C1112 on 2014-2018 Chevy Silverado 2500: Service Trailer Brake System Causes and Fixes

What's Unique About the 2014-2018 Chevrolet SILVERADO 2500

On this generation of Silverado and its GMC Sierra platform mate, the Trailer Brake Power Module (TBCM) is mounted on the driver's side frame rail, often above the spare tire, leaving it highly exposed to road spray, dirt, and de-icing chemicals. This design makes the module's main electrical connector a very common failure point due to corrosion. General Motors has issued multiple Technical Service Bulletins (TSBs), notably PIT5529A, acknowledging this pattern and advising technicians to check for low battery voltage and corroded module connectors—specifically looking for missing cavity plugs—before replacing the module itself.

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 connection not recognized or intermittently drops
• The trailer brake system is disabled and the gain control is unresponsive
• Message appears randomly, sometimes more frequently in wet or humid weather
• ABS warning light may be illuminated

Most Likely Causes

1. Low Battery Voltage or Failing Battery 🔴 High Probability → Shop Vehicle Battery TSB #PIT5521A and #PIT5529A specifically call out low battery voltage as a primary cause for DTCs C1112, C1113, C1114, and C111B. The sensitive electronics in the trailer brake system can throw fault codes when system voltage is unstable or low during startup.
   How to confirm: Test the battery's state of health and cranking amps using a professional carbon pile or digital battery tester (e.g., Midtronics GR8). A simple voltage check is often insufficient. Ensure battery terminals are clean and tight.
   Typical fix: Recharge or replace the vehicle's battery. If the truck has dual batteries, both should be tested and ideally replaced as a pair.
   Est. part cost: $150-$400
2. Corroded Trailer Brake Power Module (TBCM) Connector 🔴 High Probability → Shop Trailer Brake Control Module The module is located on the frame rail above the spare tire, where it is constantly exposed to water and road salt. TSB #PIT5529A points to water intrusion into the connector, sometimes from a missing or dislodged cavity plug, as a common cause of corrosion.
   How to confirm: Lower the spare tire for access. Locate the TBCM on the driver's side frame rail. Disconnect its large electrical connector and meticulously inspect the pins and connector housing for green or white corrosion, moisture, or bent pins.
   Typical fix: Clean minor corrosion with an electrical contact cleaner and a small brush. Apply dielectric grease to the seal before reconnecting. If corrosion is severe, the connector pigtail (e.g., ACDelco PT2715) must be spliced in, or the entire module and connector may need replacement.
   Est. part cost: $10-$350
3. Blown Fuse 🟡 Medium Probability A short in the trailer's own wiring or a momentary power surge from connecting/disconnecting the trailer can blow the fuses dedicated to the trailer brake system.
   How to confirm: Check the under-hood fuse box for fuses related to 'TRL BRK' or 'TRAILER'. Visually inspect the fuses and test for continuity with a multimeter.
   Typical fix: Replace the blown fuse with one of the exact same amperage rating. If the fuse blows again immediately, a short circuit exists in the truck or trailer wiring that needs to be diagnosed.
   Est. part cost: $1-$5
4. Faulty Trailer Brake Control Switch (In-Dash) ⚪ Low Probability → Shop Trailer Brake Control Module The switch in the dash can fail internally, often due to simple wear or liquid spills, sending erratic signals that trigger the fault message. This is a known issue and a relatively common DIY fix.
   How to confirm: If the 'Service Trailer Brake' message appears randomly, especially when it's hot inside the cab, or if the gain adjustment buttons are unresponsive, the switch is a likely suspect. The fix is often simple and cheaper than other components.
   Typical fix: Replace the trailer brake control switch assembly in the dashboard. This is a simple DIY task requiring only basic trim removal tools.
   Est. part cost: $30-$60

Rare But Worth Checking

• Faulty Trailer Brake Power Module (TBCM): → Shop Trailer Brake Control Module While often suspected, the module itself is less likely to fail than its connector. TSB #PIT4595J notes that many returned modules are found to be fully functional. Always exhaust all other diagnostic steps (battery, connector, wiring) before replacing the TBCM.
• Damaged Wiring Harness: A nick or cut in the wiring, particularly the dark blue wire (circuit 47), between the TBCM and the 7-pin bumper connector can cause intermittent shorts to ground or voltage, especially when wet. This requires a thorough visual inspection of the harness along the frame.

