U1505 on 2014-2018 GMC Sierra 1500: Service Trailer Brake System Fixes

What's Unique About the 2014-2018 Gmc SIERRA 1500

On this generation of GMC Sierra (K2XX platform), the Trailer Brake Power Module is mounted on the frame rail above the spare tire. This location makes it highly susceptible to moisture, road salt, and debris, leading to corrosion in the module's main electrical connector. GM has acknowledged this issue in Technical Service Bulletin PIT5529A, noting that missing or dislodged cavity plugs in the connector can allow water to enter and cause communication failures. The system is also very sensitive to voltage, making battery health a critical first check.

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 brake gain display on the dash may be blank or show no output
• The trailer brake system will not operate when a trailer is connected
• Warning chime may sound repeatedly while driving
• Message appears intermittently, sometimes even without a trailer connected

Most Likely Causes

1. Corroded Trailer Brake Power Module (TBCM) Connector 🔴 High Probability The module is located on the frame above the spare tire, exposing the connector to road spray and moisture. TSB #PIT5529A directly addresses this, noting that missing or dislodged cavity plugs are a primary cause for water intrusion. The pressure of mating the connector can sometimes push these small rubber plugs out.
   How to confirm: Lower the spare tire to gain access. Disconnect the electrical connector from the Trailer Brake Power Module (K133) and inspect the pins, connector housing, and rubber seals for green or white corrosion, moisture, or pushed-out cavity plugs.
   Typical fix: Clean the connector pins and module terminals carefully with electrical contact cleaner and a small brush. If corrosion is severe, the connector pigtail (ACDelco PT2867 / GM 13580822) and/or the module itself will need to be replaced. Apply dielectric grease to the connector seal upon reassembly to prevent future moisture intrusion.
   Est. part cost: $20-$60 for a connector pigtail
2. Weak or Failing Vehicle Battery 🟡 Medium Probability The control modules on these trucks are sensitive to voltage drops. A weak battery can cause various intermittent communication codes before it's weak enough to cause a no-start condition. TSB #PIT5529A explicitly recommends checking the battery's state of health with a proper load tester before any in-depth diagnostics. GM service information for LIN bus diagnostics specifically recommends connecting a battery maintainer (like the GR8) during testing to prevent false readings.
   How to confirm: Perform a load test on the battery using a professional battery tester (such as the GM-recommended EL-50313 Midtronics GR8). A simple multimeter voltage check is often insufficient. LIN bus diagnostics, in particular, require stable system voltage to get accurate readings.
   Typical fix: Replace the vehicle's battery and ensure all terminal connections are clean and tight.
   Est. part cost: $150-$350
3. Failed Trailer Brake Power Module (TBCM) ⚪ Low Probability The module can fail internally, often due to long-term moisture exposure and corrosion that has compromised the internal circuit board. Sometimes the module is replaced simply because the pins are too corroded to be cleaned effectively.
   How to confirm: This is typically diagnosed by exclusion. If the connector, wiring, power, and ground (especially G401 on the rear frame rail) are all confirmed to be good, the module itself is the likely culprit.
   Typical fix: Replace the Trailer Brake Power Module. The latest GM part number is 85073617, which supersedes 23337305. This part is plug-and-play and does not require programming.
   Est. part cost: $80-$200

Rare But Worth Checking

• Damaged Wiring Harness: A wire in the harness leading to the TBCM could be broken or shorted, especially the LIN bus communication wire (typically a Dark Blue wire). This can happen from road debris, improper repairs, or chafing against the frame.
• Poor Ground Connection: These trucks are known to have issues with main body and frame ground points. A corroded or loose ground, particularly ground G401 located on the rear frame rail near the trailer harness, can cause a host of intermittent electrical issues, including module communication faults.
• Faulty In-Dash Trailer Brake Control Switch: While often misdiagnosed as the primary cause, the dash-mounted control switch assembly can fail internally, causing communication errors. Several owner reports confirm that replacing this switch (e.g., ACDelco 84108373) resolved their "Service Trailer Brake System" message, though it's less common than the rear module/connector issue.

