C1113 on 2015-2018 GMC Sierra 2500: Service Trailer Brake System Causes and Fixes

What's Unique About the 2015-2018 Gmc SIERRA 2500

On this generation of GM heavy-duty trucks (K2XX platform), the trailer brake system is notoriously sensitive to vehicle battery voltage. GM issued service bulletin PIT5521A advising technicians to test the battery's health before any other diagnostics for this code. Furthermore, the Trailer Brake Power Module (TBPM), designated K133 by GM, is mounted on a frame crossmember directly above the spare tire. This location makes its main electrical connector (X411) highly susceptible to corrosion from water and road salt, a well-documented failure point confirmed by TSB #PIT5529A, which notes that missing or dislodged cavity plugs allow for water entry.

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
• Integrated trailer brake controller is inoperative or shows no gain
• No power output to the trailer's electric brakes
• Audible chime when the warning message appears
• Warning message appears intermittently, often during or after rain.

Most Likely Causes

1. Weak or Failing Vehicle Battery 🔴 High Probability → Shop Vehicle Battery The trailer brake control system on this platform is highly sensitive to low voltage, requiring 9.5-17 volts to operate correctly. A battery that is starting to fail may still have enough power to start the engine but not enough to keep all modules within their required voltage range, triggering this fault. This is the primary diagnostic step recommended in GM TSB #PIT5521A and later merged into #PIT5529A.
   How to confirm: Test the battery's state of health with a professional-grade load tester, such as a Midtronics GR8, as recommended by GM. A simple voltage check is often insufficient as a failing battery can show normal voltage at rest but drop significantly under load. A slow engine crank is another strong indicator.
   Typical fix: Recharge or replace the vehicle's battery. This should always be the first diagnostic step before any parts are replaced.
   Est. part cost: $150-$350
2. Corroded Trailer Brake Power Module (TBPM) Connector 🔴 High Probability → Shop Trailer Brake Control Module The module is located above the spare tire, exposing its main connector (X411) to constant moisture, dirt, and road salt. TSB #PIT5529A notes that missing or dislodged cavity plugs in the connector allow water to enter, causing corrosion on the terminals. The bulletin also mentions that 'connector hydro-locking' can occur, where pressure from plugging in the connector can force the plugs out if not seated properly.
   How to confirm: Lower the spare tire to gain access. Disconnect the large electrical connector from the Trailer Brake Power Module and visually inspect the pins and terminals on both the harness side and the module side for green or white corrosion, bent pins, or signs of water intrusion. Check that all rubber cavity plugs are present and fully seated in the connector body.
   Typical fix: If corrosion is minor, clean the terminals with a specialized contact cleaner and a small brush, apply dielectric grease to seal out moisture, and ensure all cavity plugs are present and seated before reconnecting. If corrosion is severe, the connector pigtail (e.g., ACDelco PT2715) and/or the module itself will need to be replaced. 🎬 Watch: A detailed walkthrough of the connector inspection and repair.
   Est. part cost: $30-$80 for a connector pigtail
3. Faulty Trailer Brake Power Module (TBPM) 🟡 Medium Probability → Shop Trailer Brake Control Module Internal failure of the module can occur, sometimes as a result of prolonged exposure to moisture and corrosion that has damaged the internal electronics beyond the connector pins. In some cases, the module is simply faulty from the factory.
   How to confirm: This is typically diagnosed by exclusion. If the battery is confirmed good, fuses are intact, and the wiring/connector is clean and has proper voltage/ground, the module itself is the likely point of failure.
   Typical fix: Replace the Trailer Brake Power Module. This is a plug-and-play part that does not require programming. 🎬 Watch: How to swap the module and clean the harness. The original part number may be superseded by a newer version.
   Est. part cost: $80-$200
4. Blown Fuse ⚪ Low Probability A short circuit in the trailer wiring or an internal fault in the module can cause the main power fuse to blow.
   How to confirm: Check the under-hood fuse block for blown fuses. The primary fuses are often 30-amp J-case fuses in slots F1 and F2 (labeled 'STUD 1' and 'STUD 2' or 'TRLR BRK PWR'). A video guide for a similar truck also suggests checking fuses 14, 15, and 17. 🎬 See this guide for locating and testing the correct fuses.
   Typical fix: Replace the blown fuse. If the new fuse blows immediately, a short circuit exists that must be diagnosed.
   Est. part cost: $5-$15

