C1112 on 2017-2018 Chevrolet Tahoe: Service Trailer Brake System Fixes

What's Unique About the 2017-2018 Chevrolet TAHOE

On the K2XX platform, which includes the 2017-2018 Tahoe, this system is highly sensitive to battery voltage. GM has issued multiple Technical Service Bulletins (TSBs), now consolidated into PIT5529A, advising technicians to address battery health before any other diagnostics for C1112. 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. TSB PIT5529A explicitly notes that water intrusion from missing or dislodged cavity plugs can cause the connector to 'hydro-lock', pushing the plugs out and allowing moisture to corrode the terminals.

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 by the vehicle
• Integrated trailer brake controller is inoperative
• Sluggish engine cranking or other seemingly unrelated electrical issues, indicating a weak battery.

Most Likely Causes

1. Weak or Failing Vehicle Battery 🔴 High Probability → Shop Vehicle Battery The control modules on this platform are extremely sensitive to voltage drops during startup. GM TSBs #PIT5529A and #PIT5521A specifically state to check battery health first for this code, as even a slightly weak battery can trigger it before causing a no-start condition. A user on TahoeYukonForum confirmed a new battery fixed the issue on their 2017 model. 🎬 Watch: Real-world fix for the Service Trailer Brake message.
   How to confirm: Perform a professional battery load test using a tool like a Midtronics GR8 tester, as specified in TSB #PIT5529A. A simple multimeter voltage check is often insufficient to identify a battery failing under load.
   Typical fix: Replace the vehicle's battery and clean the terminals. Ensure the new battery is fully charged before installation.
   Est. part cost: $150-$350
2. Corroded Trailer Brake Power Module Connector 🔴 High Probability → Shop Trailer Brake Control Module The module is located on the frame rail above the spare tire, an area highly exposed to road spray. TSB #PIT5529A explicitly notes that water intrusion from missing or dislodged connector cavity plugs is a common cause of corrosion and this code. The pressure of plugging in the connector can even push the seals out ('hydro-locking').
   How to confirm: Lower the spare tire to access the module. Disconnect the battery. Unplug the electrical connector (it has a red locking tab) and thoroughly inspect the pins and terminals on both the harness and module side for green or white corrosion, moisture, or bent pins.
   Typical fix: Clean the connector terminals with a specialized contact cleaner and a small brush. If corrosion is severe, the connector pigtail may need to be replaced (similar to GM P/N 23184088 or service connector 89047377). After cleaning or replacement, apply dielectric grease to the connector seal to prevent future moisture intrusion and ensure all cavity plugs are present and fully seated. 🎬 See this video for common failure points and connector inspection.
   Est. part cost: $20-$250
3. Faulty Trailer Brake Power Module 🟡 Medium Probability → Shop Trailer Brake Control Module While less common than battery or connector issues, the module can fail internally due to moisture intrusion or electronic failure. TSB #PIT4595J notes that many modules are replaced unnecessarily when the actual fault is in the wiring or grounds.
   How to confirm: This is typically diagnosed by exclusion. If the battery, all related fuses, grounds (like G401 on the frame rail), and wiring/connector integrity have been confirmed to be good, the module itself is the likely culprit. A professional scan tool can monitor module-specific data which may help confirm the failure.
   Typical fix: Replace the Trailer Brake Power Module. The new module must be programmed to the vehicle's VIN using GM's SPS2 software to function correctly.
   Est. part cost: $120-$250

Rare But Worth Checking

• Blown Fuse: Check the under-hood fuse block for any blown fuses related to the trailer brake system (often labeled 'TRLR' or similar). This is a simple but sometimes overlooked step.
• Poor Ground Connection: A corroded or loose ground connection (specifically G401 on the driver's side frame rail near the rear) can cause intermittent electrical issues for the trailer brake system.