Diagnosis Steps

1. Check Battery Health: Start by professionally testing the vehicle's battery. A weak or failing battery is a primary cause cited by GM in TSB PIT5521A. Ensure terminals are clean and tight.
2. Inspect Fuses: Check all fuses related to the trailer brake system in the under-hood fuse box. Replace any that are blown.
3. Inspect TBCM Connector: Lower the spare tire to access the module on the driver's side frame rail. Disconnect the main electrical harness (it has a locking tab) and inspect thoroughly for any signs of corrosion, moisture, or damaged pins. Pay close attention to the connector seal and ensure all cavity plugs are present.
4. Clean and Secure Connections: If minor corrosion is found, clean the terminals with electrical contact cleaner and a small wire brush. Apply dielectric grease to the connector seal before reconnecting to prevent future moisture intrusion. Ensure the connector is fully seated and the lock is engaged.
5. Inspect 7-Pin Trailer Connector: Check the 7-pin connector at the bumper for corrosion, damage, or loose pins. Issues here can send fault signals up the line.
6. Inspect In-Dash Switch: If no issues are found externally, consider the in-dash control switch, especially if symptoms are intermittent. It's a known failure point and a relatively easy replacement.
7. Professional Diagnosis: If the problem persists, a professional will need to use a scan tool (like a Tech 2 or GDS2) to monitor live data from the TBCM, such as the 'TBCM Relay Feedback Signal', to test circuit integrity and determine if the module itself has failed or if there is a wiring short.

Parts You'll Likely Need

• Trailer Brake Power Module (OEM #85073617)
  Trusted brands: ACDelco (OEM), Dorman
  OEM price range: $110-$250
  Aftermarket price range: $90-$180
• Trailer Brake Control Switch (OEM #84108373)
  Trusted brands: ACDelco (OEM), Dorman, AA Ignition
  OEM price range: $40-$65
  Aftermarket price range: $25-$45
• TBCM Connector Pigtail (OEM #ACDelco PT2715)
  Trusted brands: ACDelco
  OEM price range: $30-$50
  Aftermarket price range: $20-$40
• Vehicle Battery — A weak battery is a primary cause of this code, as specified in GM service bulletin PIT5521A.
  Trusted brands: ACDelco, Interstate, Optima
  OEM price range: $200-$350
  Aftermarket price range: $150-$250

Related Codes That Often Appear With This One

• C1113 — Relates to the trailer brake control relay circuit, often set together with C1112 due to general electrical faults like low voltage or corrosion.
• C1114 — Also relates to the trailer brake system (often for a short to power or poor ground) and is frequently cited in the same TSBs as C1112 for the same underlying causes.
• C111B — Another trailer brake system code triggered by battery or connection issues, per TSB #PIT5521A.
• U1556 / U1505 — These are communication codes that can appear alongside the C-codes when a module is malfunctioning or not communicating properly due to low voltage or a bad connection.

Technical Service Bulletins (TSBs) & Recalls

• Advises addressing any battery issues first before performing in-depth diagnostics for a 'Service Trailer Brake' message with DTCs including C1112. This bulletin was later consolidated into PIT5529A.
• A key TSB that combines previous advice. It points to low battery voltage and water intrusion into the Trailer Brake Power Module connector as primary causes for C1112. It specifically mentions to check for missing or dislodged cavity plugs in the connector that allow moisture to enter.
• Warns that many returned Trailer Brake Control Modules (TBCM) and relays are found to be fully functional. It advises thorough diagnosis of the wiring (dark blue circuit 47) for shorts or high resistance between the relay and the 7-pin connector before replacing expensive components.
• While focused on C1114, this TSB provides relevant context, explaining that the system is sensitive to wiring issues because it must first determine if the trailer has electromagnetic or electro-hydraulic brakes. A short to power or a poor trailer ground can trigger the 'Service Trailer Brake System' message.

