GMC C1112 on 2015-2018 Sierra 1500: Service Trailer Brake System Causes & Fixes

What's Unique About the 2015-2018 Gmc SIERRA 1500

On the K2XX generation of Sierra trucks (2014-2018), the trailer brake system is notoriously sensitive to battery voltage. GM has issued multiple Technical Service Bulletins, including PIT5529A (which superseded PIT5521A), advising technicians to verify the battery's health before any other diagnostics. Furthermore, the Trailer Brake Power Module is mounted on the frame rail above the spare tire, making its electrical connector highly susceptible to corrosion from road spray and water intrusion. TSB PIT5529A specifically notes that missing or dislodged cavity plugs in the connector allow moisture to enter, which is a very common failure point for this specific code.

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
• Audible warning chime that may repeat frequently and become very annoying to the driver.
• Integrated trailer brake controller is inoperative
• Manual trailer brake gain control switch (+/-) does not respond
• Message may appear intermittently, sometimes without a trailer connected.

Most Likely Causes

1. Weak or Failing Vehicle Battery 🔴 High Probability As documented in TSB PIT5529A and PIT5521A, the control modules on this truck are extremely sensitive to voltage drops, especially during startup. A weak battery can cause intermittent faults in the trailer brake system before causing a no-start condition. This is the first thing to check.
   How to confirm: Use a professional-grade battery load tester (e.g., a Midtronics GR8 tester as recommended by GM) to check the battery's health and cold cranking amps. A simple voltage check with a multimeter is often not sufficient to identify a weak battery.
   Typical fix: Replace the vehicle battery with a quality AGM (Absorbent Glass Mat) battery of the correct group size.
   Est. part cost: $150-$350
2. Corroded Trailer Brake Power Module (TBPM) Connector 🔴 High Probability The module (GM component K133) is located on the driver's side frame rail, directly above the spare tire, exposing it to road spray and moisture. TSB PIT5529A notes that missing cavity plugs in the connector can allow water to enter and cause corrosion on the pins, leading to shorts or open circuits.
   How to confirm: Lower the spare tire for access. Disconnect the 6-pin connector (X411) at the module and inspect for green or white corrosion ('blue crusties'), bent pins, or broken wires. Pay close attention to the seal and the individual wire cavity plugs.
   Typical fix: Clean the connector terminals and module pins with a specialized electrical contact cleaner and a small brush. If corrosion is severe, the connector pigtail 🎬 Watch this walkthrough on cleaning and repairing the module connector. (ACDelco PT2713 / GM Part #19329472) may need to be replaced. Apply dielectric grease to the connector seal upon reassembly to prevent future moisture intrusion.
   Est. part cost: $25-$75 for a new pigtail connector.
3. Faulty Trailer Brake Power Module (TBPM) 🟡 Medium Probability Internal failure of the module can occur, often due to long-term moisture intrusion that compromises the internal circuit board. Owners have documented replacing the module after confirming the battery and wiring are good.
   How to confirm: If the battery is confirmed good and the connector/wiring have been cleaned and tested for proper power and ground, the module itself is the likely culprit. This is a process of elimination.
   Typical fix: Replace the Trailer Brake Power Module. This is a plug-and-play part that does not typically require programming.
   Est. part cost: $70-$150
4. Blown Fuse ⚪ Low Probability A short circuit in the trailer wiring or the truck's harness can blow the fuse for the trailer brake system.
   How to confirm: Check the 30-amp fuse for the trailer brake controller (often labeled 'TRLR BRK' or similar) in the under-hood fuse box.
   Typical fix: Replace the blown fuse. If it blows again, there is an underlying short circuit that must be diagnosed, potentially on the trailer itself.
   Est. part cost: $1-$5

Rare But Worth Checking

• Damaged Wiring Harness: The harness running along the frame rail to the rear of the truck is exposed. Check for any chafed, pinched, or broken wires, especially the dark blue wire (circuit 47), which is the primary control wire monitored by the TBCM.
• Poor Ground Connection (G401): TSB PIT4595J mentions that a poor ground at location G401 on the frame can cause intermittent issues with the trailer brake system. This ground is typically located on the left rear frame rail, sometimes on the spare tire hoist bracket, and is a common point for rust and corrosion. Ensure this ground point is clean, free of rust, and tight.
• Faulty 7-Way Trailer Connector: The 7-way connector at the bumper can become corroded internally or damaged from use. This can cause intermittent shorts or poor connections that trigger the fault message, even without a trailer attached. Inspect the back of the connector for corrosion and test the pins.

