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

When the headlamp switch is in the AUTO position, the body control module (BCM) receives a ground signal through the headlamp switch headlamps OFF signal circuit and no signal on the headlamp switch headlamp ON signal circuit and headlamp switch park lamps ON signal circuit. When the headlamp switch is in the PARK or HEAD position, the BCM receives no signal through the headlamp switch headlamps OFF signal circuit and a ground signal through the headlamp switch headlamp ON signal circuit and headlamp switch park lamps ON signal circuit.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

The BCM receives a mismatched signal between the headlamp switch headlamps OFF signal circuit, the headlamps switch headlamps ON signal circuit, and the headlamp switch parklamps 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.
* The BCM receives the clear code command from the scan tool.

Circuit/System Verification

Ignition ON, headlamp switch to PARK, just the park lamps should illuminate. Headlamp switch to HEAD, the park lamps and headlamps should illuminate.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Ignition ON, verify the scan tool headlamp switch parameter is inactive.

^ If not the specified value, test the signal circuit terminal 1 for a short to ground. If the circuit tests normal, replace the BCM.

3. Install a 3A fused jumper wire between the signal circuit terminal 2 and the low reference circuit terminal 8. Verify the scan tool park lamp switch parameter is active.

^ If not the specified value, test the park lamp switch signal circuit for an open/high resistance. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, test or replace the headlamp switch.

Repair Instructions

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

* Headlamp Switch Replacement (Cadillac) Headlamp Switch Replacement (Except Cadillac)