U1505 on 2015-2018 GMC Sierra 2500: Causes and Fixes for 'Service Trailer Brake'

What's Unique About the 2015-2018 Gmc SIERRA 2500

On this generation of GMC Sierra HD trucks (and its Chevrolet Silverado platform mate), the U1505 code is strongly linked to the physical location and design of the Trailer Brake Control Module (K133). The module is mounted under the truck bed near the spare tire, leaving its electrical connector exposed to moisture, road salt, and debris. This exposure makes corrosion on the connector terminals a very common point of failure, a fact highlighted in official GM Technical Service Bulletins #PIT5529 and #PIT5529A. The TSBs specifically note that missing or dislodged cavity plugs in the connector can allow water to enter and cause these issues.

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 dashb 🎬 Watch: A quick overview of this common warning and fix.oard.
• "Check Trailer Wiring" message on the dashboard.
• Audible chimes or alarms when the message appears.
• Trailer brakes do not engage when the manual controller or vehicle's brake pedal is used.
• The gain setting for the trailer brake will not adjust.
• Trailer connection is not recognized by the truck.
• The message may appear intermittently, sometimes more frequently in wet or humid weather.

Most Likely Causes

1. Corroded or Damaged Trailer Brake Power Module Connector 🔴 High Probability → Shop Trailer Brake Control Module The module is located under the truck, above the spare tire, where its connector is exposed to the elements, leading to water intrusion and corrosion. This is a well-documented issue cited in TSBs #PIT5529A and #PIT5529. The TSBs specifically mention that missing cavity plugs on the connector can lead to a 'hydro-locking' condition where water is forced in.
   How to confirm: Lower the spare tire for access. Visually inspect the connector (both the harness side and the module side) for green or white corrosion, moisture, or any bent/broken pins. Check that all cavity plugs are present and fully seated in the connector body.
   Typical fix: Disconnect the battery. Unplug the connector and clean the terminals with an electrical contact cleaner and a small terminal brush. Let it dry completely. Apply dielectric grease to the connector seal to prevent future moisture intrusion before reconnecting. If corrosion is severe, the connector pigtail must be cut out and a new one spliced in. GM Genuine Parts pigtail connectors are available, such as part number PT2867.
   Est. part cost: $8-$75
2. Faulty Trailer Brake Control Module (TBCM) 🟡 Medium Probability → Shop Trailer Brake Control Module Internal failure of the module can occur, often secondary to moisture getting past a corroded connector seal. The module itself is an electronic component subject to failure from vibration and thermal cycles.
   How to confirm: If the connector and wiring are confirmed to be in perfect condition (clean, dry, and have good power/ground) but the code persists, the module itself is the likely culprit. A high-end scan tool may also directly indicate module failure or an inability to communicate with it.
   Typical fix: Replace the Trailer Brake Control Module. 🎬 See this step-by-step walkthrough for replacing the module. This is a bolt-on part located above the spare tire. It must be unplugged and unbolted from its bracket.
   Est. part cost: $100-$250
3. Blown Fuse ⚪ Low Probability Electrical shorts in the trailer wiring or a fault within the TBCM circuit can cause a fuse to blow as a protective measure.
   How to confirm: Check the fuses related to 'Trailer Brake' in the under-hood fuse box. On these trucks, this is often a 30A J-Case fuse in position F1. A visual inspection or a multimeter can confirm if a fuse is blown.
   Typical fix: Replace the blown fuse with one of the same amperage. If the fuse blows again immediately, a short circuit exists that needs to be traced.
   Est. part cost: $1-$5
4. Faulty In-Dash Trailer Brake Control Switch ⚪ Low Probability → Shop Trailer Brake Control Module While less common for a U1505 code than the under-truck module failure, the switch on the dashboard can fail internally, causing communication issues or other trailer brake-related codes.
   How to confirm: After confirming the main TBCM and wiring are good, this becomes a possibility. Sometimes specific codes related to the switch itself may be present, like C1117. Some owners replace this as a preventative measure due to known issues.
   Typical fix: Replace the trailer brake control switch assembly in the dashboard. This requires removing the dash trim panel to the left of the steering wheel. 🎬 Watch: How to replace the in-dash trailer brake switch.
   Est. part cost: $25-$50