Rare But Worth Checking

• Corroded Chassis Ground: The module relies on a clean chassis ground to operate correctly. On GM trucks, a common ground point is G401, located on the driver's side rear frame rail. While less common than connector corrosion, a rusty ground point can cause high resistance and trigger various electrical faults. Inspect the ground wire from the harness near the TBPM to its connection point on the frame.
• Damaged Wiring Harness: The wiring leading to the TBPM or the 7-pin bumper connector can become chafed, pinched, or damaged from road debris or improper repairs, leading to a short circuit. Inspect the harness that runs along the frame rail, especially where it is secured by clips.

Diagnosis Steps

1. Address Battery Health First: Fully charge and professionally load test the vehicle's battery using a tool like a Midtronics GR8. Resolve any battery issues before proceeding.
2. Check Fuses: Inspect the 30A J-case fuses (typically F1 and F2) related to the trailer brake system in the under-hood fuse block. Replace if blown.
3. Lower Spare Tire: Use the vehicle's tool kit to lower the spare tire to gain full access to the Trailer Brake Power Module (TBPM) mounted on the rear frame crossmember.
4. Inspect TBPM Connector (X411): Disconnect the module and thoroughly inspect the connector and module pins for any signs of green/white corrosion, moisture, or damage. Ensure all rubber cavity plugs are present and fully seated in the connector body.
5. Clean and Protect: If corrosion is found, clean terminals with a contact cleaner and a small brush. Apply dielectric grease to the connector seal before reassembly to prevent future moisture intrusion.
6. Test Power and Ground: With the key on, use a multimeter to verify battery voltage and a solid ground at the appropriate pins of the connector. Resistance between the ground pin and a clean spot on the frame should be less than 5 ohms.
7. Inspect 7-Pin Bumper Connector: Check the truck's 7-pin trailer connector at the bumper for corrosion, debris, or bent pins which can cause circuit issues.
8. Replace Components: If the battery and wiring are confirmed good, the Trailer Brake Power Module is the most likely failed component.

Parts You'll Likely Need

• Trailer Brake Power Module (OEM #85073617 (supersedes 23104453, 23114650, 23337305)) — This module is the electronic brain of the system and is a common failure point due to its vulnerable location above the spare tire.
  Trusted brands: ACDelco (GM Genuine), Dorman (e.g., 926-069)
  OEM price range: $110-$200
  Aftermarket price range: $80-$150
• Vehicle Battery — Low system voltage from a weak battery is a primary trigger for this code on GM trucks, as documented in TSB PIT5529A.
  Trusted brands: ACDelco, Interstate, DieHard
  OEM price range: $200-$350
  Aftermarket price range: $150-$250
• Trailer Brake Power Module Connector Pigtail (OEM #ACDelco PT2715 (GM 13584097)) — If the harness-side connector is severely corroded, replacing the connector pigtail is necessary to ensure a good connection to the new module.
  Trusted brands: ACDelco, Dorman
  OEM price range: $40-$70
  Aftermarket price range: $30-$50

Related Codes That Often Appear With This One

• C1112 — This code for 'Trailer Brake Control Relay Low Control Circuit' often appears with C1113 as they relate to the same relay control circuit within the TBPM.
• C1114 — This code for 'Trailer Brake Control Solenoids Control Circuit' is also part of the same system and can be triggered by the same power supply or corrosion issues.
• U1556 / U1505 — These are communication codes that can be set alongside C1113 when low battery voltage or corrosion disrupts communication with the Trailer Brake Power Module.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Combines previous bulletins. Advises checking battery health first, then inspecting the TBPM connector for corrosion due to missing cavity plugs.
• PIT5521A: Specifically addressed low battery voltage as a primary cause for the 'Service Trailer Brake' message. Now superseded by PIT5529A.

Platform-Specific Known Issues

• TSB #PIT5529A: This is the primary document for this issue, superseding PIT5529 and PIT5521A. It explicitly tells technicians to check battery health first with a specific tester (Midtronics GR8) before any other diagnostics. It then directs inspection of the TBPM connector for corrosion caused by missing cavity plugs.
• TSB #PIT5529: The earlier version of the bulletin, focused primarily on the connector corrosion issue above the spare tire.
• TSB #PIT5521A: This bulletin, now merged into PIT5529A, specifically highlighted low battery voltage as a cause for the 'Service Trailer Brake' message and associated DTCs.

