Buick Regal: DTC B258A (TT4)

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 B258A 01

Low Beam Control Circuit Short to Battery

DTC B258A 02

Low Beam Control Circuit Short to Ground

DTC B258A 04

Low Beam Control Circuit Open

Diagnostic Fault Information

* Right/Left Headlamp Low Beam Always ON
** Right/Left Headlamp Low Beam Always OFF

Circuit/System Description

The body control module (BCM) switches the headlamp low beams ON as soon it receives a signal from the headlamp switch or ambient light sensor. When this signal is received by the BCM, it applies a voltage trough the low beam relay and the low beam supply circuits which illuminates the low beams.

Conditions for Running the DTC

B258A 01 or B258A 02

The headlamp switch in OFF or AUTO position.

B258A 04

The headlamp switch in low beam position.

Conditions for Setting the DTC

B258A 01

The BCM detects a short to battery on the control circuit.

B258A 02

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

B258A 04

The BCM detects an open control circuit.

Action Taken When the DTC Sets

Headlamp low beam malfunction.

Conditions for Clearing the DTC

The condition for setting the DTC is no longer present.

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 Verification

Ignition ON, perform the appropriate Headlamp Low Beam Command test. The appropriate low beam lamp should be ON/OFF.

Circuit/System Testing

1. Ignition OFF, disconnect the KR49 Headlamp Low Beam Relay.

2. Connect a test lamp between the control circuit of KR49 Headlamp Low Beam Relay terminal 30 and ground.

3. Command the Headlamp Low Beam Command ON and OFF 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 control circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module.
• If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module.

4. Remove test lamp and reconnect the KR49 Headlamp Low Beam Relay.

5. Disconnect the connect harness of the appropriate headlamp. Connect a test lamp between the control circuit terminal 3 and ground.

6. Command the Headlamp Low Beam Command ON and OFF 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 control circuit for a short to voltage. If the circuit tests normal, replace the KR49 Headlamp Low Beam Relay.
• If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the KR49 Headlamp Low Beam Relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Relay Replacement (Attached to Wire Harness) Relay Replacement (Within an Electrical Center)
• Control Module References for BCM replacement, setup and programming

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