C1114 on 2015-2017 GMC Sierra: Trailer Brake System Fault Causes & Fixes

What's Unique About the 2015-2017 Gmc SIERRA

The 2015-2017 GMC Sierra (and its K2XX platform mates) features a sophisticated trailer brake control system that actively monitors the connection to determine the type of trailer brakes being used. This sensitivity is a double-edged sword; while it allows compatibility with different brake types, it also makes the system prone to flagging faults from minor issues on the trailer side, such as poor-quality wiring, corrosion, or non-standard connectors. GM has issued numerous evolving Technical Service Bulletins (e.g., PIT5509, PIT5311) acknowledging that the root cause is very often the trailer's wiring or the physical connection at the 7-pin plug, rather than a failure of the truck's control module.

Diagnostic Flowchart

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

Generation note: This issue is extremely common on the K2XX platform (2014-2018/19) and also affects the subsequent T1XX platform (2019+), indicating the sensitive nature of the trailer detection system is a persistent characteristic across generations.

Symptoms You May Notice

• "Service Trailer Brake System" message on the Driver Information Center (DIC).
• "Check Trailer Wiring" message on the DIC.
• No power output to the trailer's electric brakes.
• Trailer brakes do not engage when using the manual controller or pressing the brake pedal.
• Audible chime or warning alarm at startup and intermittently while driving.

Most Likely Causes

1. Poor Trailer Wiring or 7-Pin Connector Issues 🔴 High Probability The truck's sensitive monitoring system is easily faulted by common trailer issues like corroded terminals, poor ground connections (white wire), or internal wire shorts on the trailer side. TSB PIT5311 specifically calls out inspecting and cleaning the ground wire connection on the trailer side of the 7-pin connector. GM even recommends using a quality, robust plug like a "Pollak HD" to ensure a good connection.
   How to confirm: Disconnect the trailer. If the message disappears after an ignition cycle, the fault is almost certainly on the trailer. Test with a known-good trailer or a 7-pin circuit tester. Visually inspect both the truck and trailer connectors for corrosion (especially green or white powder), bent pins, or loose wires.
   Typical fix: Clean the 7-pin connectors on both the truck and trailer using a wire brush and electrical contact cleaner. Repair or replace the trailer-side 7-pin plug. Repair the trailer's ground wire connection to its frame, ensuring it's on a clean, unpainted surface.
   Est. part cost: $10-$50
2. Loose Connection at Truck's 7-Pin Connector 🟡 Medium Probability GM identified that the original connector's door spring may not be strong enough to hold the trailer plug securely, leading to intermittent connections. This is especially true with 5th-wheel harnesses that can bounce and pull on the plug.
   How to confirm: Wiggle the trailer connector while it is plugged into the truck to see if it triggers the fault message. Check if the connector door feels loose or provides weak tension.
   Typical fix: Replace the truck-side 7-pin connector with the updated GM part (P/N 23404450) 🎬 Watch: This video shows how to replace the 7-pin connector. which has a stronger spring. This part supersedes the older, weaker version (P/N 22799842).
   Est. part cost: $40-$80
3. Vehicle Battery or Charging System Issues 🟡 Medium Probability TSB #PIT5521A and its successor PIT5529A advise addressing any battery issues first before performing in-depth diagnostics for codes C1112, C1113, and C1114, indicating that low or unstable voltage can cause faults in the sensitive Chassis Control Module.
   How to confirm: Test the battery voltage with a multimeter (should be ~12.6V with engine off, ~13.7-14.7V with engine running). Have the battery and alternator professionally tested under load at an auto parts store.
   Typical fix: Recharge or replace the vehicle's battery. Address any underlying charging system problems.
   Est. part cost: $150-$350
4. Incompatible Aftermarket Trailer Devices ⚪ Low Probability Some aftermarket electronic sway control systems or electric-over-hydraulic (EOH) brake adapters can interfere with the GM system's test pulse, causing it to set C1114. The GM system is designed to work with both brake types, making external adapters unnecessary and problematic.
   How to confirm: The fault appears only when a specific trailer with an aftermarket device is connected. Check with the device manufacturer for compatibility. For certain Dexter EOH systems, a fault may occur on the first brake application after startup.
   Typical fix: Install a compatibility adapter specifically designed for this issue (e.g., Dexter K58-036-00 for some EOH systems) or remove the incompatible device from the trailer's wiring.
   Est. part cost: $50-$150

Rare But Worth Checking

• Faulty Trailer Brake Power Module (TBPM / K133): This module, located above the spare tire, can fail. TSB PIT5529 notes that water can enter the module's connector if a cavity plug is missing or unseated, causing corrosion and various trailer brake codes, including C1114. This should be suspected only after all external wiring is confirmed to be good. A YouTube user documented replacing this module to fix the issue after exhausting other options.
🎬 Watch: How to locate and replace the trailer brake module.
• Damaged Wiring Harness on the Truck: The wiring harness running along the frame to the rear 7-pin connector can become damaged or corroded, especially in areas where salt is used on roads. Inspect the dark blue wire (Circuit 47) for any visible damage, chafing, or corrosion, particularly where it runs over the frame near the spare tire.
• Blown Trailer Brake Fuse: A blown fuse for the trailer brake system can cause the error message. For 2014-2017 models, this is typically a 30A J-case fuse in the under-hood fuse box (often labeled F1 or STUD 1). One owner temporarily removed this fuse to silence the alarm when not towing.

