C1113 on 2015-2018 GMC Sierra 3500: Service Trailer Brake System Causes and Fixes

What's Unique About the 2015-2018 Gmc SIERRA 3500

On this generation of GM heavy-duty trucks (K2XX platform), the trailer brake control system is notoriously sensitive to the vehicle's electrical health. GM has issued multiple Technical Service Bulletins (TSBs) directly linking code C1113 to low battery voltage, sometimes accompanied by a slow engine crank. Furthermore, the Trailer Brake Power Module (also known as K133) is mounted on a frame crossmember above the spare tire, making its electrical connector (X411) highly susceptible to corrosion from road salt and water spray, a well-documented issue.

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 brakes do not engage when the vehicle's brakes are applied.
• The manual trailer brake controller is non-functional or shows no gain output.
• An audible chime or alarm may sound when the message appears.
• Slow engine crank may occur simultaneously if the cause is a weak battery.

Most Likely Causes

1. Weak or Failing Vehicle Battery 🔴 High Probability The trailer brake control system is very sensitive to voltage, and GM TSB #PIT5521A and the superseding #PIT5529A specifically identify low battery voltage as a primary cause for this code, sometimes accompanied by a slow engine crank. Forum users frequently report that a new battery resolved the C1113 code, even when the old battery seemed to start the truck without issue.
   How to confirm: Test the battery's state of health with a professional load tester, such as a Midtronics GR8, as recommended by GM. A resting voltage below 12.4V is a strong indicator of a problem. Many auto parts stores offer this service for free.
   Typical fix: Replace the vehicle's battery or batteries. Ensure terminals are clean and tight.
   Est. part cost: $150-$350
2. Corroded Trailer Brake Power Module (TBCM) Connector 🔴 High Probability The TBCM is located above the spare tire, exposing its connector (X411) to water, salt, and road debris. TSB #PIT5529A notes that missing or dislodged cavity plugs can allow water to enter and cause corrosion, a condition sometimes called 'hydro-locking'.
   How to confirm: Lower the spare tire and visually inspect the large electrical connector (X411) on the Trailer Brake Power Module for green or white corrosion, moisture, or pushed-out pins. Check that all rubber cavity plugs are present and fully seated.
   Typical fix: Disconnect the battery, then clean the connector and module pins with a terminal cleaning brush and electrical contact cleaner. Apply dielectric grease to the seal upon reassembly. If corrosion is severe, the connector pigtail (e.g., ACDelco PT2888) or the entire module may need replacement.
   Est. part cost: $10-$450
3. Blown Fuse 🟡 Medium Probability A short in the trailer wiring or an internal fault in a component can cause the 30-amp fuse for the trailer brake system to blow. This is a common failure point noted in owner forums.
   How to confirm: Check the under-hood fuse box for blown fuses. These are often large J-case fuses labeled 'STUD 1' and 'STUD 2' (often in slots F1 and F2 or similar, depending on the exact model year).
   Typical fix: Replace the blown fuse. If it blows again, further diagnosis is needed to find the short circuit. A shorted wire on the trailer itself is a common reason for the fuse to blow repeatedly.
   Est. part cost: $5-$15
4. Faulty Trailer Brake Power Module (TBCM) ⚪ Low Probability While less common than connector issues, the module itself can fail internally due to water intrusion or electronic failure. Water can bypass the seals and corrode the internal circuit board.
   How to confirm: This is typically diagnosed by exclusion. If the battery, fuses, and all wiring/connections test good, the module itself is the likely culprit. A scan tool may show a loss of communication with the module (e.g., code U1556).
   Typical fix: Replace the Trailer Brake Power Module. Note that some sources suggest a new module may require programming by a dealer or a properly equipped shop, though many owners have replaced it without programming.
   Est. part cost: $150-$300

Rare But Worth Checking

• Damaged Wiring Harness: The wiring harness running along the frame rail to the rear of the truck can become chafed or damaged, causing a short. Check for any areas where the harness may be rubbing against the frame, suspension components, or the spare tire mechanism.
• Poor Ground Connection: A corroded or loose main ground connection for the trailer brake system can cause high resistance and trigger faults. Inspect ground points G401/G402, which are typically located on the driver's side frame rail near the TBCM and spare tire.

