B1806 — Lamp Tail Output Circuit Failure

How do I diagnose B1806 myself?

Start with a scan: confirm B1806, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the lamp tail output circuit, and verify the suspect component against live data before buying parts.

How much does it cost to fix B1806?

BCM repair or replacement — $300–$900; Repair wiring or connectors — $80–$250; Replace the switch or actuator — $120–$450. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What does OBD2 code B1806 mean?

OBD2 code B1806 flags a problem with the body electrical system (lamp tail output circuit failure). The car is usually drivable while you plan the repair. Common culprits: body control module (BCM) fault, faulty switch, motor or actuator of the affected system, damaged wiring in door jambs or wet areas.

What are the symptoms of B1806?

Drivers most often notice: warning light on the dashboard, no impact on engine operation in most cases, the affected function (locks, windows, lighting, wipers) works intermittently or not at all.

Will B1806 clear itself?

Sometimes B1806 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

B1806 indicates the body control module has detected the tail lamp output circuit operating outside its designed parameters. This OBD2 code falls within the B-series classification, which handles body and chassis electrical systems rather than engine management. When the module sends an activation command to the tail lamp circuit and receives no response—or an incorrect response—it logs B1806 to flag the malfunction.

The tail lamp circuit receives power through a dedicated fuse and relay controlled by the body control module. When you engage the headlights or press the brake pedal, the BCM verifies the lamps illuminate as expected. If voltage does not reach the lamps, or if the module senses a short or open condition where none should exist, the fault threshold is exceeded. Most manufacturers require the condition to occur multiple times under specific operating parameters before the check engine light activates and the code solidifies in memory.

Common Symptoms of B1806

• The malfunction indicator lamp (MIL) illuminates on the instrument cluster
• One or both tail lamps fail to activate when the headlight switch is engaged
• Brake lights remain inoperative if they share the affected circuit
• Tail lamps stay on continuously without the lighting controls being activated
• Related body functions—such as interior lighting or dashboard dimming—behave intermittently
• Engine performance, fuel economy, and drivability remain unaffected in most cases
• Additional body electrical codes may accompany B1806 when a shared component is involved

What Causes B1806?

A blown fuse represents the single most frequent trigger for B1806. When the tail lamp circuit experiences a short—whether from damaged insulation, water intrusion, or a failed bulb—the excessive current blows the fuse to protect the wiring harness. Without a functioning fuse, the circuit cannot complete and the module logs the failure.

Corroded or loose electrical connectors rank as the second most common culprit. The tail lamp harness passes through areas prone to moisture accumulation, particularly in the trunk compartment and rear wheel wells. Moisture corrodes connector pins over time, creating high resistance that mimics an open circuit. Similarly, wiring that chafes against metal edges—especially near door hinges and trunk latches—eventually breaks down and interrupts current flow.

Damaged wiring in flex zones constitutes another frequent source of B1806. Each time you open or close a door, the wiring bundle inside the door jamb flexes. After countless cycles, individual wire strands fracture, creating intermittent connections that the module detects as faults. The body control module itself can fail internally, though this scenario occurs less frequently than external circuit problems.

How to Diagnose B1806

1. Connect an OBD2 scanner and retrieve all stored codes along with their freeze frame data. Recording the freeze frame helps you recreate the specific driving conditions—vehicle speed, engine temperature, load percentage—that triggered B1806.

2. Cross-reference technical service bulletins for your specific make, model, and model year. Many B-series codes have documented factory fixes or software calibrations available through the manufacturer or the NHTSA website by VIN search.

3. Perform a visual inspection of the tail lamp circuit wiring and connectors. Begin at the rear lamp housings and trace the harness toward the front, paying special attention to areas where wires bend around corners or pass through metal panels.

4. Test for voltage and ground at the tail lamp socket using a multimeter while an assistant activates the lighting controls. Compare your readings against specifications in your vehicle's service manual to identify open circuits, shorts to ground, or insufficient power supply.

5. Inspect the tail lamp fuse and relay for continuity and proper operation. Swapping the relay with an identical unit from another circuit quickly reveals whether the relay itself has failed.

6. Clear B1806, complete a full drive cycle that reproduces the freeze frame conditions, and verify the code does not return. A recurring fault confirms the repair was necessary; a code-free vehicle indicates the original condition was intermittent.

Repair Options for B1806

**