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B1809 Manufacturer-specific Basic entry

Lamp Tail Output Circuit Short To Ground

Updated 2026-10-02 · Body (manufacturer-specific)

Severity
Low
Typical repair cost
40 – 350 EUR
System
Body

This is a manufacturer-specific code. Its exact meaning can differ between makes; the definition below follows the most common usage.

What it means

B1809 is stored when the body control module measures a voltage in the Lamp Tail Output circuit that stays below the expected minimum. In practice this points to a short to ground, a damaged wire or a failed component rather than a mechanical problem.

This entry is compiled automatically from the standard definition and typical diagnostic practice for this type of fault. A detailed, reviewed article is in preparation.

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Symptoms

  • The affected function (lighting, locking, windows, seats, climate or restraint) does not work or works intermittently
  • Warning light or message on the instrument cluster
  • Possible battery drain if a module stays awake

Common causes

  1. Damaged, chafed or corroded wiring in the Lamp Tail Output circuit
  2. Corroded, loose or water-damaged connector terminals
  3. Failed Lamp Tail Output component (sensor, solenoid or actuator)
  4. Poor ground connection for the module or component
  5. Faulty control module driver (rare, confirm all else first)

How to diagnose

  1. Read freeze frame data and note when the code set (cold start, under load, after rain)
  2. Inspect the Lamp Tail Output connector for bent pins, corrosion and water; wiggle the harness with the engine running and watch live data
  3. Measure supply voltage and ground at the connector with the key on
  4. Check the signal wire for shorts to ground or battery and for continuity to the control module
  5. Test or substitute the Lamp Tail Output only after wiring and connectors are confirmed good
  6. Clear the code and complete a drive cycle to confirm the repair

Frequently asked questions

Can I keep driving with B1809?

Yes. The fault does not affect safety or driveability, but it will keep the warning light on and may fail an inspection.

Will clearing B1809 fix it?

No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.

Related codes

B1806B1807B1808

Nearby codes in this group