AutoCodes – GMC – B3891 GMC

B3891 GMC 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 GMC 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.

Save to My Repairs My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 441

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
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code B3891 GMC 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 GMC - Trailer Fog Lamps Circuit - AutoCodes.com — B3891 GMC Code - Trailer Fog Lamps Circuit

What does this mean?

What are the Possible Causes of the DTC B3891 GMC?

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 GMC?

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 GMC 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.

Need more information about the B3891 GMC code?

Need more information on how to fix the B3891 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the B3891 GMC code diagnosed by a professional: Find a repair shop in your area

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 GMC Code

1. What are the common symptoms of OBDII code B3891 GMC related to the Trailer Fog Lamps Circuit?

The common symptoms include non-functional trailer fog lamps, erratic behavior of the fog lamps, or an illuminated malfunction indicator light on the dashboard.

2. What are the possible causes of OBDII code B3891 GMC for the Trailer Fog Lamps Circuit?

The code B3891 GMC can be caused by issues such as faulty wiring connections, a defective trailer fog lamp switch, a blown fuse, or a malfunctioning control module.

3. How can I diagnose the Trailer Fog Lamps Circuit issue related to code B3891 GMC?

To diagnose the issue, check the wiring connections for any damage, test the trailer fog lamp switch for proper functionality, inspect the fuses for continuity, and scan the control module for any fault codes.

4. Can a blown fuse cause OBDII code B3891 GMC to appear for the Trailer Fog Lamps Circuit?

Yes, a blown fuse related to the trailer fog lamps circuit can trigger the OBDII code B3891 GMC. It is recommended to check and replace any blown fuses to resolve the issue.

5. What repair tips can help fix OBDII code B3891 GMC associated with the Trailer Fog Lamps Circuit?

Repair tips include inspecting and repairing any damaged wiring, replacing a faulty trailer fog lamp switch, ensuring proper fuse functionality, and addressing any issues with the control module.

6. Is it safe to continue driving with OBDII code B3891 GMC for the Trailer Fog Lamps Circuit?

It is generally safe to drive with this code as it mainly affects the trailer fog lamps functionality. However, it is recommended to address the issue promptly to ensure proper operation of the fog lamps when needed.

7. Can a DIY enthusiast fix OBDII code B3891 GMC for the Trailer Fog Lamps Circuit?

Yes, DIY enthusiasts with automotive repair experience can attempt to fix this code by following diagnostic procedures, checking components, and making necessary repairs or replacements.

8. How important are the trailer fog lamps affected by OBDII code B3891 GMC?

Trailer fog lamps play a crucial role in enhancing visibility and safety when towing a trailer in adverse weather conditions. It is important to address the code promptly to ensure proper functioning of these lamps.

9. Will fixing OBDII code B3891 GMC require specialized tools or equipment?

While basic automotive tools may be sufficient for diagnosing and repairing the Trailer Fog Lamps Circuit issue, specialized tools like a multimeter or a diagnostic scanner may be helpful in pinpointing the exact cause of the problem.

10. How can I prevent OBDII code B3891 GMC from appearing again for the Trailer Fog Lamps Circuit?

Regularly inspecting the trailer fog lamp system, ensuring proper wiring connections, using quality components, and maintaining the electrical system can help prevent the recurrence of OBDII code B3891 GMC in the future.