Diagnosis Steps

1. Check Battery Health First: Before any other step, have the vehicle's battery professionally load-tested with a device like a Midtronics GR8 tester. Do not skip this step, as it is the most common cause cited in GM service bulletins.
2. Inspect Fuses: Check the 30A fuse (F1 or F10 depending on fuse block) and other related fuses (F19, F34) for the trailer brake system in the under-hood fuse block.
3. Gain Access & Inspect: Lower the spare tire to get a clear view of the driver's side frame rail. Locate the Trailer Brake Power Module (K133, a small black box with a 6-pin connector) above the spare tire's former location. Disconnect it (Connector X411) and thoroughly inspect both the module's pins and the harness connector for any signs of green/white corrosion, moisture, or damaged pins/wires. Ensure the rubber seal and cavity plugs are intact.
4. Test for Power and Ground: With the key on, use a multimeter to verify battery voltage and a solid ground at the appropriate pins on the harness connector. Refer to a vehicle-specific wiring diagram for the pinout. A lack of power or ground points to a wiring issue upstream.
5. Inspect Wiring and Grounds: Visually trace the wiring harness from the module forward along the frame rail, looking for any signs of physical damage, chafing, or poor repairs. Locate and inspect ground G401 on the driver's side rear frame for rust and tightness.
6. Inspect 7-Way Bumper Connector: Check the truck's 7-way trailer plug for internal corrosion or damage. Unplug it from the rear and inspect the harness-side connection as well.
7. Isolate the Module: If the battery is confirmed good, the fuse is intact, and the wiring/connector/grounds are clean and show proper voltage, the Trailer Brake Power Module itself is the most likely failed component.

Parts You'll Likely Need

• Trailer Brake Power Module (OEM #85073617) — This module can fail internally from moisture or its connector can corrode, causing the C1112 code. Part number 85073617 supersedes previous versions 23104453, 23114650, and 23337305.
  Trusted brands: ACDelco (OEM), Dorman (601-237), Standard Motor Products (TBM003)
  OEM price range: $100-$150
  Aftermarket price range: $70-$120
• Vehicle Battery — A weak or failing battery is a primary cause of this code on these specific vehicles, as noted in multiple TSBs.
  Trusted brands: ACDelco, Interstate, DieHard
  OEM price range: $200-$350
  Aftermarket price range: $150-$250
• Trailer Brake Power Module Connector Pigtail (OEM #19329472) — If the original connector (X411) is heavily corroded, it must be cut out and replaced with a new pigtail. The ACDelco equivalent is PT2713.
  Trusted brands: ACDelco (PT2713), Dorman
  OEM price range: $40-$60
  Aftermarket price range: $25-$40

Related Codes That Often Appear With This One

• C1113 — This code for the 'Trailer Brake Control Relay High Control Circuit' often appears with C1112, indicating a broader electrical issue in the system. TSB PIT5529A groups them together.
• C1114 — This code for the 'Trailer Brake Control Solenoids Control Circuit' is also frequently seen with C1112 and points to similar wiring or module issues.
• C111B — Another related trailer brake system code that GM groups with C1112 in service bulletins, often pointing back to low battery voltage as the root cause.
• U1556 — A communication code indicating the Chassis Control Module lost communication with the Trailer Brake Power Control Module. This can happen if the module loses power due to corrosion or a broken wire.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Service Trailer Brake Message With DTC's C1112, C1113, C1114, C111B, U1556 and/or U1505. This is the primary document for this issue, combining previous bulletins and covering model years 2014-2018.
• PIT5521A: (Superseded by PIT5529A) Originally advised to check battery health first for these codes.
• PIT4595J: Service Trailer Brake Message With DTC C1114 And/Or C1112. Focuses on intermittent shorts and checking ground G401.

