OBD Code B1806: Lamp Tail Output Circuit Failure

Q: What does OBD code B1806 mean?

Lamp Tail Output Circuit Failure. The tail light circuit isn't working properly—the ECU can't detect voltage or current flowing through the tail lamp output. Think of it like a light switch that's either broken or not connected to the bulb.

Q: What are the symptoms of fault code B1806?

Tail lights not illuminating or dimly lit, Dashboard warning light or message indicating lighting fault, One or both tail lights inoperative while other vehicle lights function normally

Q: How do I fix OBD fault code B1806?

Replace burnt-out or failed bulbs in the affected tail light assembly.. Inspect and reseat loose or corroded connectors at the tail light and body harness.. Test and replace the lighting control module or relay if electrical continuity is broken.

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What This Actually Means

In plain language — no jargon

The tail light circuit isn't working properly—the ECU can't detect voltage or current flowing through the tail lamp output. Think of it like a light switch that's either broken or not connected to the bulb.

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Symptoms You May Notice

3 known symptoms for this code

Tail lights not illuminating or dimly lit

Dashboard warning light or message indicating lighting fault

One or both tail lights inoperative while other vehicle lights function normally

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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the tail lamp output circuit by measuring current draw and voltage feedback through the lamp control module. When you activate the lights, the ECU expects to detect a specific current range indicating proper bulb operation. If current is absent, shorted, or out of range, the fault is triggered.

Voltage & Parameter Thresholds

Parameter	Normal Range	Fault Condition
Tail Lamp Current Draw	0.5–2.5 amps per circuit	<0.1 amps or open circuit detected
Output Voltage	11–14.5 volts at full load	0 volts or shorted to ground

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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex

1

Tail lamp bulb

Replace burnt-out or failed bulbs in the affected tail light assembly.

2

Wiring harness and connectors

Inspect and reseat loose or corroded connectors at the tail light and body harness.

3

Tail lamp relay or module

Test and replace the lighting control module or relay if electrical continuity is broken.

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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code B1806 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.

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How to Clear Code B1806

What happens after you fix the fault

Once the fault is repaired, B1806 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When

Fault has been diagnosed and repaired
You want to confirm the repair worked
Code appeared after a sensor was cleaned

⚠️ Do Not Clear When

Preparing for an emissions/PUC test
Root cause is still undiagnosed
Check engine light is flashing

Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.