B257A

DTC B257A

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* 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/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

The ignition is ON.

Conditions for Setting the DTC

The BCM receives a mismatch signal between the headlamps OFF signal circuit, the headlamp switch 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 conditions for setting the DTC are no longer present.
* A history DTC clears after 100 malfunction-free ignition cycles.

Circuit/System Verification

Ignition ON with the headlamp switch in the Auto position, observe the scan tool Exterior Lighting parameters listed below. All parameters should be Inactive.

* Auto Hdlp. Disable Sw.
* Headlamp Switch
* Parklamp Switch

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Ignition ON, test for B+ between the signal circuits listed below and ground.

* Headlamp switch headlamps ON signal terminal 1
* Park lamp switch ON signal terminal 2
* Headlamp switch OFF signal terminal 3

• If less than the specified value, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

3. Install a 3A fused jumper wire between the headlamp OFF 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.

4. Install a 3A fused jumper wire between the park lamp ON signal circuit terminal 2 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.

5. Connect a 3A fused jumper wire between the park lamp ON signal circuit terminal 2 and the headlamps ON signal circuit terminal 1 to ground. Verify the scan tool Headlamp Switch parameter is Active.

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

6. If all circuits test normal, replace the headlamp switch.

Component Test

Headlamp Switch

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Test for infinite resistance between the signal terminal 3 and the ground terminal 8 with the switch in the Auto position.

• If not the specified value, replace the headlamp switch.

3. Test for infinite resistance between the signal terminal 1 and the ground terminal 8 with the switch in the Auto position.

• If not the specified value, replace the headlamp switch.

4. Test for less than 2.0 ohms of resistance between the signal terminal 3 and the ground terminal 8 with the switch in the Off position.

• If not the specified value, replace the headlamp switch.

5. Test for less than 2.0 ohms of resistance between the signal terminal 1 and the ground terminal 8 with the switch in the Headlamp position.

• If not the specified value, replace the headlamp switch.

Repair Instructions

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

* Headlamp Switch Replacement