GM Service Manual Online
For 1990-2009 cars only

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 Descriptor

DTC B257A 00: Headlamp Switch Input Signal Mismatch

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Headlamp Switch Off

B257A 00

B257A 00

B257A 00

--

Park Lamp Switch On

B257A 00

B2057A 00

B257A 00

--

Headlamp Switch On

1

2

B2575 00

--

Headlamp Ground

--

B257A 00

2

--

  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

Scan Tool Reference

    •  Scan Tool Output Controls
    •  Scan Tool Data List
    •  Scan Tool Data Definitions

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.
  3. 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.
  4. Headlamp switch to PARK, verify the test lamp is not illuminated.
  5. 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.
  6. 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.
  7. 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.
  8. Headlamp switch to OFF, verify the test lamp does not illuminate.
  9. If the test lamp does illuminate, test for a short to ground in the park lamp switch ON circuit. If the circuit/connections test normal, replace the headlamp switch.
  10. If all circuit/connections test normal, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Control Module References for BCM replacement, setup, and programming
    •  Turn Signal Multifunction Switch Replacement