C1114 on 2015-2019 GMC Sierra Denali: Trailer Brake System Fault Causes & Fixes

What's Unique About the 2015-2019 Gmc SIERRA DENALI

The K2XX platform (2014-2019) features a sophisticated trailer brake system that is highly sensitive to electrical inconsistencies. The Trailer Brake Power Module (TBPM), officially designated K133, is mounted on a frame crossmember directly above the spare tire, a location that makes its electrical connector highly susceptible to water, road salt, and dirt intrusion. This design vulnerability is a well-documented cause for C1114 and related codes, as outlined in GM Technical Service Bulletins like PIT5529. As a result, issues that might be tolerated by simpler systems, such as minor trailer wiring faults or loose connections, will quickly trigger a fault on these trucks.

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).
• "Check Trailer Wiring" message on the DIC.
• Trailer brakes do not engage or work intermittently.
• The gain setting on the in-dash controller has no effect.
• Message appears on the DIC even with no trailer connected.

Most Likely Causes

1. Corroded or Damaged Trailer Brake Power Module (TBPM) Connector 🔴 High Probability The TBPM is located above the spare tire, exposing its connector to constant moisture and road debris. TSB #PIT5529 specifically identifies water intrusion from missing or dislodged cavity plugs in the connector as a primary cause of corrosion and this code. Multiple owner videos confirm this is a top failure point.
   How to confirm: Lower the spare tire to gain access. Disconnect the module's electrical harness (connector X411) by releasing the locking tab. Carefully inspect both the male and female sides for green or white crusty corrosion, bent pins, or signs of water entry.
   Typical fix: If corrosion is minor, clean the terminals with a specialized contact cleaner and a small brush, then apply dielectric grease before reconnecting. If corrosion is severe, the connector pigtail and/or the module itself will need to be replaced. A potential service pigtail part number is 89047377, but should be verified for your specific vehicle. For 2015 models, the mating connector is listed as P/N 19329472.
   Est. part cost: $25-$75 for a connector pigtail, $100-$200 for a new module.
2. Poor Connection at the 7-Pin Trailer Connector 🔴 High Probability Many trailer-side plugs do not meet strict SAE standards and can fit loosely in the truck's socket. The bouncing of the trailer harness during driving can cause an intermittent connection, arcing, and terminal wear, leading to faults. TSBs strongly recommend using a quality, OEM-style trailer plug like a Pollak HD to ensure a secure fit.
   How to confirm: Visually inspect the truck's 7-pin socket and the trailer's plug for corrosion, dirt, or deformed pins. A loose-fitting plug is a strong indicator. Wiggle the connector while plugged in to see if the message appears or disappears. Test with a known-good trailer or a 7-pin circuit tester to see if the fault disappears.
   Typical fix: Thoroughly clean both sides of the connection. Using a high-quality, OEM-style trailer plug (e.g., Pollak) is recommended. If the truck's socket is worn, it can be replaced with an updated part (GM Part #23404450 for 2014-18 models) which has a stronger spring door to hold the plug more securely.
   Est. part cost: $15-$50
3. Trailer Wiring Fault (Ground or Brake Circuit) 🟡 Medium Probability GM notes that trailer wiring is often lower quality than automotive wiring and is susceptible to corrosion, chafing, and poor ground connections. The GM system is sensitive enough to detect these issues as a circuit fault. A poor trailer ground (white wire) is a common cause for the 'Service Trailer Brake System' message.
   How to confirm: The DIC message can help. A "Check Trailer Wiring" message often points to a short-to-ground on the brake circuit (blue wire), while "Service Trailer Brake System" can indicate a short-to-power or a poor trailer ground (white wire). Inspect the trailer's wiring harness, especially the ground connection to the frame (G401) and any junction boxes.
   Typical fix: Repair the faulty wire or clean and secure the ground connection on the trailer frame. Ensure a clean, metal-to-metal contact for the ground. One owner found the fix was repairing a corroded ground inside the trailer's junction box.
   Est. part cost: $0-$20

