B257A

DTC B257A

DTC Descriptor
DTC B257A 00
- Headlamp Switch Input Signal Mismatch

Diagnostic Fault Information

Important: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview





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.

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.

5. Headlamp switch to OFF, verify the test lamp does not illuminate.

• 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.

6. If the circuit/connections test normal, replace the BCM.

Repair Instructions

Important: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.

* Headlamp Switch Replacement (Impala) Headlamp Switch Replacement (Monte Carlo)
* Programming and Relearning Programming and Relearning