Circuit/System Verification

Ignition ON.
Verify that DTCs U1505, U1556, U2504, P2534, P2535, or any other communication DTCs are not set.
- If any of these DTCs are set
  Refer to Diagnostic Trouble Code (DTC) List - Vehicle .
- If none of these DTCs are set
Verify the Service Trailer Brake System message is not displayed in the driver information center
- If the Service Trailer Brake System message is displayed and the customer's trailer is connected to the vehicle
1. Disconnect the customer's trailer.
2. Verify the Service Trailer Brake System message is not displayed in the driver information center.
  - If the Service Trailer Brake System message is not displayed after disconnecting the customer's trailer, the circuit fault causing the DTC to set is located in the customer's trailer wiring harness. This is not diagnosed in service information, nor is it covered by GM warranty.
  - If the Service Trailer Brake System message is displayed after disconnecting the customer's trailer
3. Refer to Circuit/System Testing.
- If the Service Trailer Brake System message is displayed and the customer's trailer is not connected to the vehicle
  Refer to Circuit/System Testing.
- If the Service Trailer Brake System message is not displayed and the customer's trailer is available
Verify the customer's trailer connection is recognized with a Trailer Connected message in the driver information center.
- If the Trailer Connected message is not displayed when the customer's trailer is connected to the vehicle
  Refer to Circuit/System Testing.
- If the Trailer Connected message is displayed when the customer's trailer is connected to the vehicle
All OK.