U1505 on 2015-2018 Chevy Silverado 2500: 'Service Trailer Brake' Causes and Fixes

What's Unique About the 2015-2018 Chevrolet SILVERADO 2500

The third-generation Silverado HD trucks (2014-2019) and related GM SUVs are known for this issue due to the placement of the Trailer Brake Power Module and its main electrical connector. Located on the driver's side frame rail, often near the spare tire, the connector is highly exposed to road spray, salt, and moisture. TSB PIT5529A specifies that the issue is often caused by missing or dislodged cavity plugs in the connector, which allows water to enter and cause corrosion. This can lead to a condition called 'connector hydro-locking,' where water pressure prevents a secure connection. This makes it a very common and platform-specific failure point.

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
• Integrated trailer brake controller is inoperative
• ABS and Traction Control lights may illuminate in some cases

Most Likely Causes

1. Corroded Trailer Brake Power Module (TBPM) Connector 🔴 High Probability The module's connector is located on the driver's side frame rail, making it highly susceptible to water and road salt intrusion. GM TSB #PIT5529A identifies this as the primary cause, specifically noting that missing or dislodged cavity plugs allow water to enter the connector.
   How to confirm: Visually inspect the multi-pin connector (GM P/N 15491307) at the Trailer Brake Power Module, located above the spare tire area on the driver's frame rail. Look for green or white crusty deposits, bent pins, or signs of water entry. Check that all wire cavities have a plug seated in them.
   Typical fix: Disconnect the battery. Unplug the connector, clean the terminals meticulously with a terminal cleaning brush and electrical contact cleaner. If corrosion is severe, the connector pigtail and terminals must be replaced with a service connector (GM P/N 89047377). Apply dielectric grease to the connector seals before reassembly to prevent future moisture intrusion.
   Est. part cost: $25-$75
2. Corroded Battery Terminals or Low Battery Voltage 🟡 Medium Probability TSB #PIT5529A explicitly states that low battery voltage can cause these communication issues and diagnostics should begin with a battery health check. Poor connections can cause a range of electrical and communication issues across the vehicle.
   How to confirm: Inspect the positive and negative battery terminals for corrosion, looseness, or fraying cables. A professional battery test (like the Midtronics GR8 tester mentioned in the TSB) is recommended over a simple voltage check.
   Typical fix: Clean the battery terminals and cable ends thoroughly with a wire brush. Ensure connections are tight. If the battery fails a load test, it must be replaced.
   Est. part cost: $0-$10
3. Damaged Wiring Harness ⚪ Low Probability The wiring harness runs along the frame and is exposed to the elements. While durable, it can be damaged by road debris or chafe against the frame. One documented repair found the corroded wire not at the module, but further up the harness near the running board motor.
   How to confirm: Visually inspect the wiring harness leading to the Trailer Brake Power Module for any signs of chafing, cuts, or breaks. Pay close attention to where the harness is secured to the frame and near other components like running board motors.
   Typical fix: Repair the damaged section of wire using solder and heat-shrink tubing or appropriate weatherproof butt connectors.
   Est. part cost: $5-$25

Rare But Worth Checking

• Failed Trailer Brake Power Module (TBPM): While possible, the module itself is more robust than its connector. This should only be considered after all wiring and connection issues have been ruled out. A diagnostic scan showing a good LIN signal but no communication would point to a faulty module.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm U1505 and any other related codes are present.
2. Perform a battery state of health check using a professional battery tester. TSB #PIT5529A recommends this as the first step before any other diagnostics. Address any battery issues first.
3. Disconnect the negative terminals on both batteries for safety before proceeding.
4. Locate the Trailer Brake Power Module (TBPM) on the driver's side frame rail, generally above or near the spare tire.
5. Carefully unplug the large electrical connector from the TBPM. This may have a red locking tab that needs to be slid out first.
6. Thoroughly inspect the pins and cavities on both the module side and the harness connector side for any green/white corrosion, moisture, or damage. Specifically check if any cavity plugs are missing or pushed out.
7. If corrosion is found on the TBPM connector, clean it using a terminal cleaning tool and electrical contact cleaner. If the damage is severe, the connector pigtail will need to be replaced.
8. Apply a generous amount of dielectric grease to the connector seal to prevent future water intrusion.
9. Reconnect the connector firmly, ensuring it clicks into place and the lock is engaged. Reconnect the batteries.
10. Clear the trouble codes with the scanner and start the vehicle to see if the "Service Trailer Brake System" message returns.