Diagnosis Steps

1. Read all fault codes from the Chassis Control Module (CCM) with a capable OBD-II scanner.
2. As per TSB #PIT5529A, the first step is to fully charge and professionally test the vehicle's battery/batteries. Replace if it fails a load test, regardless of whether it still starts the engine.
3. Inspect the under-hood fuse box. Check the 30A J-case fuses for the trailer brake system (often labeled 'STUD 1' and 'STUD 2') and any other related fuses. Replace if blown.
4. Lower the spare tire to gain access to the Trailer Brake Power Module (TBCM) on the rear frame crossmember.
5. Disconnect the battery/batteries to de-energize the system.
6. Carefully disconnect the large electrical connector (X411) from the TBCM. This may have a red locking tab that needs to be disengaged first. Inspect both the connector and the module pins for any signs of moisture or corrosion (green/white powder).
7. Check that all cavity plugs are present and fully seated in the connector to prevent future water intrusion, as noted in TSB #PIT5529A.
8. If corrosion is found, clean the terminals thoroughly with electrical contact cleaner and a small brush. Apply dielectric grease to the connector seal before reassembly. If damage is severe, the connector pigtail may need to be replaced.
9. Inspect the main ground wire for the TBCM (often G401/G402) on the frame rail. Ensure the connection is clean, tight, and free of corrosion.
10. If no issues are found with the battery, fuses, or connector, test the power and ground circuits at the TBCM connector with a multimeter to ensure proper voltage and low resistance to ground.
11. If all wiring and power/ground circuits test normal, the Trailer Brake Power Module is likely faulty and requires replacement.

Parts You'll Likely Need

• Vehicle Battery — A weak battery is the most common trigger for this code on this platform, as confirmed by GM TSBs.
  Trusted brands: ACDelco, Interstate, DieHard
  OEM price range: $200-$350
  Aftermarket price range: $150-$250
• Trailer Brake Power Module (OEM #85073617) — This module is replaced if it fails internally or if the connector pins are corroded beyond repair. It is the central component of the system located above the spare tire.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $200-$300
  Aftermarket price range: $150-$250
• TBCM Connector Pigtail (OEM #ACDelco PT2888 (GM 13587179)) — If the connector at the TBCM is severely corroded, the pigtail must be spliced into the harness. This part number is a widely cited service connector for this application.

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 both relate to the control relay's circuitry and are cited together in TSBs.
• C1114 — This code for 'Trailer Brake Control Solenoids Control Circuit' can be set alongside C1113 when there is a general power or communication issue with the TBCM.
• C111B — This is another related trailer brake system code often triggered by the same root causes, like low battery voltage, and is listed in the same service bulletins.
• U1556 / U1505 — These are communication (U-prefix) codes that indicate a loss of communication with the Trailer Brake Power Module, often caused by the same corrosion or power supply issues that set C1113.

Technical Service Bulletins (TSBs) & Recalls

• This is the most comprehensive TSB for this issue, superseding PIT5521A. It advises that a 'Service Trailer Brake' message with codes C1112, C1113, C1114, C111B, U1556, and/or U1505 can be caused by either low battery voltage or water/dirt ingress into the Trailer Brake Power Module (K133) connector. It specifically calls out missing or dislodged cavity plugs as a cause for water entry and advises technicians to check battery health with a Midtronics tester before performing other diagnostics.
• This earlier TSB specifically advises that a 'Service Trailer Brake Message' with DTCs C111B, C1112, C1113, and C1114 can be caused by low battery voltage, possibly noticed as a slow engine crank. It strongly recommends addressing any battery issues first. This TSB was later merged into PIT5529A.

