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9B-35 Lighting Systems:
DTC B257A
Diagnostic Instructions
• Perform the "Diagnostic System Check - Vehicle Diagnostic Information: " prior to using this diagnostic procedure.
• Review "Strategy Based Diagnosis: " for an overview of the diagnostic approach.
• "Diagnostic Procedure Instructions - Vehicle Diagnostic Information: " provides an overview of each diagnostic
category.
DTC Descriptor
DTC B257A 00: Headlamp Switch Input Signal Mismatch
Diagnostic Fault Information
Circuit
Headlamp Switch Off
Park Lamp Switch On
Headlamp Switch On
Headlamp Ground
1. Headlamps Always On - Low or High Beam
2. Headlamps Inoperative - Low Beam
Circuit/System Description
When the headlamp switch is in the OFF position, the
body control module (BCM) receives a ground signal
through the headlamp switch OFF circuit and an open
signal on the park lamp ON circuit. When the headlamp
switch is in the PARK or HEAD position, the BCM
receives an open signal through the headlamp switch
OFF circuit and a ground signal through the park lamp
ON circuit.
Conditions for Running the DTC
The ignition is ON.
Conditions for Setting the DTC
The BCM receives an unexpected signal along either the
headlamp switch OFF circuit or the park lamp 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"
Connector End View Reference
"Lighting Systems Connector End Views"
Electrical Information Reference
• "Circuit Testing: "
• "Connector Repairs: "
• "Testing for Intermittent Conditions and Poor
Connections: "
• "Wiring Repairs: "
Short to
Open/High
Ground
Resistance
B257A 00
B257A 00
B257A 00
B2057A 00
1
2
—
B257A 00
Scan Tool Reference
• "Scan Tool Output Controls - Lighting"
• "Scan Tool Data List - Lighting"
• "Scan Tool Data Definitions - Lighting"
Circuit/System Verification
Ignition ON, headlamp switch to PARK, just the park
lamps should illuminate.
Circuit/System Testing
1. Headlamp switch to OFF, disconnect the BCM
connector C2.
2. Ignition to ON, connect a test lamp between the
headlamp OFF switch circuit and the positive battery
terminal. Verify the test lamp illuminates.
➢ If the test lamp does not illuminate:
– Test for a short to voltage or an open in the
headlamp OFF switch circuit.
– Test for an open in the headlamp ground circuit.
– If the circuit/connections test normal replace the
headlamp switch.
3. Headlamp switch to PARK, verify the test lamp is not
illuminated.
➢ If the test lamp illuminates, test for a short to
ground in the headlamp OFF switch circuit. If the
circuit/connections test normal, replace the
headlamp switch.
4. Headlamp switch to PARK, connect a test lamp
between the park lamp switch ON circuit and the
positive battery terminal. Verify the test lamp
illuminates.
➢ If the test lamp does not illuminate, test for a short
to voltage or an open in the park lamp switch ON
circuit. If the circuit/connections test normal,
replace the headlamp switch.
Signal
Short to Voltage
Performance
B257A 00
B257A 00
B2575 00
2
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Troubleshooting