Platform-Specific Known Issues

• TSB #PIT5529A: Warns of 'Service Trailer Brake' messages and codes C1112, C1113, C1114, C111B, U1556, and/or U1505. It explicitly states the two primary causes are a weak battery or water intrusion into the Trailer Brake Power Module connector due to missing cavity plugs. It advises testing the battery with a Midtronics tester before any other diagnostics.
• TSB #PIT4595J: Pertains to slightly older models but is still relevant. It advises checking ground G401 on the frame and monitoring the trailer brake control wire (dark blue, circuit 47) for shorts that can cause C1112 or C1114.

Mechanic-Grade Diagnostic Values

• Control Circuit Voltage at TBPM connector (X411) — expected: 3.2 - 4.1 Volts. Failure: A reading outside this range when testing between Terminal A and ground, or Terminal C and ground, suggests a short to ground, short to voltage, or an open/high resistance in the control circuit from the main chassis module.
• B+ Voltage at TBPM connector (X411) — expected: Battery Voltage (approx. 12.6V KOEO). Failure: Using a test lamp between the B+ circuit terminal (Terminal F) and ground, the lamp should illuminate brightly. If it does not, there is an open or short in the power feed circuit from the underhood fuse.
• Ground Circuit Resistance at TBPM connector (X411) — expected: Less than 5 ohms. Failure: A reading greater than 5 ohms between the ground circuit terminal (Terminal B) and a clean chassis ground indicates high resistance or an open in the ground circuit.

Hidden / Shadow Codes Worth Checking

• C1112 00: The '00' is a symptom byte that specifies the fault type. For C1112, the '00' symptom byte corresponds directly to 'Trailer Brake Control Relay Low Control Circuit', confirming the primary definition. (see via Professional scan tools like the GM Tech 2 or GDS2 will display the DTC along with its symptom byte.)

Scan Tool Commands That Help

• GDS2: Data Display -> Trailer Brake Control Output Circuit — This live data parameter is crucial for diagnosis. It will display 'Okay', 'Short to Ground', or 'Short to Power'. This can instantly tell a technician the nature of the fault on the control circuit (Circuit 47) without extensive manual testing, helping differentiate between a wiring short and a module fault.
• GDS2: Data Display -> Trailer Brake Type — This parameter shows if the system has identified a trailer and what type ('Electro-magnetic', 'Electro-over hydraulic brakes', or 'Undefined'). If it shows 'Undefined' with a trailer connected, it points to a problem in the initial communication handshake, often related to the wiring or connector.

Wiring & Ground Locations

• K133 — Mounted to the driver's side frame rail, directly above the spare tire.. This is the Trailer Brake Power Module itself. Its location makes its connector (X411) extremely vulnerable to corrosion, which is a primary cause of C1112.
• X411 — The 6-pin electrical harness connector that plugs into the Trailer Brake Power Module (K133).. This is the single most common point of failure. All voltage and resistance tests for the module are performed at this connector. TSB PIT5529A specifically calls out inspecting this connector for corrosion and missing cavity plugs.
• G401 — On the rear of the vehicle frame, typically on the driver's side near the trailer harness breakout or on the spare tire hoist bracket.. This is a critical chassis ground for the trailer brake system and other rear components. A corroded or loose connection at G401 can cause intermittent power loss or erratic behavior of the K133 module, triggering this code.
• Circuit 47 — Dark Blue wire running from the Trailer Brake Power Module (K133) to Pin 2 of the 7-way trailer connector at the bumper.. This is the main trailer brake control output circuit. The system sends a test pulse on this wire to detect a connected trailer and continuously monitors it for faults like shorts or opens, which can trigger C1112 and related codes.

Real Owner Repair Stories

• YouTube channel 'biker mike' (2015 GMC Sierra) — "Service Trailer Brake System" message on the DIC.
  ❌ Tried (didn't work) Inspected the wiring harness and found it to be in good condition.
  ✅ What actually fixed it Replacing the Trailer Brake Power Module (GM Part #85073617) located above the spare tire resolved the issue. The code was cleared and did not return.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — Standard part evolution by the manufacturer to improve reliability or integrate new materials.
  Heads up: All previous part numbers are superseded by 85073617, which is the correct and current service replacement.

Model Year Variations Within This Range

• 2015-2018: The primary module that communicates with the K133 Trailer Brake Power Module may have different names depending on the year and options. It can be called the Chassis Control Module (CCM, K38), Chassis Control Module Auxiliary (K38A), or Brake System Control Module (BSCM, K160). This is important for navigating scan tool menus and wiring diagrams correctly.