Diagnosis Steps

1. Read all Diagnostic Trouble Codes (DTCs) from the Chassis Control Module.
2. CRITICAL FIRST STEP: Professionally test the vehicle's battery state of health using a load tester (e.g., Midtronics GR8). Do not proceed until the battery is confirmed to be good. Address any battery issues first.
3. Inspect all fuses related to the trailer brake system in the under-hood fuse block.
4. Lower the spare tire to gain access to the rear frame rail.
5. Locate the Trailer Brake Power Module (K133) on the driver's side frame rail.
6. Disconnect the battery. Disconnect the electrical connector from the module. Thoroughly inspect the connector and module pins for any signs of corrosion, water intrusion, or damage. Check that all cavity plugs are present and fully seated.
7. Inspect the G401 ground connection on the driver's side frame rail for tightness and corrosion. Clean if necessary.
8. If cleaning the connector resolves the issue, apply dielectric grease to the seal before reconnecting to prevent future moisture damage.
9. If all wiring, connectors, grounds, and the battery check out, the Trailer Brake Power Module is the most likely remaining cause.
10. If the module is replaced, it will require programming by a dealer or a qualified shop with the proper tools (SPS2).

Parts You'll Likely Need

• Vehicle Battery — A weak or failing battery is the most common trigger for this code on the 2017-2018 Tahoe, as specified in multiple GM service bulletins.
  Trusted brands: ACDelco, Interstate, DieHard
  OEM price range: $200-$350
  Aftermarket price range: $150-$250
• Trailer Brake Power Module (OEM #85073617) — This module is the second most likely failure point, either from internal failure or, more commonly, from corrosion on its connector pins due to its exposed location. This part number supersedes 23104453, 23114650, and 23337305.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $120-$250
  Aftermarket price range: $100-$180
• Trailer Brake Module Connector Pigtail (OEM #89047377 (Service Connector)) — If the harness-side connector is severely corroded, it must be cut out and replaced. This pigtail allows for splicing in a new, clean connector. 🎬 Watch: Step-by-step guide for replacing the module and connector.
  Trusted brands: ACDelco, EFI Connection
  OEM price range: $40-$70
  Aftermarket price range: $25-$50

Related Codes That Often Appear With This One

• C1113 — This code is for the trailer brake control relay high control circuit and is often set with C1112 due to shared power sources and wiring, as noted in TSB #PIT5529A.
• C1114 — This code relates to the trailer brake control solenoids and is frequently seen with C1112 as they are part of the same system and susceptible to the same voltage and corrosion issues.
• C111B — Also related to the trailer brake system, this code is mentioned in the same TSBs as C1112 and often points to battery voltage problems.
• U1556 — This is a LIN Bus communication code that can be triggered by the Trailer Brake Power Module going offline due to power or ground issues, as mentioned in TSB #PIT5529A.
• U1505 — Another communication code that can be set when the Trailer Brake Power Module has a fault, as listed in TSB #PIT5529A.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Service Trailer Brake Message With DTC's C1112, C1113, C1114, C111B, U1556 and/or U1505. Advises checking battery and connector corrosion. This TSB supersedes PIT5521A.
• PIT5521A: Diagnostic Tip - Service Trailer Brake Message with DTC C111B C1112 C1113 C1114.
• PIT4595J: Diagnostic tips for a service trailer brake message with DTC C1114 and C1112.

Platform-Specific Known Issues

• TSB #PIT5529A: This is the primary TSB for this issue, superseding PIT5521A. It warns that a 'Service Trailer Brake' message with code C1112 is most likely caused by either low battery voltage or water getting into the Trailer Brake Power Module (K133) connector from a missing cavity plug. It specifically instructs to test the battery with a Midtronics GR8 tester before

Mechanic-Grade Diagnostic Values

• TBCM Relay Feedback Signal voltage — expected: 3.2 - 4.1 Volts. Failure: A reading below 3.2V can indicate a short to ground or open circuit; a reading above 4.1V can indicate a short to voltage.
• Voltage on Dark Blue wire (circuit 47) at the 7-pin trailer connector — expected: 3.7 - 4.4 Volts (with module connected, no trailer attached). Failure: Voltage being too high or too low indicates a short or other circuit integrity issue.

