B257A
DTC B257A
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC B257A 00
- Headlamp Switch Input Signals Correlation Malfunction
Diagnostic Fault Information
Circuit/System Description
When the headlamp switch is placed in the low beam or park position, the body control module (BCM) will receive a ground signal through the headlamp switch headlamps signal circuit and the park lamp switch signal circuit.
Conditions for Running the DTC
The ignition is in ON, ACCESSORY or RUN position.
Conditions for Setting the DTC
The BCM receives an unexpected signal along either the headlamp switch OFF circuit or the headlamp switch ON 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 BCM clears this code from current when the fault is removed from the system.
Reference Information
Schematic Reference
Headlights/Daytime Running Lights (DRL) Schematics Headlights/Daytime Running Lights (DRL) Schematics
Connector End View Reference
Component Connector End Views Hybrid/EV Battery Pack X1 A4
Description and Operation
Exterior Lighting Systems Description and Operation Exterior Lighting Systems
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Scan Tool Reference
Control Module References Control Module References for scan tool information
Circuit/System Verification
Ignition ON, turn the headlamp switch in the low beam position. The low beam lamps should be ON.
Turn the headlamp switch in the OFF position. The low beam lamps should be OFF.
Circuit/System Testing
1. Ignition OFF, disconnect the S78 Turn signal/Multi-Function Switch harness connector. Test for less than 5 ohm between the ground circuit terminal 6 and ground.
• If greater then the specified value, test the ground circuit for an open/high resistance.
2. Connect the S78 Turn signal/Multi-Function Switch harness connector.
3. Disconnect the X1 harness connector at the K9 Body Control Module.
4. Connect a test lamp between B+ and the signal circuit terminal 16.
5. Turn the headlamp switch between the low beam and OFF position. The test lamp should turn ON and OFF as the switch is cycled.
• If the test lamp is always OFF, test the signal circuit for a short to voltage or an open/high resistance. If the circuit test normal, replace the S78 Turn signal/Multi-Function Switch.
• If the test lamp is always ON, test the signal circuit for a short to ground. If the circuit test normal, replace the S78 Turn signal/Multi-Function Switch.
6. If all circuits test normal, replace the K9 Body Control Module.
Component Testing
Static Test
1. Ignition OFF, disconnect the S78 Turn signal/Multi-Function Switch harness connector. Turn the S78 Turn signal/Multi-Function Switch in OFF position, test for less than 5 ohm between the terminal 6 and terminal 9.
• If greater than the specified value, replace the S78 Turn signal/Multi-Function Switch.
2. Turn the S78 Turn signal/Multi-Function Switch in PARK position, test for less than 5 ohm between the terminal 6 and terminal 7.
• If greater than the specified value, replace the S78 Turn signal/Multi-Function Switch.
3. Turn the S78 Turn signal/Multi-Function Switch in low beam position, test for less than 5 ohm between the terminal 6 and terminal 2.
• If greater than the specified value, replace the S78 Turn signal/Multi-Function Switch.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
Control Module References Control Module References for trailer brake control module replacement, programming and setup