Platform-Specific Known Issues

• TSB #PIT5529A is the most critical document for this issue, combining previous bulletins. It explicitly links the 'Service Trailer Brake' message and code C1112 to either a weak battery or water intrusion in the TBCM connector near the spare tire. It highlights that missing cavity plugs in the connector are a key source of water entry.

Mechanic-Grade Diagnostic Values

• TBCM Relay Feedback Signal Voltage — expected: 3.2 - 4.1 Volts. Failure: A voltage reading higher or lower than the specified range indicates a short or open in the circuit. Higher voltage may be addressed by a TBCM software update per TSB PIT4595J.
• Battery Voltage for DTC Condition — expected: 9.5 - 17 Volts. Failure: The code may set if the voltage is outside this range during module initialization.
• TBCM Ground Circuit Resistance — expected: Less than 5 ohms. Failure: Resistance greater than 5 ohms between the ground circuit terminal at the TBCM connector and a known good ground indicates high resistance or an open in the ground circuit.

Scan Tool Commands That Help

• Tech 2 / GDS2: Monitor 'TBCM Relay Feedback Signal' in the Trailer Brake Control Module data display. — This is a primary diagnostic step to determine if there is a short to voltage or ground on the dark blue wire (circuit 47) without having to physically probe the wire. It helps differentiate between a wiring issue and a module fault.
• GM SPS (Service Programming System): Trailer Brake Control Module Programming — If the Trailer Brake Power Module (TBCM) is replaced, the new module must be programmed to the vehicle's VIN using GM's online Service Programming System. Failure to program a new module can result in a P0602 (Control Module Not Programmed) code and continued system malfunction.

Wiring & Ground Locations

• G401 — On the rear of the frame, typically near the spare tire or trailer hitch.. This is a primary ground point for the rear chassis components, including the trailer wiring harness. TSB #PIT4595J specifically calls for examining G401 for proper grounding and cleaning it as required when diagnosing C1112 or C1114. A poor ground here can cause a variety of trailer-related electrical faults.
• Circuit 47 (Dark Blue wire) — Runs from the Trailer Brake Power Module (TBCM) to pin 2 of the 7-pin trailer connector at the bumper.. This is the trailer brake output supply voltage circuit. Code C1112 is often set when the TBCM detects a short to ground, short to voltage, or an open on this specific wire.
• TBCM Connector (X411) — Connects to the Trailer Brake Power Module, located on the driver's side frame rail, often above the spare tire.. This connector is the most common failure point due to corrosion. Diagnostic procedures involve checking for B+ voltage at terminal F and ground integrity at terminal B.

Real Owner Repair Stories

• YouTube user 'C Farmer' (2015 Chevrolet Silverado 2500 HD 6.0L) — Intermittent 'Service Trailer Brake System' warning that had been occurring for years and recently became almost constant.
  ❌ Tried (didn't work) Initially unplugged the in-dash control switch to stop the annoying message, but this did not resolve the underlying fault.
  ✅ What actually fixed it Replacing the Trailer Brake Power Module (TBCM) located under the spare tire resolved the issue. The owner noted that while this fixed their problem, other system issues could cause the same warning.

OEM Part Supersession History

• 23337305, 23114650, 23104453 → 85073617 — Part revision and update by the manufacturer. The newer part may have improved internal components or sealing to prevent failures.
  Heads up: The newest part number (85073617) is the correct replacement for the earlier numbers and is functionally compatible.
• 23145874 → 84108373 — Updated design for the in-dash trailer brake control switch to address a known issue with the original switches malfunctioning and causing random warnings.