Buick Regal: DTC B258A (TT6, TT2)

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 headlamp low beam relay is supplied with battery voltage at all times. The headlamp switch signal circuit is grounded by turning the headlamp switch. The body control module (BCM) energizes the headlamp low beam relay by applying ground to the headlamp low beam relay control circuit. When the headlamp low beam relay is energized, the relay switch contacts close and battery voltage is applied through the low beam lamp fuses to the low beam lamp supply voltage circuit which illuminates the low beam lamps.

Conditions for Running the DTC

B258A 01 or B258A 04

The headlamp switch in low beam position.

B258A 02

The headlamp switch in OFF or AUTO 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 X2 harness connector at the X50A Fuse Block - Underhood Primary.

2. Connect a test lamp between the B+ and the control circuit X50A Fuse Block - Underhood Primary terminal 24 X2.

3. Command the Low Beam Lamps 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 ground. 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 voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module.

4. If all circuits test normal, test or replace the X50A Fuse Block - Underhood Primary.

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

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