C1114 on 2016-2020 GMC Sierra 3500: 'Service Trailer Brake System' Causes and Fixes

What's Unique About the 2016-2020 Gmc SIERRA 3500

On this generation of Sierra HD trucks, the trailer brake control system is highly sophisticated and sensitive to electrical fluctuations to accommodate both electromagnetic and electro-over-hydraulic (EOH) brakes. The manufacturer has issued multiple Technical Service Bulletins (TSBs) highlighting that issues often stem from simple causes like low battery voltage or corrosion on connectors, rather than a failure of the control module itself. The system's sensitivity means it is quick to set a C1114 code in response to poor trailer grounds, non-standard connectors, or faulty trailer wiring that older, simpler systems might have ignored.

Diagnostic Flowchart

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

Generation note: This year range covers the end of the third generation (K2XX; 2016-2019) and the start of the fourth generation (T1XX; 2020). The trailer brake system architecture and its related issues, including the C1114 code, are largely consistent across these model years, as evidenced by TSBs that cover both generations. A specific recall, N232414840, affects some 2020 models where voltage feedback from a trailer's battery can cause a false short-to-ground, requiring a new Trailer Brake Power Module.

Symptoms You May Notice

• "Service Trailer Brake System" message on the Driver Information Center (DIC).
• "Check Trailer Wiring" message on the DIC.
• Trailer brake output is disabled.
• Inability to adjust trailer brake gain using the in-cab controller.
• Intermittent "Trailer Connected" and "Trailer Disconnected" messages, especially on rough roads.
• Warning message appears randomly, even with no trailer connected.

Most Likely Causes

1. Poor Battery Health or Low Voltage 🔴 High Probability → Shop Vehicle Battery As per TSB #PIT5529A, the control modules on this platform are sensitive to system voltage. A weak or failing battery can cause various electrical gremlins, including faults in the trailer brake system, before any other starting or running issues become apparent. This should be the first check.
   How to confirm: Test the battery with a multimeter or a professional battery tester (like the EL-50313 Midtronics tester specified by GM). Voltage should be above 12.4V with the engine off. Check for any battery-related DTCs. On dual-battery diesel models, test both batteries individually.
   Typical fix: Recharge or replace the vehicle's battery/batteries. Ensure all battery terminals and ground connections are clean and tight.
   Est. part cost: $150-$700
2. Corroded Trailer Brake Power Module Connector 🔴 High Probability → Shop Trailer Brake Control Module The Trailer Brake Power Module (GM P/N 84402332 or 23337305 for some years) is located above the spare tire, exposing it to moisture and road debris. TSB #PIT5529A specifically calls out inspecting this connector for corrosion or missing cavity plugs as a primary diagnostic step.
   How to confirm: Lower the spare tire for access. Disconnect the module's electrical connector and inspect the pins and terminals on both the module and the harness side for any green or white corrosion or backed-out pins. A user on YouTube demonstrated this exact fix.
   Typical fix: Clean the terminals with an electrical contact cleaner and a small brush. Apply dielectric grease upon reassembly to prevent future corrosion. If corrosion is severe, the connector pigtail or the module itself may need to be replaced.
   Est. part cost: $20-$250
3. Trailer Wiring or Connector Issues 🔴 High Probability The truck's sensitive monitoring system can easily be tripped by faults on the trailer side. Poor grounds (white wire), damaged wires (especially the blue brake wire), or low-quality 7-pin plugs on the trailer are extremely common triggers. TSBs repeatedly state that trailer wiring is a more likely cause than a truck fault.
   How to confirm: Test the truck with a known-good trailer or a 7-pin circuit tester. If the code does not return, the fault is on the trailer. Inspect the trailer's plug and wiring harness, paying close attention to the ground (white wire) and brake (blue wire) circuits. Cleaning the trailer frame ground connection to bare metal is a common fix.
   Typical fix: Repair the trailer's wiring. Replace the trailer-side 7-pin plug, preferably with a high-quality unit like those from Pollak, which is an OEM supplier recommended by GM in service bulletins.
   Est. part cost: $15-$50
4. Poor Connection at Truck's 7-Pin Connector 🟡 Medium Probability The bumper connector is exposed to the elements. A weak connector door spring can lead to a loose connection that vibrates and triggers faults. GM issued an updated part with a stronger spring to address this.
   How to confirm: Inspect the truck's 7-pin connector for corrosion, spread pins, or damage. Wiggle the trailer plug when connected; it should feel secure. Also check the large harness connector on the back of the 7-pin socket for corrosion.
   Typical fix: Clean the connector terminals. If the connection is loose or the connector is damaged, replace it with the updated GM part (P/N 23404450, which supersedes 22799842).
   Est. part cost: $50-$100

