector terminal listed below and ground:

•	E42L Tail lamp left harness connector terminal D

•	E42R Tail lamp right harness connector terminal E

⇒	If greater than the specified value, test the appropriate ground circuit for an open/high resistance.

3. Connect a test lamp between the appropriate control circuit harness connector terminal listed below and ground :

•	E42L Tail lamp left harness connector terminal A

•	E42R Tail lamp right harness connector terminal A

4. Command the appropriate Auxiliary Tail and Brake Lamp test with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the appropriate control circuit for a short to voltage. If the circuit tests normal, replace the K68 Trailer Lighting Converter.

⇒	If the test lamp is always OFF, test the appropriate control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K68 Trailer Lighting Converter.

5. If all circuits test normal, test or replace the appropriate tail lamp.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.