Diagnosis Steps

1. Read all Diagnostic Trouble Codes (DTCs) from all modules to get a full picture of the faults.
2. Begin with the simplest checks. Per TSB PIT5529A, have the vehicle's battery professionally load-tested. Address any battery issues first.
3. Check underhood fuses related to the trailer brake system. A blown 30A fuse (e.g., Fuse #2 in some underhood blocks) can cause this issue.
4. Lower the spare tire to gain access to the Trailer Brake Power Module (TBCM) on the frame rail.
5. Disconnect the TBCM electrical connector. Thoroughly inspect both the harness side and the module side for any signs of green/white corrosion, moisture, or pushed-out pins. Pay close attention to the small rubber cavity plugs to ensure they are all present and fully seated.
6. If corrosion is found, clean it thoroughly with electrical contact cleaner. If damage is severe, the connector pigtail may need to be replaced.
7. If no corrosion is visible, use a multimeter to check for proper power and ground at the connector. Check the integrity of ground G401 on the rear frame.
8. If power, ground, and the connector are all good, the issue is likely a faulty TBCM or a break in the LIN bus communication wire. Advanced diagnostics may involve checking the LIN bus for a toggling voltage between ~1V and ~12V.
9. After repairs, apply dielectric grease to the connector to protect it from future moisture intrusion.

Parts You'll Likely Need

• Trailer Brake Power Module (OEM #85073617 (supersedes 23337305, 23114650, 23104453)) — The module itself can fail due to internal corrosion from moisture, or it may be replaced if the corroded terminals cannot be salvaged.
  Trusted brands: ACDelco (Genuine GM), Dorman (e.g., 926-068)
  OEM price range: $130-$220
  Aftermarket price range: $80-$150
• Trailer Brake Module Connector Pigtail (OEM #ACDelco PT2867 / GM 13580822) — If the original connector is heavily corroded, it must be cut out and a new pigtail spliced in to ensure a solid connection. This is a 16-way connector.
  Trusted brands: ACDelco, Duralast, EFI Connection
  OEM price range: $40-$60
  Aftermarket price range: $25-$45
• Dielectric Grease — Essential for preventing moisture from re-entering the connector after the repair is completed.
  Trusted brands: Permatex, CRC
  OEM price range: $5-$15
  Aftermarket price range: $5-$15

Related Codes That Often Appear With This One

• U1556 — This is another LIN Bus communication code, often set alongside U1505, indicating a communication loss with the TBCM. TSB PIT5529A lists them together.
• C1112, C1113, C1114 — These are chassis codes related to the trailer brake control circuit (relay faults, output circuit faults) that are frequently triggered by the same root causes of corrosion or wiring faults at the TBCM.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Service Trailer Brake Message With DTC's C1112, C1113, C1114, C111B, U1556 and/or U1505. Points to low battery voltage or a corroded Trailer Brake Power Module connector with missing cavity plugs.
• PIT5529: The predecessor to PIT5529A, also identifying the corroded TBCM connector as the primary cause.
• PIT5698C: Provides general LIN Bus diagnostic information, relevant for testing the communication circuit for the U1505 code.

Platform-Specific Known Issues

• GM Technical Service Bulletin #PIT5529A explicitly details the cause of this code being corrosion in the Trailer Brake Power Module connector due to its location and potential for water intrusion from missing or dislodged cavity plugs.
• A real-world repair documented on the 'How To DIY Moore' YouTube channel showed a user with an intermittent 'Service Trailer Brake' warning who resolved it by replacing the TBCM (part #23337305 with the updated #85073617).
• A user on the gm-trucks.com forum described the classic symptoms appearing, and the fix was cleaning heavy corrosion from the TBCM connector and applying dielectric grease. This is a very common owner experience.

