Code Definition

B2509 — Rear Fog Lamp Switch Circuit Failure

System: Body  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About B2509

B2509 is a generic (obd-ii standard) body diagnostic trouble code. It belongs to the B-prefix family of codes, which covers body, interior, airbag, or comfort-electronics faults.

When B2509 is stored, the module has detected an electrical-circuit fault in the wiring, connector, or driver associated with the named component.

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.

Because B2509 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Body Control Module (BCM) monitors the rear fog lamp switch input circuit to detect when the driver requests rear fog lamps. DTC B2509 is set when the BCM detects an out of range or implausible voltage on the rear fog lamp switch input, typically caused by an open in the switch circuit, a short to power, or a short to ground that the BCM cannot interpret as a valid switch position. The switch is usually a multifunction lighting stalk or a dedicated dash switch that pulls the input either to ground or to a reference voltage when activated. When this code is set the rear fog lamps may not op...

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