Diagnosis Steps

1. Verify code C1114 is present with a scan tool. Note the exact message on the DIC.
2. A 'Service Trailer Brake System' message with C1114 often points to a short to power or a poor trailer ground.
3. A 'Check Trailer Wiring' message with C1114 often points to a short to ground on the brake output circuit (Circuit 47).
4. Disconnect the trailer. If the code and message clear (after an ignition cycle), the fault is on the trailer side. This is the most common scenario.
5. Thoroughly inspect the trailer's 7-pin plug. Look for green or white corrosion, pushed-back pins, or loose wires. Pay special attention to the ground terminal (white wire) and its connection to the trailer frame.
6. Inspect the truck's 7-pin receptacle for corrosion or damage. Ensure the spring-loaded door provides firm tension. If not, consider replacing it with the updated GM part 23404450.
7. If possible, test the truck with a known-good trailer or a dedicated 7-pin circuit tester to definitively isolate the fault to the truck or trailer.
8. Test the vehicle's battery and charging system to rule out voltage-related issues as noted in TSB #PIT5529A.
9. If the fault remains with a known-good trailer, inspect the truck's wiring harness from the 7-pin connector back, looking for any chafing, corrosion, or damage, particularly to the dark blue wire (Circuit 47).
10. Inspect the Trailer Brake Power Module (TBPM), located above the spare tire, and its electrical connector for signs of water intrusion or corrosion, as detailed in TSB PIT5529.
11. Only after all external wiring and connection possibilities have been exhausted should the Trailer Brake Power Module be considered as the potential cause.

Parts You'll Likely Need

• 7-Pin Trailer Connector Socket (Truck Side) (OEM #23404450) — The original connector's spring-loaded door can weaken, causing a poor connection. GM released this updated part with a stronger spring to fix the issue. It supersedes part number 22799842.
  Trusted brands: ACDelco (Genuine GM)
  OEM price range: $45-$70
  Aftermarket price range: $20-$40
• 7-Pin Trailer Connector Plug (Trailer Side) — This is the most common failure point due to exposure to the elements, causing corrosion and poor connections. Using a quality, SAE-compliant plug like those from Pollak is recommended by GM in TSBs.
  Trusted brands: Pollak, Hopkins, Bargman
  OEM price range: $20-$40
  Aftermarket price range: $10-$25
• Trailer Brake Power Module (TBPM) (OEM #23337305) — A rare but possible cause, this module located above the spare tire can fail due to internal faults or water intrusion at its connector. Replacement often requires programming.
  Trusted brands: ACDelco (Genuine GM)
  OEM price range: $100-$180
  Aftermarket price range: $70-$120

Related Codes That Often Appear With This One

• C1112 — TSB #PIT5529A lists C1112, C1113, and C1114 together, indicating they are all related to trailer brake system faults and can be triggered by common causes like battery issues or water intrusion in the TBPM connector.
• C1113 — TSB #PIT5529A lists C1112, C1113, and C1114 together, indicating they are all related to trailer brake system faults and can be triggered by common causes like battery issues or water intrusion in the TBPM connector.
• U1556 — This code indicates a loss of communication with the Trailer Brake Power Module. TSB PIT5529 lists this code alongside C1114 as a possible result of water intrusion and corrosion in the module's connector. A user on YouTube found this code when diagnosing a water-damaged module.

Technical Service Bulletins (TSBs) & Recalls

• PIT5509 (A, C, D, E, J): An evolving TSB that is the primary diagnostic guide for C1114. It details how to differentiate between DIC messages, common trailer wiring failures, and issues with aftermarket accessories.
• PIT5311: Specifically calls out poor trailer ground connections and recommends using a high-quality 'Pollak HD' trailer-side plug.
• PIT5529A: Supersedes PIT5521A. Advises checking for low battery voltage first and also points to water intrusion in the Trailer Brake Power Module (K133) connector as a cause for C1114 and other related codes.
• 23-NA-044: A general bulletin that provides a part number for reflective heat tape for wiring harness repairs, which can be useful if repairing the harness near the exhaust.

