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 mismatched signal between the headlamps OFF signal circuit, the headlamp switch headlamps ON signal circuit, and the park lamp switch 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 condition responsible for setting the DTC no longer exists.
* A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.
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
* Park Lamp 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 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.
4. If all circuits test normal, test or replace the headlamp switch.
Component Test
Headlamp Switch
Important: If the switch tests open in any switch position other than Auto, test the switch signal circuits for a short to voltage before replacing the 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 Headlamps ON 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 (Impala).