Code Definition

B124B — Light Source Sensor

System: Body  |  Type: Manufacturer-Specific  |  Make: Lexus

About B124B

B124B is a manufacturer-specific body diagnostic trouble code recorded for Lexus vehicles. It belongs to the B-prefix family of codes, which covers body, interior, airbag, or comfort-electronics faults.

When B124B is stored, the module has detected a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: Body (B) codes are often stored in the body control module, airbag module, or climate module. A basic OBD-II tool usually will not read body codes — an enhanced or OEM-level scan tool is typically required.

B124B is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The Body Control Module (BCM), referred to on Lexus vehicles as the Main Body ECU, monitors the ambient light sensor (also called the light source sensor) to determine external lighting conditions for the automatic headlamp system and for interior illumination fade control. The sensor is a photo diode mounted on the upper dashboard near the windscreen and returns an analogue voltage proportional to ambient light intensity. DTC B124B is logged when the sensor signal is outside the expected operating range, such as an open circuit, short to ground, short to battery, or a reading that remains sta...

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