For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diangostic 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 | B257A 00 | B257A 00 | --
Headlamp Switch On | B257A 00 | B257A 00 | B257A 00 | --
Headlamp Switch Ground | -- | B257A 00 | -- | --

### 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 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.

### Reference Information

#### Schematic Reference

Headlights 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. Headlamp switch to HEAD, the park lamps and headlamps should illuminate.

### Circuit/System Testing

Circuit | AUTO | PARK | HEAD
--- | --- | --- | ---
Headlamp Switch Headlamps Off Signal BCM Terminal 17 | Ground | Open | Open
Headlamp Switch Park Lamps Signal BCM Terminal 8 | Open | Ground | Ground
Headlamp Switch Headlamps On Signal BCM Terminal 12 | Open | Open | Ground

1. Ignition OFF, disconnect the BCM connector C2 and start with the headlamp switch in the AUTO position. Use the chart above to determine when and where a ground signal should be present. Test each connector circuit listed in the chart for the proper signal.
2. If voltage is present on any circuit, test that circuit for a short to voltage.

⇒ | If the circuit/connections test normal, replace the headlamp and panel dimmer switch.

3. If no ground signals are present, test the headlamp switch ground circuit for an open.

⇒ | If the circuit/connections test normal, replace the headlamp and panel dimmer switch.

4. If all signal functions of the headlamp and panel dimmer switch circuits test normal, replace the BCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Headlamp Switch Replacement

• | Control Module References for BCM replacement, setup, and programming