Rare But Worth Checking

• Incompatible Trailer Brake System (Electric-Over-Hydraulic): Some electric-over-hydraulic (EOH) brake actuators on trailers are not fully compatible with the GM control system's test sequence, causing a C1114 code. This is a known issue, especially with certain brands like Hydrastar, which can generate a voltage spike. Dexter EOH systems may also have issues. The EOH manufacturer may offer a compatibility adapter module (e.g., Hydrastar HBA-CAM P/N 381-7073 or Dexter P/N K58-036-00) to resolve the problem.
• Faulty Chassis Control Module (CCM) / Trailer Brake Power Module: → Shop Trailer Brake Control Module While possible, the control module is rarely the cause. A YouTube user documented replacing the Trailer Brake Power Module (located above the spare tire) to successfully fix the code after checking wiring. However, all other wiring, power, ground, and trailer-side issues should be exhaustively ruled out before condemning the control module, as it is an expensive part that may require programming.
• Aftermarket Trailer Sway Control Devices: Some aftermarket electronic sway control devices installed on the trailer can interfere with the truck's trailer brake system. When the device activates, it can cause the truck to set a C1114 code and display the 'Service Trailer Brake System' message. The device manufacturer should be contacted to confirm compatibility with the GM system.

Diagnosis Steps

1. Address any battery issues first. Per TSB #PIT5529A, this is the primary step. Load test the battery (or batteries) and charging system. Clean battery terminals and check chassis grounds.
2. Note the exact message on the DIC. Per TSB #PIT5509J, "Check Trailer Wiring" + C1114 often points to a short to ground on the brake output circuit (Circuit 47). "Service Trailer Brake System" + C1114 often indicates a short to power or a poor trailer ground.
3. If a trailer is connected, disconnect it and clear the codes. If the code does not return, the fault is on the trailer side.
4. Test the truck with a known-good trailer or a dedicated 7-pin circuit tester (like GM tool EL-52641) to isolate the fault.
5. Inspect the truck's 7-pin bumper connector for corrosion, bent pins, or a loose fit. Clean with contact cleaner and apply dielectric grease. If the plug fit is loose, consider replacing it with the updated GM P/N 23404450.
6. Lower the spare tire to access the Trailer Brake Power Module (K133). Disconnect and inspect its electrical connectors for corrosion, as highlighted in TSB #PIT5529A.
7. If all connections and trailer-side components are good, test the wiring for the trailer brake control output circuit (dark blue wire, Circuit 47) for shorts to power or ground between the module and the 7-pin connector.
8. If all wiring and connections are confirmed good, diagnosis of the Trailer Brake Power Module or Chassis Control Module may be required, which is best l

Parts You'll Likely Need

• Vehicle Battery — A weak or failing battery is a primary cause of this code, as noted in TSB #PIT5521A and PIT5529A.
  Trusted brands: ACDelco, Interstate, Optima
  OEM price range: $200-$350
  Aftermarket price range: $150-$300
• 7-Pin Trailer Connector Socket (OEM #23404450) — The original connector can become loose or corroded. GM released this updated part for a more secure connection.
  Trusted brands: ACDelco (GM Genuine), Pollak
  OEM price range: $50-$70
  Aftermarket price range: $30-$50
• Trailer Brake Power Module (OEM #84402332) — This module, located above the spare tire, can fail or its connector can corrode. It is a common replacement part after wiring and battery issues are ruled out.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $100-$150
  Aftermarket price range: $70-$120

Related Codes That Often Appear With This One

• C1112 — Often appears with C1114, relates to the trailer brake control relay circuit. TSB #PIT5529A lists them together.
• C1113 — Often appears with C1114, relates to the trailer brake control relay circuit. TSB #PIT5529A lists them together.
• U1556 or U1505 — These are communication (U-series) codes that can be set alongside C1114, often pointing to a broader electrical issue like low system voltage or a bad connection affecting multiple modules. TSB #PIT5529A lists them together.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Addresses trailer brake messages and codes C1112, C1113, C1114, C111B, U1556, U1505. Recommends checking battery health first, then inspecting the Trailer Brake Power Module connector for corrosion.
• PIT5521A: Specifically advises addressing any battery issues before performing in-depth diagnostics for a 'Service Trailer Brake' message. This TSB was later merged into PIT5529A.
• PIT5509 (A-J): A series of bulletins providing diagnostic tips for C1114. Explains how the system differentiates between DIC messages ('Check Trailer Wiring' vs. 'Service Trailer Brake System') and discusses issues with EOH brakes, loose connectors, and trailer-side wiring faults.
• PIT6212A: Focuses on C1114 caused by Hydrastar EOH trailer brake actuators generating a voltage spike. Confirms this is a trailer-side issue and recommends using a trailer simulator tool for diagnosis.

Platform-Specific Known Issues

• Trailer Brake Power Module Location: → Shop Trailer Brake Control Module The placement of the Trailer Brake Power Module (K133) above the spare tire makes it highly susceptible to water intrusion and corrosion from road spray. Lowering the spare tire for a visual inspection of the module and its connectors is a critical, vehicle-specific diagnostic step.
• DIC Message Specificity: GM TSBs clarify that the specific message displayed on the DIC provides a diagnostic clue. 'Check Trailer Wiring' often means a short to ground on the blue wire, while 'Service Trailer Brake System' points toward a short to power or a bad trailer ground. This helps narrow down the electrical fault type.

