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DTC B257B

DTC B257B

Diagnostic Instructions


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

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• 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 Short to Ground Open/High Resistance Short to Voltage Signal Performance
Headlamp Switch B+ Reference B257B 03* B257B 03* - -
Instrument Panel Lamps Dimmer Switch Signal B257B 03* B257B 03* B257B 07* -
* Interior backlighting at full intensity and cannot be dimmed


Circuit/System Description

The body control module (BCM) supplies a reference voltage to the backlight dimmer switch, which is part of the headlamp switch. When the backlight dimmer switch is placed in a desired brightness position, reference voltage is applied 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 dimming voltage circuit.

B257B 07

The BCM detects a voltage that is greater than 12.2 V on dimming voltage circuit.

Action Taken When the DTC Sets

The BCM will not dim or illuminate any LED backlighting supplied by the backlight lamp control circuits.

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

Connector End View Reference

Component Connector End Views

Description and Operation

Interior Lighting Systems Description and Operation

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON, headlamps ON, vary the interior backlight dimming from low to full bright by using the dimmer switch. The interior backlighting should start 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 B+ between the B+ reference circuit terminal 1 and ground.

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

4. Ignition ON, test for less than 0.3 V between signal circuit terminal 12 and ground.

5. If greater than the specified range, test the signal circuit pin 12 for a short to voltage. If the circuit tests normal, replace the BCM K9.

6. Reconnect the harness connector at the S30 Headlamp Switch.

7. Dim the interior backlighting with the S30 Headlamp Switch.

8. Disconnect the harness connector at the S30 Headlamp Switch.

9. Install a 3 A fused jumper wire between the signal circuit terminal 12 and the B+ reference circuit terminal 1. The interior backlighting should now be dimmed.

10. If the interior backlighting is not dimmed, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 BCM.

11. If all circuits test normal, replace the S30 Headlamp Switch.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


• Headlamp Switch Replacement

• Control Module References for BCM replacement, programming and setup