Mechanic-Grade Diagnostic Values

• Ground circuit resistance at TBPM connector (X411) — expected: Less than 5.0 Ohms. Failure: A reading higher than 5.0 Ohms indicates high resistance or an open in the ground circuit (G401).
• Voltage drop on ground circuit at TBPM connector (X411) — expected: Less than 0.3 Volts. Failure: A reading higher than 0.3V while the circuit is under load points to high resistance in the ground path.
• TBCM operating voltage — expected: 9.5 - 17 Volts. Failure: System voltage below 9.5V, especially during engine cranking, can trigger this fault before other symptoms of a weak battery appear.
• Voltage at Solid State Relay Enable circuit (Pin 33, Yellow/Black wire at X411) — expected: Varies, but should not be shorted to battery voltage.. Failure: Code C1113 is specifically defined as a short to voltage on this circuit; a reading near battery voltage confirms the fault condition described by the code.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Data Display > Trailer Brake Type — Use this to see how the main control module (CCM/BSCM) identifies a connected trailer or tester. It can show 'Electro-magnetic', 'Electro-over hydraulic brakes', or 'Undefined', helping to confirm if the truck is communicating correctly.
• GDS2 (GM Global Diagnostic System 2): Data Display > Trailer Brake Control Output Circuit — This parameter shows the real-time status of the output circuit (Circuit 47). It will display 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown'. This is useful for diagnosing C1114, which often appears with C1113.

Wiring & Ground Locations

• K133 / X411 — The Trailer Brake Power Module (K133) and its main connector (X411) are mounted to a frame crossmember directly above the spare tire.. This is the component and connector most susceptible to corrosion, which is a primary cause of code C1113.
• G401 — A chassis ground point located on the left (driver's side) rear frame rail.. This is a common ground point for several rear components, including the trailer wiring harness and the Trailer Brake Power Module. Corrosion here can cause high resistance and lead to this fault.
• Pin 33 at X411 — The Yellow/Black wire at the main connector for the Trailer Brake Power Module.. This is the specific 'Solid State Relay Enable' circuit. The definition for C1113 is a short to voltage on this exact circuit, making it the primary wire to test for this fault.
• Pin 32 at X411 — The Red/White wire at the main connector for the Trailer Brake Power Module.. This is the main battery positive voltage feed to the module. Verifying full battery voltage at this pin is a critical step in electrical diagnosis.

Real Owner Repair Stories

• YouTube video by user 'snomuncher' (2015 GMC Sierra 2500 HD) — "Service Trailer Brake System" message appeared on the dash, accompanied by a constant warning chime.
  ❌ Tried (didn't work) The owner temporarily pulled the 30A 'Trailer Brake Control' fuse from the under-hood fuse box to silence the alarm while driving without a trailer.
  ✅ What actually fixed it Replacing the Trailer Brake Power Module located above the spare tire fixed the issue. The owner noted that his intuition told him the problem was with the externally mounted module, not the in-dash switch, which proved correct.

When the Usual Fixes Don't Work

• While some owners may first suspect the in-dash trailer brake controller switch, real-world evidence strongly points away from this as a common fix for code C1113. One owner documented replacing the in-dash switch, which did not resolve the 'Service Trailer Brake' message. Another successful repair was achieved by correctly diagnosing and replacing the rear-mounted Trailer Brake Power Module, bypassing the in-dash switch entirely. This confirms that for code C1113, diagnostic efforts should be focused on the battery and the components located at the rear of the truck.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — Standard part revision and consolidation by the manufacturer.
  Heads up: No known incompatibilities. The newest part number (85073617) is the correct service replacement for all previous versions.

Model Year Variations Within This Range

• 2015-2018: While the Trailer Brake Power Module (K133) is consistent, the primary module it communicates with may be designated differently depending on the specific model year and vehicle options. It can be the Chassis Control Module (K38), Chassis Control Module Auxiliary (K38A), or the Brake System Control Module (K160). This is primarily a service manual terminology difference but is important for accurate diagnosis using schematics.