C1114 on 2021-2025 GMC Sierra 3500: Trailer Brake System Fault Causes and Fixes

What's Unique About the 2021-2025 Gmc SIERRA 3500

The T1 platform (2020+) Sierra HD features a highly sensitive trailer brake control system designed to be compatible with multiple trailer brake types. This sensitivity makes it prone to setting code C1114 when connected to trailers with their own power systems (like batteries, generators, or solar chargers) or specific types of Electric-Over-Hydraulic (EOH) actuators. GM has issued multiple Technical Service Bulletins (TSBs) acknowledging that voltage can back-feed from the trailer to the truck, causing the system to register a fault. The problem is so specific that trailer parts manufacturers, like Dexter, have developed adapter modules in collaboration with GM to resolve the incompatibility. Furthermore, TSB 23-NA-149 and Customer Satisfaction Program N232414840 address the voltage back-feed issue from trailer batteries by recommending the replacement of the truck's Trailer Brake Power Module with an updated part.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• "Service Trailer Brake System" message on the Driver Information Center (DIC).
• "Trailer Brake Disabled" message on the DIC.
• "Check Trailer Wiring" message on the DIC.
• Integrated trailer brake controller is inoperative and gain cannot be adjusted.
• No braking force applied to trailer wheels when using manual override or vehicle brakes.
• Code C1114 stored in the Chassis Control Module (CCM).

Most Likely Causes

1. Trailer with Onboard Battery, Generator, or Solar Charger 🔴 High Probability If the trailer's battery is at a higher state of charge than the truck's battery, voltage can back-feed into the Trailer Brake Power Module (TBPM), which interprets this as a fault and shuts down. This is a known issue addressed by GM Customer Satisfaction Program N232414840.
   How to confirm: The issue occurs only when connected to a trailer with its own power source. The DIC may display "Trailer Brake Disabled." The issue is specifically described in GM TSB #23-NA-149 and program N232414840.
   Typical fix: Per TSB #23-NA-149, the official fix is to replace the truck's original Trailer Brake Power Module with an updated part (GM P/N 85073615) designed to tolerate this voltage feedback.
   Est. part cost: $100-$160
2. Incompatible Electric-Over-Hydraulic (EOH) Trailer Brakes 🔴 High Probability Certain EOH systems, particularly older Hydrastar or Dexter models, can send a voltage spike or present an electrical signature that the GMC's Chassis Control Module misinterprets as a fault.
   How to confirm: The problem only occurs with a specific EOH trailer. The issue is documented in TSB #PIT6212D for Hydrastar systems and acknowledged by Dexter for their E/H actuators.
   Typical fix: Install a manufacturer-recommended compatibility adapter on the trailer's wiring. For Dexter systems, this is part number K58-036-00, as noted in TSB PIT5509J.
   Est. part cost: $90-$120
3. Poor Connection at 7-Pin Connector 🟡 Medium Probability The system's sensitivity can interpret intermittent connections from corrosion, dirt, spread terminals, or non-SAE compliant trailer plugs as a circuit fault. GM specifically recommends using high-quality Pollak brand trailer-side plugs to ensure a robust connection.
   How to confirm: Visually inspect both the truck and trailer 7-pin connectors for green/white corrosion, bent pins, or debris. Wiggle the connector while plugged in to see if the error message appears.
   Typical fix: Clean both sides of the connector with a contact cleaner and a terminal brush. Apply dielectric grease to prevent future corrosion. Replace the trailer-side plug if it is worn or low-quality. If the truck's socket is worn, replace it with an OEM part.
   Est. part cost: $15-$80
4. Trailer Wiring Fault (Shorts or Poor Ground) 🟡 Medium Probability Many trailers have wiring that is exposed and not built to automotive standards, making it susceptible to chafing, corrosion, and bad splices that can cause shorts or poor grounds which the sensitive T1 platform detects immediately.
   How to confirm: A 'Service Trailer Brake System' message often indicates a short to power or a bad ground, while a 'Check Trailer Wiring' message often points to a short to ground on the blue wire. Test by connecting to a known-good trailer; if the code disappears, the fault is in the trailer's wiring.
   Typical fix: Inspect the entire length of the trailer's wiring harness, especially the brake circuit (blue wire) and ground circuit (white wire). Pay close attention to the ground connection point on the trailer frame; clean it to bare metal. Repair any chafed wires and replace any low-quality splices with soldered, heat-shrunk connections.
   Est. part cost: $5-$50