Rare But Worth Checking

• Blown Fuse: A blown fuse for the trailer brake system in the under-hood fuse block can cause this code. This is often a symptom of a short circuit elsewhere (like on the trailer), not the root cause. One owner reported a blown 30-amp fuse at position #2 was the culprit after other attempted fixes failed. Also check fuses 1, 14, 15, 16, and 17.
🎬 See this video to quickly locate all trailer brake fuses.
• Faulty Trailer Brake Power Module (TBPM): While the connector is the most common failure point, the module itself can fail internally due to prolonged water damage or an electrical surge. This is usually diagnosed after all wiring and connection issues have been ruled out. A technician who opened a failed module found extensive corrosion on the internal circuit board.
• Incompatible Electric-Over-Hydraulic (EOH) Trailer Brakes: As noted in TSB #PIT5509J and others, some EOH brake systems can have an internal load or create voltage characteristics that the truck's Chassis Control Module interprets as a fault, setting code C1114. The solution is often a specific compatibility adapter from the trailer brake manufacturer.

Diagnosis Steps

1. Connect a scan tool to confirm C1114 is the active code. Note the specific message on the DIC. 'Check Trailer Wiring' suggests a short to ground on Circuit 47; 'Service Trailer Brake System' suggests a short to power or poor trailer ground.
2. If a trailer is connected, disconnect it. If possible, connect the truck to a different, known-good trailer or a dedicated trailer simulator (like GM tool EL-52641). If the code does not return, the fault is with the original trailer's wiring.
3. Thoroughly inspect the truck's 7-pin connector and the trailer's plug for corrosion, debris, or bent/loose pins. Clean as needed and apply dielectric grease. GM specifically recommends using a high-quality Pollak plug on the trailer side.
4. Lower the spare tire to access the Trailer Brake Power Module (TBPM) on the rear frame crossmember.
5. Disconnect the main electrical connector (X411) from the TBPM. Carefully inspect both the module's pins and the harness connector for any signs of green/white corrosion or water intrusion.
6. If corrosion is found, clean the terminals. If severe, replace the connector pigtail (verify part #89047377) and/or the TBPM.
7. Inspect the wiring harness leading to the 7-pin connector and the TBPM for any visible damage, chafing, or previous poor repairs.
8. Check fuses related to the trailer brake system in the under-hood fuse block. Specifically check the 30A fuses (e.g., position #2) and other trailer-related fuses (e.g., 1, 14, 15, 16, 17).

Parts You'll Likely Need

• Trailer 7-Pin Connector Socket (OEM #23404450) — The original socket's spring can weaken, leading to a loose connection. This updated part (for 2014-18 models) has a stronger spring door to hold the trailer plug more securely, as recommended by GM TSBs.
  Trusted brands: ACDelco, Pollak
• Trailer Brake Power Module (TBPM) (OEM #85073617) — This module, located above the spare tire, can fail due to internal corrosion from water intrusion. Part number 85073617 is the current service replacement for several older part numbers including 23337305.
  Trusted brands: ACDelco, GM Genuine Parts

Related Codes That Often Appear With This One

• C1112 — This code is also related to the trailer brake control circuit and is often triggered by the same wiring and corrosion issues, as noted in TSB #PIT5529.
• C1113 — Like C1112, this code points to a fault in the trailer brake control system and shares the same common causes, per TSB #PIT5529.
• U1556 — This code indicates a loss of communication with the Trailer Brake Power Module. It can be set alongside C1114 if the connector corrosion is severe enough to interrupt communication lines (LIN bus) in addition to the brake control circuit.

Technical Service Bulletins (TSBs) & Recalls

• TSB #PIT5509 (multiple revisions A-K): This is the primary diagnostic bulletin for this issue. It crucially clarifies that a 'Check Trailer Wiring' message with C1114 indicates a short to ground on the brake control circuit (47), while a 'Service Trailer Brake System' message points to a short to power or a poor trailer ground. It also recommends using a quality Pollak HD trailer plug.
• TSB #PIT5529: Specifically calls out water intrusion and corrosion at the Trailer Brake Power Module (TBPM) connector as a primary cause for C1114 and related codes.
• TSB #PIT5311: Recommends replacing a worn truck-side 7-pin connector with the updated part #23404450, which has a stronger spring door to hold the trailer plug more securely.