Rare But Worth Checking

• Damaged Wiring Harness: Check for chafing or breaks in the wiring harness leading from the front of the truck to the TBCM in the rear, especially anywhere it passes through the frame or near suspension components. Repairs often involve splicing in new sections of wire.
• Poor Battery Terminal Connection or Weak Battery: TSB #PIT5529A explicitly mentions inspecting battery terminals for corrosion and checking the battery's state of health first. Low system voltage can cause communication errors across various modules, including the TBCM.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm U1505 is present and see if other related codes exist.
2. Check for the 'Service Trailer Brake System' message on the driver information center.
3. Per TSB #PIT5529A, first inspect the battery terminals for corrosion and test the battery's health. A weak battery can cause various module communication issues.
4. Inspect the fuses for the trailer brake system in the under-hood fuse panel. Specifically check the 30A fuse in slot F1 labeled 'Trailer Brake'.
5. Lower the spare tire to gain full access to the Trailer Brake Control Module (TBCM) and its connector.
6. Disconnect the negative battery terminals.
7. Carefully unplug the electrical connector from the TBCM. Inspect both the connector and the module's pins for any signs of green or white corrosion, moisture, or physical damage like bent pins. Pay close attention to see if any small rubber cavity plugs are missing from the connector body, as this is a known entry point for water.
8. If corrosion is found, clean the terminals thoroughly with an electrical contact cleaner and a dedicated terminal cleaning brush. Let it dry completely.
9. Apply a small amount of dielectric grease to the connector seal to prevent future moisture intrusion.
10. Reconnect the connector, ensuring the locking tab clicks into place. Reconnect the battery terminals.
11. Clear the codes and start the truck to see if the message returns. If it does not, the issue is resolved.
12. If the connector was clean or cleaning it did not solve the problem, the TBCM itself is the next most likely failure point and should be replaced.

Parts You'll Likely Need

• Trailer Brake Control Module (OEM #85073617) — This module is the most common point of failure for code U1505, either due to internal faults or as a result of damage from a corroded connector.
  Trusted brands: ACDelco, Dorman
  OEM price range: $150-$250
  Aftermarket price range: $100-$180
• TBCM Connector Pigtail (OEM #PT2867) — If the connector is too corroded to be cleaned, it must be replaced. This involves cutting the old connector off and splicing the new pigtail wire by wire.
  Trusted brands: ACDelco, Duralast
  OEM price range: $60-$75
  Aftermarket price range: $25-$50
• Trailer Brake Control Switch (OEM #84109447) — If the main module and wiring are confirmed to be good, the switch in the dash is the next logical part to replace, though it's a less common cause for U1505 specifically.
  Trusted brands: ACDelco
  OEM price range: $30-$50
  Aftermarket price range: $20-$40
• Electrical Contact Cleaner & Dielectric Grease — Required for cleaning corrosion from the TBCM connector, which is the most common cause of the problem, and protecting the connection from future moisture.
  Trusted brands: CRC, WD-40, Permatex
  OEM price range: $8-$15
  Aftermarket price range: $5-$10

Related Codes That Often Appear With This One

• C1112, C1113, C1114, C111B, U1556 — These codes are listed alongside U1505 in TSBs #PIT5529 and #PIT5529A, all relating to faults within the trailer brake control system.

Technical Service Bulletins (TSBs) & Recalls

• PIT5529A: Addresses DTCs C1112, C1113, C1114, C111B, U1556, and/or U1505 with a 'Service Trailer Brake' message, pointing to corroded terminals on the battery and Trailer Brake Power Module (K133) connector. It specifically calls out checking for missing cavity plugs.
• PIT5529: An earlier version of the bulletin with similar guidance, focusing on the Trailer Brake Power Module connector corrosion due to water intrusion from missing plugs.

Platform-Specific Known Issues

• GM issued Technical Service Bulletin #PIT5529A, which directly addresses code U1505 along with a 'Service Trailer Brake' message. It instructs technicians to inspect the Trailer Brake Power Module connector and battery terminals for corrosion and replace parts as necessary. A key detail is to check for missing or dislodged cavity plugs on the connector, which allows water intrusion.