Platform-Specific Known Issues

• Multiple TSBs (PIT5509 series, PIT5311) have been issued by GM for the "Service Trailer Brake System" message and code C1114, highlighting that the issue is frequently caused by trailer wiring, poor grounds, or a loose 7-pin connection rather than a fault with the truck itself.
• TSB #PIT5521A specifically recommends checking for and resolving any vehicle battery issues before proceeding with further diagnostics for this code.
• TSB #PIT5311 points directly to inspecting and cleaning the ground wire (typically white) on the trailer side of the 7-pin connector as a primary diagnostic step.

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 brake magnets at the 7-pin connector — expected: 0.75 to 1.5 Ohms for a typical tandem axle (4 brakes) trailer.. Failure: A reading outside this range suggests a problem with the trailer's brake magnets or wiring.
• 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. A reading significantly outside the range indicates a faulty magnet.
• Trailer Brake Power Module (K133) Connector Pin Voltages — expected: Pin 1: Battery Power, Pin 9: Ground.. Failure: Absence of power or a bad ground at these pins will prevent the module from operating.

Hidden / Shadow Codes Worth Checking

• C1114 + 'Service Trailer Brake System' message: 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 Driver Information Center (DIC) and a standard OBD-II scanner. Further detail is visible with a GM GDS2 scan tool.)
• C1114 + 'Check Trailer Wiring' message: Indicates the Chassis Control Module has detected a short to ground on the trailer brake output circuit (Circuit 47). (see via Driver Information Center (DIC) and a standard OBD-II scanner. Further detail is visible with a GM GDS2 scan tool.)

Scan Tool Commands That Help

• GDS2: Data Display -> Trailer Brake Control Output Circuit — This is the most critical diagnostic parameter. It directly displays the fault the module has detected: 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown', telling the technician exactly what type of electrical fault to look for.
• GDS2: Data Display -> Trailer Brake Type — Use this to verify if the Chassis Control Module is correctly identifying the connected trailer as 'Electro-magnetic' or 'Electro-over hydraulic brakes'. An incorrect or 'Undefined' reading with a trailer connected points to a detection issue.

Wiring & Ground Locations

• K133 Trailer Brake Power Module — Located on a bracket above the spare tire.. This module sends test pulses and power down Circuit 47. It is susceptible to water intrusion and corrosion at its connector, which can cause C1114 and other communication codes like U1556.
• Circuit 47 (Dark Blue Wire) — Runs from the Trailer Brake Power Module (K133) to pin 2 of the 7-way trailer connector at the bumper.. This is the primary trailer brake output circuit. The Chassis Control Module continuously monitors this wire for shorts to ground, shorts to power, or open circuits, which are the direct triggers for code C1114.
• Trailer Ground (White Wire) — Pin 1 of the 7-way trailer connector. On the trailer, this wire must have a clean, solid connection to the trailer frame.. A poor trailer ground is a common cause for the 'Service Trailer Brake System' message with C1114, as the system interprets the resulting voltage anomaly as a fault.
• Under-hood Fuse Box - Fuse F1 or STUD 1 — In the main fuse box on the driver's side of the engine bay.. This 30A J-case fuse provides power to the trailer brake control system. If it's blown, the system will be inoperative and may display a service message.

Real Owner Repair Stories

• YouTube user '67c10owner' (2015 GMC Sierra 2500 HD) — Intermittent 'Service Trailer Brake System' warning that became almost constant.
  ❌ Tried (didn't work) Unplugging the in-dash control switch to silence the alarm.
  ✅ What actually fixed it Replacing the Trailer Brake Power Module (located under the spare tire) resolved the issue.
• YouTube user 'Just Fix It' (2019 GMC Sierra Elevation (T1XX Platform, but relevant diagnosis)) — 'Service Trailer Brake System' message on every startup, accompanied by DTC U1556 (Lost Communication With Trailer Brake Power Control Module).
  ❌ Tried (didn't work) Checked for power, ground, and LIN bus continuity at the module connectors, all of which tested good.
  ✅ What actually fixed it The Trailer Brake Power Module (K133) had failed internally due to water intrusion, despite the connector pins looking clean. Replacing the module fixed the fault. The water entered because a rubber plug for an empty pin cavity in the connector was compromised, a known issue mentioned in a GM bulletin.

OEM Part Supersession History

• 22799842 → 23404450 — The updated truck-side 7-pin connector has a stronger door/cover spring to ensure a more secure connection with the trailer plug, reducing intermittent faults.
• Unknown/Varies by year → 84441240 (example for some models) — Part numbers for the Trailer Brake Power Module (K133) have been updated over time due to revisions and potential fault corrections.
  Heads up: When replacing the Trailer Brake Power Module, it is critical to use the latest part number specified by GM for the vehicle's specific VIN. Programming by a dealer may be required after installation.