Platform-Specific Known Issues

• TSB #PIT5529 highlights that water and dirt can enter the Trailer Brake Power Module connector due to a missing or dislodged cavity plug, leading to corrosion and codes C1112, C1113, C1114, C111B, U1556, and/or U1505.
• TSB #PIT5509E explains that the Chassis Control Module sets C1114 when it has trouble determining the type of trailer brake system being used, often due to circuit faults like shorts or poor grounds.

Mechanic-Grade Diagnostic Values

• Trailer Brake Control Relay Feedback Signal Voltage — expected: 3.2 - 4.1 Volts with no trailer connected. Failure: A voltage > 4.1V suggests a short to voltage; a voltage < 3.2V suggests a short to ground or an open/high resistance.
• 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.
• Ground Circuit Resistance at TBPM Connector — expected: Less than 5.0 Ohms between the ground circuit terminal and a known good chassis ground. Failure: Resistance greater than 5.0 Ohms indicates an open or high resistance in the ground 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. A reading significantly outside the range indicates a faulty magnet.

Hidden / Shadow Codes Worth Checking

• C1114 + 'Check Trailer Wiring' Message: This specific combination indicates the Chassis Control Module has detected a short to ground on the trailer brake output circuit (Circuit 47). (see via Read the DTC with a scan tool and observe the specific warning message on the Driver Information Center (DIC).)
• 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. (see via Read the DTC with a scan tool and observe the specific warning message on the Driver Information Center (DIC).)
• GDS2 Parameter: 'Trailer Brake Control Output Circuit': This is a live data parameter, not a code, that provides a specific fault status. It will read 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown', providing the most precise fault identification. (see via Using a GM GDS2 scan tool, navigate to CCM -> Data Display -> 'Trailer Brake Control Output Circuit'.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Output Controls > Trailer Brake Control Output Voltage — This command allows a technician to manually apply a specific voltage (e.g., 5.0V) to the trailer brake output circuit (Circuit 47). By measuring with a multimeter at the 7-pin connector, a technician can verify the integrity of the truck's wiring from the module to the bumper without needing a trailer connected.

Wiring & Ground Locations

• K133 — On the driver's side frame rail, on a crossmember above the spare tire.. This is the official identifier for the Trailer Brake Power Module, which sends test pulses and controls output on Circuit 47, setting code C1114 when a fault is detected.
• X411 — The main inline harness connector for the rear chassis, located on the driver's side frame rail.. This connector is a potential point of failure for the trailer brake output circuit (Circuit 47) due to its exposed location. Corrosion or loose pins within X411 can cause an open or short, triggering C1114.
• G401 — A primary ground point associated with the rear chassis and trailer connector wiring.. A poor connection at this ground can cause a 'Service Trailer Brake System' message and a C1114 code, as the system may interpret the poor ground as a circuit fault.
• TBPM Connector Pins — At the K133 Trailer Brake Power Module connector.. For manual testing: Pin 1 is power, Pin 9 is ground, and Pin 12 is the LIN bus communication wire. Verifying power and ground here is a key diagnostic step.

Real Owner Repair Stories

• YouTube user '8K' (2015+ GMC Sierra/Chevy Silverado) — 'Service Trailer Brake System' message on the dash.
  ❌ Tried (didn't work) Thoroughly cleaning the original Trailer Brake Power Module (P/N 23337305) and its connector.
  ✅ What actually fixed it Replacing the original Trailer Brake Power Module with the updated, superseded part number (85073617). The new module resolved the error message immediately upon connection, even though it had a different part number.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — GM regularly updates service parts to integrate new materials, technologies, or fix known issues.
  Heads up: The new part number 85073617 is the correct service replacement for the older numbers and has been confirmed by owners to work correctly.
• N/A → 19329472 — This is the mating connector for the TBPM, required if the harness-side connector needs replacement.
  Heads up: This part is specifically called out for the 2015 model year when replacing the TBPM connector.

Model Year Variations Within This Range

• 2017 (specific VINs): A small population of 2017 trucks from the Ft. Wayne Assembly plant may have improperly hardened body and chassis ground bolts. This can cause a poor ground connection at locations like G400, leading to C1114 and other electrical faults. This was addressed in TSB PIT5562.