Rare But Worth Checking

• Faulty Trailer Brake Power Module (TBPM):
• Internal Break in Truck Wiring Harness:

Diagnosis Steps

1. Read the specific message on the Driver Information Center (DIC). Per TSB PIT5509, "Check Trailer Wiring" with C1114 often points to a short-to-ground on Circuit 47, while "Service Trailer Brake System" with C1114 suggests a short-to-power or a poor trailer ground.
2. Identify the type of trailer connected. Does it have its own battery, solar panels, or generator? Is it an Electric-Over-Hydraulic (EOH) system?
3. If the trailer has its own power source, consult TSB #23-NA-149 or GM Customer Satisfaction Program N232414840. The likely fix is an updated Trailer Brake Power Module (P/N 85073615) for the truck.
4. If the trailer has EOH brakes, consult TSB #PIT6212D and trailer brake manufacturer instructions. The likely fix is a trailer-side adapter like the Dexter K58-036-00.
5. If the trailer is a standard electric brake type, disconnect it. If the code clears, the fault is on the trailer side.
6. Thoroughly inspect the 7-pin connectors on both the truck and trailer for corrosion, damage, or loose pins. Clean with a terminal brush and apply dielectric grease.
7. Inspect the trailer's wiring harness for physical damage, especially the ground wire (white) connection to the frame and the brake output wire (blue).
8. If possible, test the truck with a known-good trailer or a dedicated 7-way trailer circuit tester/simulator (GM tool EL-52641) to definitively isolate the problem to the truck or the trailer.
9. If the truck is confirmed to be the issue after simulator testing, inspect the truck's wiring harness from the 7-pin connector to the control module for damage before suspecting the module itself.
🎬 See this walkthrough for replacing the trailer brake controller and module.

Parts You'll Likely Need

• E/H Compatibility Adapter (OEM #K58-036-00) — This specific adapter, made by Dexter Axle, is required to resolve incompatibility between the GM integrated brake controller and certain Electric-Over-Hydraulic trailer brake actuators, as noted in TSB PIT5509J.
  Trusted brands: Dexter Axle
  OEM price range: $90-$120
• Trailer Brake Power Module (Updated) (OEM #85073615) — This is the updated module specified in TSB 23-NA-149 and Customer Satisfaction Program N232414840 to fix issues with voltage back-feeding from trailers with their own power sources (batteries, solar). It supersedes older part numbers like 23306694. 🎬 Watch: Step-by-step guide to replacing the brake module.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $100-$160

Technical Service Bulletins (TSBs) & Recalls

• 23-NA-149
• PIT6212D
• PIT5509K

Platform-Specific Known Issues

• TSB #23-NA-149: Directly addresses the "Trailer Brake Disabled" message and DTC C1114 when towing a trailer equipped with its own battery, generator, or solar charger. The fix is to replace the truck's trailer brake module with an updated part.
• TSB #PIT6212D: Identifies a voltage spike from Hydrastar Electric Over Hydraulic trailer brake systems as a cause for C1114 and advises using a trailer simulator to confirm the truck is not at fault.
• TSB #PIT5509 (series): A long-running diagnostic guide that emphasizes the fault is most often with the trailer wiring or connection, not the truck. It details how to interpret different DIC messages associated with C1114.
• An owner filed NHTSA complaint #11589523 after purchasing the recommended EOH adapter part (K58-036-00) and still having the C1114 code, indicating that diagnosis can be complex and the adapter is not a guaranteed fix for all scenarios.

Mechanic-Grade Diagnostic Values

• Resistance of Trailer Brake Output Circuit (Circuit 47, Dark Blue Wire) to Ground — expected: Infinite resistance (Open Line / O.L.) with trailer disconnected.. Failure: Any reading less than infinite resistance indicates a short to ground in the truck's wiring harness.

Hidden / Shadow Codes Worth Checking

• C1114 + 'Check Trailer Wiring' Message: This combination indicates the Chassis Control Module has detected a short to ground on the trailer brake output circuit (Circuit 47). (see via Read the specific message on the Driver Information Center (DIC) when the C1114 code is set.)
• C1114 + 'Service Trailer Brake System' Message: This combination indicates the Chassis Control Module has detected a short to power on the trailer brake output circuit (Circuit 47) or a poor trailer ground connection. (see via Read the specific message on the Driver Information Center (DIC) when the C1114 code is set.)
• GDS2 Parameter: 'Trailer Brake Control Output Circuit': This is not a DTC, but a live data parameter that provides the specific fault status the module is seeing. It will read 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown', providing a more precise diagnosis than the code alone. (see via Using a GM GDS2 scan tool, navigate to Chassis Control Module -> Data Display -> 'Trailer Brake Control Output Circuit'.)

