B257A

DTC B257A

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.



DTC Descriptor
DTC B257A 00
- Headlamp Switch Input Signal Mismatch




Diagnostic Fault Information






Circuit/System Description

The body control module (BCM) supplies a B+ reference voltage through each of the headlamp switch signal circuits to the headlamp switch. When the headlamp switch is in the AUTO position, all headlamp switch signal circuits to the BCM are open and the BCM operates in the automatic mode. When the headlamp switch is turned to the PARK position, only the park lamp switch signal circuit is pulled low signaling the BCM park lamps ON request. When the headlamp switch is turned to the HEAD position, the park lamp and headlamp switch signal circuits are both pulled low, signaling the BCM the headlamps ON request. When the headlamp switch is turned to the OFF position, only the headlamp OFF switch signal circuit is pulled low, signaling the BCM the lights off request.


Conditions for Running the DTC

* Battery voltage must be between 9-16 volts.
* Headlamp and panel dimmer switch cycled between the AUTO, PARK, HEAD, and OFF positions.



Conditions for Setting the DTC

The BCM receives a mismatch signal between the headlamps OFF signal circuit, the headlamps ON signal circuit, and the park lamps ON signal circuit.


Action Taken When the DTC Sets

The BCM disregards the headlamp switch input and defaults to automatic headlamp function.


Conditions for Clearing the DTC

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



Reference Information
Schematic Reference

Headlights/Daytime Running Lights (DRL) Schematics (Headlights/Daytime Running Lights (DRL) Schematics)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Exterior Lighting Systems Description and Operation (Exterior Lighting Systems Description and Operation)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information


Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the headlamp and panel dimmer switch.
2. Ignition ON, verify the scan tool Headlamp Switch parameter is Inactive.

• If not the specified value, test the signal circuit terminal 1 for short to ground. If the circuit tests normal, replace the BCM.

3. Verify the scan tool Auto Hdlp. Disable Sw. parameter is Inactive.

• If not the specified value, test the signal circuit terminal 3 for short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Parklamp Switch parameter is Inactive.

• If not the specified value, test the signal circuit terminal 5 for short to ground. If the circuit tests normal, replace the BCM.

5. Install a 3A fused jumper wire between the signal circuit terminal 1 and ground. Verify the scan tool Headlamp Switch parameter is Active.

• If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuits test normal replace the BCM.

6. Install a 3A fused jumper wire between the signal circuit terminal 3 and ground. Verify the scan tool Auto Hdlp. Disable Sw. parameter is Active.

• If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

7. Install a 3A fused jumper wire between the signal circuit terminal 5 and ground. Verify the scan tool Parklamp Switch parameter is Active.

• If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

8. If all circuits test normal, test or replace the headlamp and panel dimmer switch.



Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Headlamp Switch Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming