B257B

DTC B257B

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

DTC Descriptors
DTC B257B 03
- Lighting Control Switch Signal Low Voltage

DTC B257B 07
- Lighting Control Switch Signal High Voltage

Diagnostic Fault Information





Circuit/System Description

The body control module (BCM) supplies a reference voltage to the backlight dimmer switch, which is part of the headlamp switch. This switch control the fog lamps and the backlight dimming. When the backlight dimmer switch is placed in a desired brightness position, reference voltage is applied through the backlight dimmer switch resistor ladder to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage to the LED dimming supply circuit to all related interior lamps, illuminating them to the desired level of brightness.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

B257B 03

The BCM detects a voltage that is lower than 0.3 V on the lighting control switch signal circuit.

B257B 07

The BCM detects a voltage that is greater than 12.2 V on lighting control switch signal circuit.

Action Taken When the DTC Sets

* All cluster/backlighting defaults full bright.
* Steering wheel cluster/backlighting dimming controls inoperative.
* Front fog lamps are inoperative.

Conditions for Clearing the DTC

* The condition responsible for setting the DTC no longer exists.
* A history DTC will clear once 100 consecutive malfunction ignition cycles have occurred.

Reference Information
Schematic Reference

Interior Lights Dimming Schematics (Interior Lights Dimming Schematics)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
Description and Operation

Interior Lighting Systems Description and Operation (Interior Lighting Systems)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

Ignition ON, switch ON exterior lighting, vary the interior backlight from dim to full bright by using the dimmer switch. The interior backlighting should dim and then become full bright.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the S30 headlamp switch.
2. Ignition ON, test for greater than 10 V between the 12 V reference circuit terminal 1 and ground.

• If less than the specified range, test the 12 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 BCM.

3. Test for 0.4 Kohm to 1 Kohm between the signal circuit terminal 12 and ground.

• If greater than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 BCM.
• If less than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the K9 BCM.

4. If all circuits test normal, test or replace the S30 headlamp switch.

Repair Instructions

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

* Headlamp Switch Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming