Code Definition

B1809 — Lamp Tail Output Circuit Short To Ground

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

About B1809

B1809 is a manufacturer-specific body diagnostic trouble code. It belongs to the B-prefix family of codes, which covers body, interior, airbag, or comfort-electronics faults.

When B1809 is stored, the module has detected a short-circuit (voltage pulled to ground or to supply) in the monitored circuit.

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.

B1809 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) controls the tail lamp output and monitors the circuit by checking current draw and the voltage state on the controlled side of the driver. DTC B1809 sets when the BCM detects that the tail lamp output line is being pulled to ground at a time when the output should be at battery voltage during the on state, or floating during the off state, indicating that the wiring or load has shorted to chassis ground somewhere between the BCM driver and the lamp supply path. To protect the internal switching transistor, the BCM disables the output once the short is detected si...

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