Parts You'll Likely Need

• Trailer Brake Power Module Connector Pigtail (OEM #89047377 (Service Connector)) — If the original connector is too corroded to be cleaned, it must be cut out and replaced with a new pigtail to ensure a reliable connection.
  Trusted brands: ACDelco, Dorman
  OEM price range: $40-$70
  Aftermarket price range: $20-$40
• Dielectric Grease — Essential for preventing future moisture intrusion and corrosion in the repaired connector.
  Trusted brands: Permatex, CRC
  OEM price range: $5-$15
  Aftermarket price range: $5-$10

Related Codes That Often Appear With This One

• U1556 — This is another LIN bus communication code, often set alongside U1505, indicating a general communication loss with the trailer brake module.
• C1112, C1113, C1114, C111B — These are chassis codes related to the trailer brake system. They are listed in TSB #PIT5529A as codes that may appear with U1505, all pointing to the same underlying electrical fault.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Addresses a "Service Trailer Brake" message and DTCs C1112, C1113, C1114, C111B, U1556, and/or U1505, pointing to corrosion at the Trailer Brake Power Module connector (due to missing cavity plugs) and low battery voltage.
• PIT5529: An earlier version of the same bulletin with similar diagnostic advice.

Platform-Specific Known Issues

• GM issued Technical Service Bulletin #PIT5529A, which directly addresses the "Service Trailer Brake" message and codes including U1505. It explicitly calls for technicians to inspect the Trailer Brake Power Module connector for corrosion, often caused by missing cavity plugs allowing water entry.
• The bulletin also makes a point to check battery health first, as low voltage can mimic the symptoms of a communication failure.

Mechanic-Grade Diagnostic Values

• LIN Bus Signal Voltage — expected: A square wave data signal switching between approximately 0V and 12V when viewed on a lab scope.. Failure: Signal is stuck near 0V, indicating a loss of communication from the master module or a short on the line.
• TBPM Connector Power and Ground — expected: With key on, there should be battery voltage at Pin 32 (RD/WH wire) and continuity to ground at Pin 1 (BK wire) of the TBPM connector.. Failure: Absence of voltage or a high resistance to ground indicates a wiring fault between the fuse block/ground point and the module.

Hidden / Shadow Codes Worth Checking

• Fault differentiation via DIC message + DTC: According to GM TSB PIT5509L, the combination of messages and codes helps pinpoint the fault type on the trailer output circuit (Circuit 47). 'Check Trailer Wiring' with NO DTC indicates an open or high resistance. 'Check Trailer Wiring' WITH DTC C1114 indicates a short to ground. 'Service Trailer Brake System' WITH DTC C1114 indicates a short to power or a poor trailer ground. (see via Observe the Driver Information Center (DIC) message and read codes with a capable OBD-II scanner.)

Scan Tool Commands That Help

• GDS2 (GM Dealer Scan Tool): Data Display > Trailer Brake Control Output Circuit — To check the live status of the trailer brake output circuit as seen by the Chassis Control Module. The status will display as 'Okay', 'Short to Ground', 'Short to Power', or 'Unknown'. This helps confirm an electrical fault without needing a trailer connected.
• GDS2 (GM Dealer Scan Tool): Data Display > Trailer Brake Type — To verify if the truck is correctly identifying a connected trailer (or tester). It will display 'Electro-magnetic', 'Electro-over hydraulic brakes', or 'Undefined' (when nothing is connected). This confirms the initial handshake between the truck and trailer is working.
• Any: Module Replacement Programming — No programming is required when replacing the Trailer Brake Power Module (TBPM). It is a plug-and-play component.

Wiring & Ground Locations

• Trailer Brake Power Module (TBPM) — On the driver's side frame rail, located above the spare tire.. This is the module that fails to communicate, setting code U1505. Its exposed location is the primary reason for the common corrosion failures at its connector.
• TBPM Connector (X133) — The 47-way connector that plugs directly into the TBPM.. This is the most common failure point. Key pins for testing are Pin 1 (Ground), Pin 32 (Battery Positive Voltage), and the LIN bus wire.
• TBPM Ground — The ground connection for the TBPM is provided through Pin 1 (Black wire) in the main connector, which is grounded at a shared chassis ground point.. A poor ground connection can cause a loss of communication or erratic module behavior, mimicking other failures.
• Aftermarket Controller Connector Port — For trucks without the factory ITBC, the plug-in port for an aftermarket controller harness is in a black junction box under the driver's side dash, above the driver's left foot and behind the parking brake pedal.. This is not directly related to the U1505 code for the factory system, but is critical information for anyone looking to bypass a faulty factory system with an aftermarket controller.

Real Owner Repair Stories

• YouTube: JL's Auto Solution (2015+ Chevrolet Silverado) — "Service Trailer Brake System" message on the dash. Scan tool showed no communication with the Trailer Brake Control Module.
  ❌ Tried (didn't work) Initial visual inspection did not immediately reveal the problem.
  ✅ What actually fixed it A broken power wire was found right at the entry point to the TBPM connector. The technician de-pinned the affected terminal from the connector, soldered a new section of wire to the terminal, re-inserted the repaired pin into the connector, and sealed the repair with heat shrink. This restored communication and cleared the code.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — Standard part evolution to integrate new materials, technologies, or manufacturing processes.