Scan Tool Commands That Help

• GDS2: Data Display: 'Trailer Brake Type' — Use this to verify what type of brake system the truck thinks is connected ('Electro-magnetic', 'Electro-over hydraulic', or 'Undefined'). This is critical for diagnosing compatibility issues, especially when using the GM trailer simulator tool EL-52641.

Wiring & Ground Locations

• Trailer Brake Power Module (TBPM / K133) — Located at the rear of the vehicle, mounted to a bracket above the spare tire, often on the driver's side frame rail.. This is the module that sends the test pulse, detects faults on Circuit 47, and sets code C1114. It is also the part that is replaced per TSB 23-NA-149.
• Circuit 47 — A Dark Blue wire that runs from the Trailer Brake Power Module (TBPM) to Pin 2 of the 7-pin trailer connector at the bumper.. This is the specific trailer brake output circuit that is monitored for faults. All C1114 diagnostics revolve around testing this wire for shorts or opens.
• G401 — A primary frame ground point, typically located on the driver's side frame rail near the rear of the vehicle, associated with the rear lighting and trailer connector harness.. A poor ground at this location can cause a variety of trailer-related electrical issues, including those that trigger a C1114 code.
• TBPM Connector Pins (T1XX Platform) — At the connector for the module located above the spare tire.. For manual testing: Pin 1 is power, Pin 9 is ground, and Pin 12 is the LIN bus communication wire (Green/Violet). This allows a technician to verify power, ground, and communication to the module itself.

Real Owner Repair Stories

• YouTube user, referencing GM Customer Satisfaction Program N232414840 (2024 GMC Sierra 2500 HD) — Intermittent loss of trailer brakes and 'Service Trailer Brake System' message immediately after taking delivery of the new truck and towing for the first time.
  ❌ Tried (didn't work) The owner immediately recognized the issue was likely the truck, not the trailer, due to the known service campaign.
  ✅ What actually fixed it The dealer performed the fix for GM Customer Satisfaction Program N232414840, which involved replacing the original Trailer Brake Power Module (TBPM) with the updated part. The owner confirmed this resolved the issue completely.
• YouTube user 'si-performance' (2019 GMC Sierra Elevation) — 'Service Trailer Brake System' warning message on the dash on every startup, even without a trailer connected. Code U1556 (Lost communication with brake power module) was also present.
  ❌ Tried (didn't work) Checked wiring continuity between the Chassis Control Module and the Trailer Brake Power Module, which tested good., Inspected connector pins, which were clean and not compromised per a known bulletin.
  ✅ What actually fixed it The Trailer Brake Power Module (located above the spare tire) was replaced. Upon opening the old module, it was found to be full of water and corrosion from road salt, causing the internal failure. Replacing the module resolved the fault.

"I Checked Everything" — The Actual Cause

• The issue described in TSB 23-NA-149 and Customer Satisfaction Program N232414840 is a prime example. A technician can test all truck and trailer wiring for shorts or opens and find no issues. The root cause is that when a trailer's own charging system (like solar or a generator) charges its battery to a voltage higher than the truck's battery, this voltage feeds back into the truck's Trailer Brake Power Module. The module's original software incorrectly interprets this higher voltage as a fault (false short to ground) and disables the trailer brakes. The wiring is not actually shorted; the module's logic is the problem, which is corrected by installing the updated module (P/N 85073615).

OEM Part Supersession History

• 23306694 → 85073615 — The original module could misinterpret voltage feedback from a trailer's own charging system (solar, battery) as a short circuit. The updated module (85073615) contains revised logic to tolerate this voltage feedback without setting a fault.

Model Year Variations Within This Range

• 2020, 2024: Certain 2020 and 2024 models are specifically covered by GM Customer Satisfaction Program N232414840 for a false short-to-ground condition caused by trailer battery voltage feedback. The official fix is the replacement Trailer Brake Power Module (P/N 85073615).
• 2025: For the 2025 model year, the integrated trailer-brake controller becomes a standard feature on all trim levels of the Sierra HD, including the base Pro trim.