GMC Sierra Trailer Lights Not Working: Solutions For A Frustration-Free Towing Experience

Are you experiencing issues with your GMC Sierra’s trailer lights not working? This common problem can be frustrating, but worry not! We’ve got you covered with this blog post. 

The most likely culprit is the K68 Trailer Lighting Control Module, which controls all trailer lighting through a pulse width modulated (PWM) Voltage.

Read on as we delve into the possible causes, troubleshooting tips, and solutions to get your Sierra’s trailer lights back in working order.

Understanding the K68 Trailer Lighting Control Module on GMC Sierra

GMC Sierra Trailer Lights Not Working

The K68 Trailer Lighting Control Module is a vital component in GMC Sierra models, particularly those manufactured in 2019 and later with the U1D equipment package. 

This module plays a crucial role in managing and regulating the trailer lighting system. To gain a deeper understanding, let’s break down its features and functions:

Pulse Width Modulation (PWM) Voltage Control

The K68 module uses PWM voltage control to manage the lighting signals sent to the trailer. 

PWM is a technique that involves turning the voltage on and off rapidly, simulating varying voltage levels.

This method allows for precise control of the lighting intensity and can help maintain consistent brightness across all trailer lights.

Current Handling Capability

The K68 module is designed to handle a current of approximately 9 amps on the running lamp circuit.

This current capacity is generally sufficient for most trailer lighting systems. 

However, trailers with a large number of incandescent lights may require more current, leading to potential issues with the module’s capacity to handle the in-rush current.

Safety Features

The K68 module also incorporates safety features to protect the lighting system from potential damage. 

For instance, if it detects a short circuit or overload, the module will shut down the affected circuit to prevent further damage.

Compatibility with Vehicle Systems

The K68 Trailer Lighting Control Module is designed to work seamlessly with other vehicle systems, such as Sierra’s onboard diagnostics and the towing package’s various features.

This integration ensures that the trailer lights function correctly and provide crucial information to the driver.

6 Common Causes Of GMC Sierra Trailer Lights Not Working

GMC Sierra Trailer Lights Not Working

Trailer light issues in GMC Sierra models can result from various factors. Understanding these common causes can help you identify and address the problem more effectively. 

Here are some of the primary reasons for trailer light malfunctions and possible solutions for how you can fix that:

#-1. Incompatible Lighting Systems

The K68 Trailer Lighting Control Module in GMC Sierras is designed to work with a specific range of lighting systems. 

If your trailer has a large number of incandescent lights, the in-rush current may exceed the module’s 9-amp capacity, leading to intermittent lighting issues or complete failure.

Ensure that your trailer’s lighting system is compatible with the K68 Trailer Lighting Control Module in your GMC Sierra.

If necessary, consider upgrading your trailer lights to a system with lower in-rush current or use an adapter specifically designed for LED trailer lights, like the Trailer Connector Adapter w/ Lamp-Out Sensor Bypass

#2. Faulty Wiring

Damaged or corroded wiring can cause poor electrical connections, resulting in inconsistent or non-functioning trailer lights.

Inspect the wiring harness for visible signs of wear, corrosion, or damage.

Repair or replace any damaged sections, and ensure all connections are secure and free from corrosion.

#3. Grounding Issues

A weak or faulty ground connection can lead to erratic trailer light behavior. Ensure that the ground wire is securely connected to the trailer’s frame and that there is no corrosion or damage at the contact points.

Check the ground wire and its connection to the trailer frame. Clean and tighten the connection, and remove any corrosion to ensure proper ground.

#4. Burned-out Bulbs or Blown Fuses

Check the trailer light bulbs and fuses for any signs of damage or burnout. If you find any damaged or burned-out components, replace them to restore functionality.

#5. Vehicle Sensor Issues

Some GMC Sierra models require a load to be sensed before sending power to the 7-way connector. 

If the vehicle doesn’t detect a load, it may not send power to the trailer lights. In this case, using an older vehicle with a 7-way connector or a 12v battery may help.

If your GMC Sierra model requires a load to be sensed before sending power to the 7-way connector, consider using an older vehicle with a 7-way connector or a 12v battery to bypass this requirement.

#6. Faulty Connectors

Check the 7-way connectors on both the vehicle and the trailer for damage or corrosion, which may hinder proper electrical connections. Clean or replace the connectors if necessary.

Examine the 7-way connectors on both the vehicle and the trailer for damage or corrosion. Clean the connectors using a specialized electrical contact cleaner or replace them if necessary.

gmc sierra Faulty Connectors

By following these troubleshooting steps and solutions, you can effectively address common trailer light issues in your GMC Sierra. 

Regular maintenance and inspection of your trailer’s lighting system will help ensure safe, reliable performance during your travels.


As you navigate the complexities of trailer lighting issues with your GMC Sierra, you may have additional questions that have not been covered in this blog. 

In this FAQ section, we aim to address those concerns with concise and helpful information.

Let’s dive into four common questions that may arise while troubleshooting your trailer lights.

How do I check for proper grounding in my trailer lighting system?

To check for proper grounding, inspect the ground wire connections at the trailer frame, ensuring they are clean and free of corrosion. If necessary, sand the connections to create better contact.

Can I use aftermarket LED lights on my GMC Sierra trailer without issues?

Yes, you can use aftermarket LED lights as long as they are compatible with your vehicle’s K68 Trailer Lighting Control Module. Ensure you choose high-quality, reliable LED lights for optimal performance.

What should I do if only one side of the trailer lights is not working?

If only one side is not working, inspect the wiring and connections on the affected side, and check for any blown fuses. Replace or repair any damaged components as needed.

How often should I inspect and maintain my trailer lighting system?

Regular inspections and maintenance of your trailer lighting system should be conducted at least once a year or before any long trips to ensure safety and optimal performance on the road.


Addressing trailer light issues in your GMC Sierra is crucial for maintaining safety and visibility on the road. 

By being proactive in identifying and resolving problems, you can enjoy a seamless towing experience. 

We recommend that you conduct regular inspections and maintenance of your trailer’s lighting system to prevent issues from arising. 

Additionally, investing in high-quality, compatible lighting systems, adapters, and connectors can further enhance the reliability and performance of your vehicle and trailer. Stay safe, and happy towing!

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