B1808 Tail Lamp Circuit Short to Battery
The B1808 code indicates a short to battery voltage in the tail lamp output circuit. This manufacturer-specific code is triggered when the Body Control Module (BCM) or lighting module detects an abnormally high voltage on the tail lamp control circuit, exceeding the expected threshold during normal operation. A short to battery means the control wire is in direct contact with a constant positive voltage source, bypassing the module's pulse-width modulation (PWM) or switching control.
1. Commonly Affected Makes
2. Most Common Causes
- Damaged tail lamp circuit wiring insulation shorting to a battery power wire
- Internally damaged tail lamp relay or lighting control module with a shorted output transistor
- Corroded or bent pins in the tail lamp harness connector causing circuit contact
- Tail lamp with an internal filament short or deformed metal base
- Incorrect aftermarket electrical modification to the rear lighting system
3. Symptoms You Will Notice
- Tail lamp stays on continuously or operates erratically
- Blown tail lamp fuse or repeated fuse failure
- BCM or lighting module malfunction affecting other lighting functions
4. Step-by-Step Diagnosis
Visually inspect tail lamp wiring and connectors for damage, corrosion, or bent pins
Check for aftermarket modifications to the rear lighting system
Measure voltage at the tail lamp control circuit with a multimeter; compare to expected PWM signal
Test the tail lamp bulb and socket for internal shorts
Inspect the BCM or lighting module for signs of internal failure; perform module self-test if available
5. Frequently Asked Questions
Is it safe to drive?▼
Driving with this code is not recommended as it may cause a short circuit, blow fuses, or damage the BCM. The tail lamp may not function properly, reducing visibility and safety.
How much does it cost to fix?▼
Repair costs typically range from $100 to $400, depending on the cause. Simple wiring repairs may cost $100-$200, while replacing a BCM or lighting module can be $300-$400 or more.