GM Service Trailer Brake Bulletin for Silverado EV & Sierra EV

GM has issued a service bulletin that may help explain “Service Trailer Brake” messages on several Chevrolet, GMC, and Cadillac trucks and SUVs, including the Silverado EV, Sierra EV, Hummer EV, Escalade IQ, and other towing-capable GM vehicles.

The bulletin, listed as PIT6523, is titled “Diagnostic Tip: K67 Trailer Brake Control Module DTC C1114 C159C C1158 C1596” and was released in June 2026.

This is not a recall. It is a GM service bulletin intended to guide dealer diagnosis when a customer reports a “Service Trailer Brake” message or a related driver information center message.

What vehicles are listed in the bulletin?

The bulletin applies to a wide range of GM vehicles in North America, including:

  • 2024-2026 Chevrolet Silverado EV
  • 2024-2026 GMC Sierra EV
  • 2022-2026 GMC Hummer EV Pickup
  • 2024-2026 GMC Hummer EV SUV
  • 2025-2026 Cadillac Escalade IQ
  • 2026 Cadillac Escalade IQL
  • 2023-2026 Chevrolet Colorado
  • 2023-2026 GMC Canyon
  • 2020-2026 Chevrolet Silverado HD
  • 2021-2026 Chevrolet Silverado LD
  • 2020-2026 GMC Sierra HD
  • 2021-2026 GMC Sierra LD
  • 2025-2026 Chevrolet Tahoe and Suburban
  • 2025-2026 GMC Yukon and Yukon XL
  • 2025-2026 Cadillac Escalade and Escalade ESV

As always, owners should not assume the bulletin applies to their specific vehicle just because the model is listed. A GM dealer can check the vehicle and confirm whether this bulletin is relevant to the concern.

What is the issue?

Some customers may see a “Service Trailer Brake” message or a similar message on the driver information center.

GM says engineering is actively working on a design improvement related to diagnostic trouble codes C1114, C159C, C1158, and C1596.

In plain English, this bulletin is about helping dealers determine whether the trailer brake warning is coming from the vehicle, the trailer, the trailer connector, the wiring, or the trailer brake control module itself.

Why this matters for owners who tow

The trailer brake system is not something to ignore. If you are towing a travel trailer, equipment trailer, boat trailer, enclosed trailer, or anything heavy enough to rely on trailer brakes, a trailer brake warning can affect towing confidence and stopping performance.

If the message appears while towing, you should treat it seriously and avoid assuming it is just a harmless warning.

What owners can check first

This bulletin is written for technicians, but there are still a few practical things owners can do before going to the dealer.

1. Pay attention to when the warning appears

Try to note whether the “Service Trailer Brake” message appears:

  • Only when a trailer is connected
  • Even when no trailer is connected
  • Right after plugging in the trailer
  • After driving in rain or wet conditions
  • Only with one specific trailer
  • With multiple trailers

This information can help the dealer separate a vehicle-side issue from a trailer-side issue.

2. Inspect the trailer plug and 7-pin connector

Before heading to the dealer, check the trailer connector for obvious issues:

  • Loose connection
  • Corrosion
  • Water intrusion
  • Damaged pins
  • Debris inside the connector
  • A trailer plug that does not fully seat into the truck’s connector

A poor connection at the trailer plug can create trailer brake messages even when the truck itself is operating normally.

3. Look over the trailer wiring

If the warning only happens with one trailer, inspect the trailer-side wiring as best you can. Look for damaged wiring near the tongue, loose grounds, frayed wires, exposed conductors, pinched wiring, or wiring that may have been damaged by road debris.

If your trailer has electric brakes, a weak ground or wiring problem can create symptoms that look like a truck problem.

4. Check whether the problem follows the trailer

If you have safe access to another compatible trailer, or if the trailer can be checked by a trailer shop, it may help to determine whether the problem follows the trailer or stays with the vehicle.