Hidden / Shadow Codes Worth Checking

• C1112 00: The '00' is a symptom byte that specifies the fault as 'Trailer Brake Control Relay Low Control Circuit', pointing directly to the relay's control side. (see via A professional scan tool like the GM GDS2 or Tech 2 is required to view symptom bytes.)
• Trailer Brake Control Output Circuit Status (Live Data): This is a live data parameter, not a code, that provides real-time status of the main output circuit. It will display 'Okay', 'Short to Ground', or 'Short to Power', which is extremely helpful for pinpointing the electrical fault. (see via Available in the Chassis Control Module (CCM) data display on a GDS2 scan tool.)

Scan Tool Commands That Help

• GDS2 / Tech 2: Monitor 'TBCM Relay Feedback Signal' — Use this as a primary electrical test with the ignition on to see if the feedback voltage is within the expected 3.2-4.1V range, which quickly diagnoses shorts or open circuits.
• GDS2 / Tech 2: Command Trailer Brake Relay ON/OFF — This bidirectional control allows a technician to manually cycle the trailer brake relay to test its function and the circuit's integrity, often used with a test lamp at the 7-pin bumper connector.
• TIS2WEB: Reprogram TBCM (Trailer Brake Control Module) — According to TSB PIT4595J, updated calibrations were released to improve diagnostics for high-resistance shorts. This should be considered if the 'TBCM Relay Feedback Signal' is consistently above 4.1V and no physical short is found.

Wiring & Ground Locations

• G401 — On the driver's side frame rail, located behind the left rear tire.. This is a primary ground point for rear-body electrical components, including the Trailer Brake Power Module. Corrosion or a loose connection here can cause a variety of trailer system faults, including C1112.
• K133 Trailer Brake Power Module Connector — Above the spare tire, mounted to the driver's side frame rail.. This is the main connector for the module. Key pins for testing C1112 are Pin 28 (Dark Blue, Circuit 47 - Solid State Relay Feedback), Pin 32 (Red/White - Battery Positive Voltage), and Pin 33 (Yellow/Black - Solid State Relay Enable).
• Circuit 47 (Dark Blue wire) — Runs from the Trailer Brake Power Module (K133) to the 7-pin trailer connector at the bumper.. This is the trailer brake control output circuit. The module constantly sends a test pulse on this wire to detect a connected trailer and monitor for faults. A short or open on this wire is a direct cause for codes C1112 and C1114.

Real Owner Repair Stories

• YouTube channel 'The Batri' (2018 Chevrolet Silverado 3500HD (K2XX Platform)) — 'Service Trailer Brake System' message appeared on the DIC immediately after putting the truck in gear with a foot on the brake. The trailer gain control was inoperative.
  ❌ Tried (didn't work) Replaced the in-dash trailer brake controller switch assembly (P/N 84108373)., Disconnected batteries and cleaned the connector at the Trailer Brake Power Module.
  ✅ What actually fixed it The Trailer Brake Power Module (located above the spare tire) was faulty. Replacing the old module (P/N 23337305) with the updated part (P/N 85073617) resolved the issue immediately.

When the Usual Fixes Don't Work

• While the most common causes are a weak battery and corroded connectors, these should be treated as the first diagnostic steps, not the only possibilities. A documented repair on a 2018 Silverado showed that after replacing the in-dash switch and cleaning the module connector to no effect, the problem was ultimately a failed Trailer Brake Power Module itself. This confirms that if the common, simple fixes do not resolve the code, the module can and does fail internally and should be considered the next likely point of failure.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — GM regularly updates service parts to integrate new materials, technologies, or fix previously identified failure modes.
  Heads up: The new part number (85073617) is a direct, backward-compatible replacement for the older numbers on the K2XX platform. A real-world repair confirmed a 23337305 module was successfully replaced by an 85073617 module.