Mechanic-Grade Diagnostic Values

• LIN Bus Voltage (at TBCM connector) — expected: Toggling waveform between approx. 1V and system voltage (approx. 12V) when communicating. A steady voltage of approx. 12V indicates the bus is at rest or a device is not communicating.. Failure: Voltage significantly lower than 12V at rest (e.g., 4-5V) suggests a short to ground on the LIN circuit. No voltage or a steady low voltage indicates an open or a faulty module pulling the bus down.
• TBCM Power Supply Voltage — expected: 11 volts or greater between the ignition voltage circuit terminal and a good ground with Key On.. Failure: Voltage below 11V indicates a short or open/high resistance in the power feed circuit or a blown fuse.
• TBCM Ground Circuit Resistance — expected: Less than 5.0 Ohms between the ground circuit terminal at the TBCM connector and a known good chassis ground (like the frame).. Failure: Resistance greater than 5.0 Ohms points to a corroded, loose, or broken ground connection (check G401).

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Module Diagnostics > Chassis Control Module > Data Display — To monitor the status of the LIN bus and see if the master module (CCM) is attempting to communicate with the Trailer Brake Power Module (TBCM). This can help determine if the fault is with the master, the slave (TBCM), or the wiring between them.
• GDS2 (GM Global Diagnostic System 2): Control Functions — To perform bi-directional tests, which may allow a technician to command specific outputs on the trailer brake system to verify functionality of the module and wiring, separate from the in-dash controls.

Wiring & Ground Locations

• G401 — Mounted to the left rear body mount, behind the left rear tire on the frame.. This is a primary ground point for rear-body electrical components, including the trailer wiring harness and potentially the TBCM. Corrosion or a loose connection here can cause a loss of ground to the module, leading to communication codes like U1505.
• K133 Trailer Brake Power Module Connector (ACDelco PT2867) — Above the spare tire, mounted to a bracket on the frame rail.. This is the main connection point for the module. Key pins for testing are: Pin 1 (Power), Pin 9 (Ground), and Pin 12 (LIN Bus communication wire, typically Green/Violet). Some service documents for similar models show Pin 32 (RD/WH) for Battery Positive Voltage. Always verify with the specific diagram for the vehicle.
• Underhood Fuse Block — Under the hood, typically on the driver's side.. Contains fuses for the trailer brake system. Specifically, a 30A fuse in position #2 (for Trailer Battery) has been identified as a cause. Other relevant fuses can be in positions 1, 14, 15, 16, and 17.

Real Owner Repair Stories

• YouTube channel '59 Motorsports' (2014-2018 GM Truck (specific year not stated, but had previously fixed a 2016 Sierra with the same issue)) — "Service Trailer Brake" message on the dash.
  ❌ Tried (didn't work) Replacing the in-dash trailer brake controller switch assembly.
  ✅ What actually fixed it After replacing the dash switch did not solve the problem, the owner checked the underhood fuse box and found a blown 30 amp fuse in position #2, labeled for the trailer battery system. Replacing the fuse cleared the message.
• YouTube channel 'Fixing cars and stuff' (2017 Suburban (applies to Sierra)) — "Service Trailer Brake System" message, code U1556 (Loss of communication with device on LIN bus) set by the Chassis Control Module.
  ✅ What actually fixed it The technician verified good power and ground at the TBCM connector and used an oscilloscope to confirm a good LIN signal was being sent from the master module. Since the TBCM had everything it needed to communicate but was not responding, the module itself was condemned and replaced. The new module was plug-and-play and fixed the issue.
• YouTube channel 'Just a dude with a wrench' (2019 GMC Sierra Elevation (newer generation, but same TBCM part #23337305 and principles apply)) — "Service Trailer Brake System" warning on every startup. Code U1556 was present.
  ❌ Tried (didn't work) Checked for power and ground at the TBCM, which were both present., Checked for continuity on the LIN wire between the TBCM and the master module, which was good.
  ✅ What actually fixed it Upon physical inspection, the Trailer Brake Power Module (TBCM) was found to be full of water due to a compromised seal. Replacing the water-damaged module (part #23337305) resolved the communication code.

OEM Part Supersession History

• 23104453 → 23114650, then 23337305, then finally 85073617 — GM regularly updates service parts to integrate new materials, technologies, or fix known issues. TSB 23-NA-149 suggests an updated part is needed to resolve issues with trailers that have their own charging systems.
  Heads up: All listed part numbers are considered interchangeable for the 2014-2018 model years, but using the latest version (85073617) is recommended to get all design updates.