If the same truck only has the warning with one specific trailer, that points more toward the trailer or trailer wiring. If the warning appears with multiple trailers, the truck side becomes more suspect.

5. Do not keep towing heavy if trailer brakes may not be working

If you are towing a heavy trailer and the truck shows a trailer brake warning, use caution. Trailer brakes are a major part of safe towing, especially with larger travel trailers or cargo trailers.

If braking performance feels abnormal, stop towing when safe and have the system inspected.

What the dealer may do

GM’s bulletin gives dealers a step-by-step diagnostic path. The main takeaway is that GM does not want technicians to immediately replace the trailer brake control module without checking the rest of the system first.

1. Check for specific diagnostic trouble codes

The dealer may check whether any of the following DTCs are present as current or history codes:

  • C1114
  • C159C
  • C1158
  • C1596

If none of those codes are present, the bulletin says no issue has been identified related to the K67 Trailer Brake Control Module.

2. Perform normal service diagnostics

If the codes are present, the dealer is directed to perform GM service diagnostics for the applicable code.

If a fault is found and corrected during those diagnostics, the dealer is instructed to clear the codes, verify the repair, and return the vehicle to the customer without replacing the K67 Trailer Brake Control Module.

3. Inspect trailer connector and wiring

The bulletin also directs dealers to confirm whether another GM bulletin, 21-NA-155, related to trailer connector and wiring inspection, has been performed.

If an issue is found during the trailer connector or wiring inspection, the dealer is instructed to repair it, clear the codes, verify the repair, and not replace the trailer brake control module.

4. Use a trailer presence simulator tool

GM also tells dealers to use the EL-52641 Trailer Presence Simulator Tool. This helps determine whether the concern appears to come from the trailer side or the vehicle side.

If the issue appears to originate from the trailer, the bulletin tells dealers to educate the customer on the trailer concern and return the vehicle. Again, GM says not to replace the K67 Trailer Brake Control Module in that case.

5. Replace the trailer brake control module only after other checks

The K67 Trailer Brake Control Module may be replaced only if the concern remains after the diagnostic steps have been completed and the diagnostic trouble codes reset or cannot be cleared.

That is an important detail. GM’s process appears designed to prevent unnecessary module replacement when the real issue may be trailer wiring, connector condition, or a trailer-side problem.

What to tell your dealer

If you are seeing a “Service Trailer Brake” message, it may help to give the dealer a clear summary rather than just saying the warning came on.

Useful details include:

  • Your vehicle model and model year
  • Whether the warning appears with or without a trailer connected
  • The type of trailer you were towing
  • Whether the trailer uses electric brakes
  • Whether the warning appears with more than one trailer
  • Photos of the warning message, if available
  • Whether the issue happens after rain, washing, or wet roads
  • A request that the dealer review GM bulletin PIT6523

Bottom line

GM bulletin PIT6523 gives dealers a structured process for diagnosing “Service Trailer Brake” messages on many GM towing vehicles, including the Silverado EV and Sierra EV.

For owners, the practical takeaway is that the warning may not automatically mean the trailer brake control module has failed. It could be related to the trailer, trailer wiring, the 7-pin connector, or the vehicle-side trailer brake system.

If you tow and see this warning repeatedly, document when it happens, inspect the trailer connector and wiring for obvious issues, and schedule service with a GM dealer. Ask whether bulletin PIT6523 applies to your vehicle and concern.

Got questions? Drop a comment I love hearing from all of you.

If you found this useful:

Source: GM PIT6523

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.

Ken, EV Outdoors

Ken, EV Outdoors

I'm an EV analyst focused on a real-world, objective take on electric vehicle ownership. I've been driving EVs for over a decade and have owned 8 across multiple brands, with seat time in many more. EV Outdoors is independent analysis of EV towing, road-tripping, and the realities manufacturers leave out of the brochure.

Subscribe to EV Outdoors →

EV Outdoors is independent and unsponsored. Some links may be affiliate links; this never changes the assessment.