Mechanic-Grade Diagnostic Values

• Resistance of Trailer Brake Output Circuit (Circuit 47) 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.
• Total resistance of all trailer brakes at the 7-pin connector — expected: 0.8 to 1.5 Ohms for a typical tandem axle (4 brakes) trailer.. Failure: A significantly higher reading suggests an open or high resistance in the wiring. A reading near zero indicates a short circuit.
• Resistance of a single trailer brake magnet assembly — expected: 3.0 to 4.0 Ohms (when measured cold).. Failure: A reading of 'O.L.' indicates an open circuit/bad magnet.
• Trailer Brake Power Module (K133) Connector Pin Voltages/Continuity — expected: Pin 1: Battery Positive Voltage. Pin 9: Ground (less than 5 ohms to chassis ground). Pin 12: LIN Bus (Communication wire, check continuity to Chassis Control Module).. Failure: Loss of power, high resistance on ground, or open/short on LIN wire.

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 The message appears on the Driver Information Center (DIC) and the code is read by a standard OBD-II scanner. The specific fault type can be confirmed with a GDS2 scan tool.)
• 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 The message appears on the Driver Information Center (DIC) and the code is read by a standard OBD-II scanner. The specific fault type can be confirmed with a GDS2 scan tool.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Data Display -> 'Trailer Brake Type' — Use this to verify if the truck is correctly identifying the connected trailer as 'Electro-magnetic', 'Electro-over hydraulic brakes', or 'Undefined'. An incorrect or 'Undefined' reading with a trailer connected points to a detection or compatibility issue.
• GDS2 (GM Global Diagnostic System 2): Data Display -> 'Trailer Brake Control Output Circuit' — This is a critical live data parameter that directly displays the fault status detected by the module. It will read 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown'. This tells the technician exactly what type of electrical fault to look for on Circuit 47, saving significant diagnostic time.
• GM Special Tool EL-52641: Trailer Presence Simulator Tester — This is the definitive tool to determine if the fault is in the truck or the trailer. If the C1114 code does not set while this simulator is connected, the truck's system is functioning correctly, and the fault lies with the trailer wiring, brakes, or accessories.

Wiring & Ground Locations

• K133 — The Trailer Brake Power Module is located above the spare tire, mounted to a bracket.. This module is the heart of the power delivery for the trailer brakes. Its location makes its connectors susceptible to corrosion from road spray, which is a primary cause of C1114.
• Circuit 47 — Dark Blue wire running from the Trailer Brake Power Module (K133) to the 7-pin trailer connector at the bumper.. This is the trailer brake control output circuit. The module sends test pulses and the brake command signal on this wire. Shorts to ground or power on this specific circuit are what directly trigger code C1114.
• K133 Connector Pins — At the connector for the Trailer Brake Power Module, above the spare tire.. Pin 1 is Battery Power, Pin 9 is Ground, and Pin 12 is the LIN communication wire to the Chassis Control Module. Testing for power, ground, and communication at these specific pins is a key diagnostic step.

Real Owner Repair Stories

• YouTube user 'Fix it Angel' (2019 GMC Sierra 1500 Elevation) — Persistent 'Service Trailer Brake System' message on the dash with every startup. DTC U1556 (Lost Communication with Power Brake Control Module) was also present.
  ❌ Tried (didn't work) Visual inspection of the module exterior and connector pins, which appeared clean., Testing power (Pin 1), ground (Pin 9), and LIN communication wire continuity (Pin 12) at the module connector, all of which tested good.
  ✅ What actually fixed it The Trailer Brake Power Module (K133) was replaced. Upon opening the old module, significant internal corrosion and water damage were found, despite the exterior and connector appearing clean. Replacing the module resolved the fault codes and warning message.
• YouTube user 'Keep It Simple Stupid' (2023/2024 GMC Denali 2500HD/3500HD) — 'Check Trailer Wiring' message, trailer brake icon on dash not illuminated, indicating brakes are not connected/working.
  ❌ Tried (didn't work) Inspecting truck wiring., Replacing the 7-pin connector on the truck.
  ✅ What actually fixed it The issue was caused by voltage feedback from the trailer's solar charging system. Disconnecting the solar charger's input to the trailer battery (by installing a simple cutoff switch) immediately resolved the problem, allowing the truck to recognize the trailer brakes correctly. This aligns with GM Recall N232414840.

OEM Part Supersession History

• Various → 85073615 — To correct a false short-to-ground condition caused by voltage feedback from a trailer battery with a higher state of charge than the truck battery.
  Heads up: This is the fix for recall N232414840 affecting certain 2020-2024 models. The older module is overly sensitive to this voltage feedback.
• 22799842 → 23404450 — The updated truck-side 7-pin connector has a stronger door/cover spring to more securely hold the trailer plug, preventing intermittent connections.

Model Year Variations Within This Range

• 2016-2019 (K2XX Platform): These models are part of the K2XX generation. While the trailer brake system is functionally similar to the next generation, some part numbers for modules and connectors may differ. The primary issues are consistent (wiring, connections, battery voltage).
• 2020 (T1XX Platform): This is the first year of the T1XX generation. Certain 2020 models are subject to recall N232414840 for the Trailer Brake Power Module being sensitive to voltage feedback from trailer batteries, a problem less documented on the K2XX platform.