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DTC B257C or B257D

DTC B257C or B257D

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 B257C 00

Left Headlamp Assembly Malfunction

DTC B257D 00

Right Headlamp Assembly Malfunction

Diagnostic Fault Information

Circuit Short to Ground Open/High Resistance Short to Voltage Signal Performance
Left Headlamp Low Beam Supply Voltage B257C 00 B257C 00 B257C 00 -
Right Headlamp Low Beam Supply Voltage B257D 00 B257D 00 B257D 00 -


Circuit/System Description

The headlamp control module calculates the headlamp angle and sends commands to the left and right adaptive forward lighting system headlamp actuators. The headlamp actuators drive the headlamps to the position commanded by the headlamp control module. The headlamp control module monitors the headlamp actuator motor control circuits for proper circuit continuity and for shorts to ground or voltage. If a malfunction is detected, a DTC will be stored in memory and the driver will be notified with a message displayed over the driver information center located on the instrument cluster.

Conditions for Running the DTC

The headlamps are turned ON.

Conditions for Setting the DTC

B257C 00

The headlamp control module detects a fault in the left low beam control circuit or an open/high resistance is the left low beam ground circuit.

B257D 00

The headlamp control module detects a fault in the right low beam control circuit or an open/high resistance is the right low beam ground circuit.

Action Taken When the DTC Sets

The appropriate headlamp is inoperative.

Conditions for Clearing the DTC


• The DTC will be current for as long as the fault is present.

• When the fault is no longer present, the DTC will be a history DTC.

• A history DTC will clear after 50 ignition cycles.


Reference Information

Schematic Reference

Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference

Component Connector End Views

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, observe the function of the low beam headlamps while commanding the Low Beam Lamps ON and OFF with a scan tool. The E4G headlamp-left low beam and the E4H headlamp-right low beam should turn ON and OFF while changing between the commanded states.

Circuit/System Testing


Left Low Beam Inoperative

1. Ignition OFF, disconnect the X118 inline harness connector at the E4G headlamp-left low beam.

2. Connect a test lamp between the control circuit terminal G and ground.

3. Ignition ON, command the Low Beam Lamps ON with a scan tool, the test should illuminate while commanding the headlamp ON.

4. If the test lamp does not illuminate and the circuit fuse is good, test the control circuit for an open/high resistance.

5. If the test lamp does not illuminate and the circuit fuse is open, test the control circit for a short to ground.

6. If all circuits test normal, replace the E4G headlamp-left low beam.



Right Low Beam Inoperative

1. Ignition OFF, disconnect the X120 inline harness connector at the E4H headlamp-right low beam.

2. Connect a test lamp between the control circuit terminal G and ground.

3. Ignition ON, command the Low Beam Lamps ON with a scan tool, the test should illuminate while commanding the headlamp ON.

4. If the test lamp does not illuminate and the circuit fuse is good, test the control circuit for an open/high resistance.

5. If the test lamp does not illuminate and the circuit fuse is open, test the control circit for a short to ground.

6. If all circuits test normal, replace the E4H headlamp-right low beam.



Headlamps Malfunction

1. Ignition OFF, disconnect the KR49 headlamp low beam relay.

2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 30 and ground.

3. If the test lamp illuminates, test the control circuit for a short to voltage

4. Verify that a test lamp illuminates between the B+ circuit terminal 87 and ground.

5. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

6. Verify that a test lamp illuminates between the B+ circuit terminal 86 and ground.

7. If the test lamp does not illuminate, test the B+ circuit for an open/high resistance.

8. Connect a 15A fused jumper wire between the B+ circuit terminal 87 and the control circuit terminal 30. Verify the E4G headlamp-left low beam and the E4H headlamp-right low beam are activated.

9. If the E4G headlamp-left low beam and the E4H headlamp-right low beam do not activate, test the control circuit for a short to ground or an open/high resistance.

10. Connect a test lamp between the B+ circuit terminal 86 and the control circuit terminal 85.

11. Ignition ON, 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.

12. If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the K9 BCM.

13. 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 BCM.

14. If all circuits test normal, test or replace the KR49 headlamp low beam relay.


Component Testing


Relay Test

1. Ignition OFF, disconnect the KR49 headlamp low beam relay.

2. Test for 60-180 Ω between terminals 85 and 86.

3. If not within the specified range, replace the KR49 headlamp low beam relay.

4. Test for infinite resistance between the following terminals:

• 30 and 86

• 30 and 87

• 30 and 85

• 85 and 87

5. If not the specified value, replace the KR49 headlamp low beam relay.

6. Install a 15A fused jumper wire between relay terminal 85 and 12 V. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 Ω between terminals 30 and 87.

7. If greater than the specified range, replace the KR49 headlamp low beam relay.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


• Headlamp Replacement (T84 T85) Headlamp Replacement (TT6 HID)

• Relay Replacement (Attached to Wire Harness) Relay Replacement (Within an Electrical Center)

• Control Module References for headlamp control module replacement, setup and programming