AutoCodes – Cadillac – B3891 Cadillac

B3891 Cadillac Code - Trailer Fog Lamps Circuit

Quick Answer

The Trailer Lighting Control Module receives a serial data message from the Body Control Module (BCM) indicating that the vehicle rear fog lamp have been activated. Common causes include Faulty Trailer Fog Lamps, Trailer Fog Lamps harness is open or shorted, Trailer Fog Lamps circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: B3891 Cadillac indicates Trailer Fog Lamps Circuit.
Common causes: Common causes include Faulty Trailer Fog Lamps, Trailer Fog Lamps harness is open or shorted, Trailer Fog Lamps circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 456

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Mar 24, 2020

Description
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
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Code B3891 Cadillac Description

The Trailer Lighting Control Module receives a serial data message from the Body Control Module (BCM) indicating that the vehicle rear fog lamp have been activated. The Trailer Lighting Control Module responds by applying battery voltage to the trailer fog lamp control circuit illuminating the trailer fog lamps. The Diagnostic Trouble Code (DTC) sets when the Trailer Lighting Control Module detects a short to ground or an open/high resistance in the trailer fog lamps control circuit.

OBDII Code B3891 Cadillac - Trailer Fog Lamps Circuit - AutoCodes.com — B3891 Cadillac Code - Trailer Fog Lamps Circuit

What does this mean?

What are the Possible Causes of the DTC B3891 Cadillac?

Faulty Trailer Fog Lamps
Trailer Fog Lamps harness is open or shorted
Trailer Fog Lamps circuit poor electrical connection
Faulty Trailer Lighting Control Module

How to Fix the DTC B3891 Cadillac?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

B3891 02 Trailer Fog Lamps Circuit Short to Ground
B3891 04 Trailer Fog Lamps Circuit Open

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the B3891 Cadillac code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about B3891 Cadillac Code

1. What does the OBDII code B3891 mean on a Cadillac?

B3891 indicates an issue with the Trailer Fog Lamps Circuit, typically related to a fault in the wiring, connectors, or control module managing the trailer fog lamp system.

2. What are the common symptoms of the B3891 code?

Symptoms may include non-functioning trailer fog lamps, a warning light on the dashboard, or error messages related to trailer lighting.

3. What causes the B3891 code to appear?

The code can be caused by damaged or corroded wiring, a faulty trailer fog lamp, defective connectors, or a malfunctioning control module.

4. Can the B3891 code affect other systems in my Cadillac?

While it primarily affects the trailer fog lamp circuit, issues with wiring or the control module could potentially interfere with other trailer lighting functions.

5. How do I diagnose a B3891 code on my Cadillac?

Start by inspecting the trailer fog lamp wiring and connectors for damage or corrosion. Use a multimeter to test circuit continuity and check for power at the lamp. If no issues are found, test or replace the control module.

6. Can I drive my Cadillac with a B3891 code?

Yes, the vehicle is typically drivable, but the trailer fog lamps may not function as intended, which could be a safety concern when towing in low visibility conditions.

7. How do I fix the B3891 code on my Cadillac?

Repairs may involve replacing damaged wiring, cleaning or replacing corroded connectors, installing a new trailer fog lamp, or replacing a faulty control module.

8. Is the B3891 code specific to Cadillac vehicles?

No, the code can appear on other vehicles with a trailer lighting system, but the diagnosis and repair procedure may vary depending on the make and model.

9. Do I need special tools to fix the B3891 code?

Basic tools like a multimeter and wiring diagrams are typically sufficient for diagnosing and repairing this issue. Advanced diagnostic tools may be needed to test the control module.

10. How much does it cost to repair a B3891 code on a Cadillac?

The cost depends on the root cause. Simple fixes like repairing wiring or replacing a lamp might cost under $100, while replacing a control module could cost several hundred dollars.