Mechanic-Grade Diagnostic Values

• Battery Positive Voltage at TBCM Connector (X411) — expected: 9-17 Volts, typically ~12.6V with key on, engine off.. Failure: Voltage below 9V or absent, indicating a problem with the power feed circuit or fuse.
• Ground Circuit Resistance at TBCM Connector (X411) — expected: Less than 1.0 ohm resistance to a clean chassis ground point.. Failure: High resistance (O.L. or several ohms) indicates a corroded or broken ground wire at a location like G401.
• High-Speed GMLAN Bus Resistance — expected: Approximately 60 ohms when measured across pins 6 and 14 of the DLC with the battery disconnected.. Failure: A reading of ~120 ohms indicates an open in the bus or a missing termination resistor (one module is offline). A reading near 0 ohms indicates the CAN High and CAN Low wires are shorted together.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Data Display > Trailer Brake Type — When a trailer is connected, this parameter in the Chassis Control Module (CCM/BSCM) should display 'Electro-magnetic' or 'Electro-over hydraulic brakes'. If it shows 'Undefined' with a trailer connected, it indicates a primary connection detection failure.
• GDS2 (GM Global Diagnostic System 2): Data Display > Trailer Brake Control Output Circuit — This parameter helps diagnose related code C1114 by showing the circuit status as 'Okay', 'Short to Ground', or 'Short to Power', which can help pinpoint the electrical fault type.

Wiring & Ground Locations

• K133 — The Trailer Brake Control Module itself, located on a bracket above the spare tire.. This is the module that is losing communication, setting the U1505 code. Its exposed location is the root of most failures.
• X411 — The main electrical connector that plugs into the Trailer Brake Control Module (K133) above the spare tire.. This connector is the single most common failure point. Corrosion or damage to its pins (especially power, ground, and CAN bus lines) directly causes the U1505 communication loss.
• G401 — A primary chassis ground point located on the rear of the frame, often near the trailer hitch or bumper area.. The TBCM requires a solid ground to function. Corrosion or looseness at G401 can cause an open in the ground circuit, leading to communication and power issues for the module.
• TBCM Connector Pin 32 (RD/WH wire) — At the K133 module connector (X411).. This is the Battery Positive Voltage circuit. A voltage test here confirms if the module is receiving power from the fuse block.
• TBCM Connector Pins 21 (TN) & 22 (TN/BK) — At the K133 module connector (X411).. These are the High-Speed GMLAN (CAN bus) serial data lines. A break or short in these specific wires will cause a direct loss of communication and set code U1505.

Real Owner Repair Stories

• YouTube user '59 Motorsports' (2016 GMC Sierra) — "Service Trailer Brake" message on the dash.
  ❌ Tried (didn't work) Replacing the in-dash trailer brake control switch did not fix the issue on this particular truck (though it had fixed it on a previous truck he owned).
  ✅ What actually fixed it Found a blown 30-amp fuse (Fuse #2) in the under-hood fuse box. Replacing the fuse cleared the message.
• YouTube user 'Blessed Beyond Measures' (2018 Chevrolet Silverado 3500 HD) — "Service Trailer Brake System" message, gain control not working.
  ❌ Tried (didn't work) Replacing the in-dash switch., Cleaning the TBCM connector under the truck.
  ✅ What actually fixed it Replacing the entire Trailer Brake Control Module (Part No. 85073617) located above the spare tire finally resolved the problem. The gain control started working immediately after replacement.
• YouTube user 'Water Damaged Trailer Brake Control Module' (2019 GMC Sierra Elevation (similar system)) — "Service Trailer Brake System" message, code U1556 (Lost Communication With Power Brake Control Module).
  ❌ Tried (didn't work) Visual inspection of the connector pins, which looked clean.
  ✅ What actually fixed it The technician confirmed power, ground, and communication line continuity were all good at the connector. Replacing the module fixed the fault. Upon opening the old module, it was found to be full of water and corrosion internally, despite the external pins looking clean.

OEM Part Supersession History

• 23104453, 23114650, 23337305 → 85073617 — Standard part evolution, potentially for improved sealing or internal components.
• 22811723 → 84109447 — Updated design for the in-dash trailer brake control switch.