Platform-Specific Known Issues

• TSB #PIT5529A: This bulletin from GM advises that a 'Service Trailer Brake Message' with code C1113 can be caused by low battery voltage or water intrusion into the TBCM connector (K133) due to missing cavity plugs. It instructs technicians to check the battery first before any other diagnostics.
• TBCM Location: The placement of the Trailer Brake Power Module directly above the spare tire makes it extremely vulnerable to road spray, salt, and debris, leading to a high incidence of connector corrosion for trucks in wet or winter climates.

Mechanic-Grade Diagnostic Values

• Ground Circuit Resistance at TBCM Connector — expected: Less than 5.0 ohms between the ground circuit terminal and a good chassis ground.. Failure: A reading higher than 5.0 ohms indicates high resistance in the ground circuit, which can cause various faults.
• Battery Positive Voltage at TBCM Connector (Pin 32) — expected: A test lamp should illuminate when connected between the B+ circuit terminal (Pin 32, Red/White wire) and a good ground.. Failure: If the test lamp does not light, it indicates a loss of power to the module, likely from a blown fuse or an open in the power wire.
• Solid State Relay Enable Circuit Voltage (Pin 33) — expected: 3.2-4.1 volts between the control circuit terminal (Pin 33, Yellow/Black wire) and ground with the ignition ON.. Failure: Code C1113 specifically flags a short to voltage on this circuit. A reading higher than the specified range indicates a short to voltage. A reading lower than the range could indicate a short to ground or an open.

Scan Tool Commands That Help

• GDS2: Data Display > Trailer Brake Type — Use this to see how the Chassis Control Module (CCM) is identifying a connected trailer. It can show 'Electro-magnetic', 'Electro-over hydraulic brakes', or 'Undefined'. This helps confirm if the truck is correctly detecting a trailer or tester.
• GDS2: 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 especially useful for diagnosing the related C1114 code which often appears with C1113.

Wiring & Ground Locations

• K133 / TBCM — On the frame crossmember, directly above the spare tire.. This is the Trailer Brake Power Module itself. Its location makes it and its connector (X411) highly vulnerable to corrosion.
• X411 — The main electrical connector on the Trailer Brake Power Module (K133).. This is the primary point of failure due to corrosion. All power, ground, and control signals for the trailer brakes pass through this connector.
• X411 Pin 33 — At the TBCM connector. The wire is typically Yellow with a Black stripe (YE/BK).. This is the 'Solid State Relay Enable' circuit. DTC C1113 is set when the module detects a short to voltage on this specific pin and circuit.
• X411 Pin 32 — At the TBCM connector. The wire is typically Red with a White stripe (RD/WH).. This is the main Battery Positive Voltage feed for the module. Verifying battery voltage at this pin is a critical step in diagnosis.
• G401 — On the left rear of the frame, often near the spare tire hoist bracket or rear body mount.. This is a common ground point for several rear vehicle components, including the trailer wiring harness. A poor connection at G401 can cause intermittent and difficult-to-diagnose trailer brake faults.

Real Owner Repair Stories

• YouTube channel '59 Motorsports' (2014-2018 GM Truck (specific year not mentioned)) — 'Service Trailer Brake' message on the dashboard.
  ❌ Tried (didn't work) Replacing the in-dash trailer brake control switch module.
  ✅ What actually fixed it After replacing the in-dash switch did not solve the problem, the owner inspected the under-hood fuse box and found that the 30-amp fuse in position #2 for the 'Trailer Battery' was blown. Replacing this fuse cleared the message.
• YouTube channel 'Service Trailer Brake System Warning? Here's The FIX!' (2018 Chevrolet Silverado 3500 HD) — 'Service Trailer Brake System' message and associated fault codes.
  ❌ Tried (didn't work) Cleaning the TBCM connector (X411) provided a temporary fix, but the problem returned.
  ✅ What actually fixed it The owner replaced both the in-dash switch and the Trailer Brake Power Module (TBCM) above the spare tire. After replacement, the system functioned correctly, indicating the module itself had failed.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — GM regularly updates service part designs to integrate new materials and technologies.