GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra - 2WD 🢒 Brakes 🢒 Trailer Brake Controls 🢒 DTC C056D or C056E

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC C056D 33: ECU Hardware Performance Special Memory Failure

DTC C056D 34: ECU Hardware Performance RAM Failure

DTC C056D 35: ECU Hardware Performance ROM Failure

DTC C056D 36: ECU Hardware Performance EEPROM Failure

DTC C056D 39: ECU Hardware Performance Internal Electronic Failure

DTC C056E 41: ECU Software Performance Operational Software/Calibration Set Not Programmed

Circuit/System Description

This diagnostic applies to internal microprocessor integrity conditions within the trailer brake control module (TBCM). This diagnostic also addresses if the TBCM is not programmed.

Conditions for Running the DTC

C056D 33, C056D 34, C056D 35, C056D 36 or C056D 39

    • Ignition ON.
    • Runs once an ignition cycle.

C056E 41

    •  Ignition ON, engine OFF.
    • Runs continuously.

Conditions for Setting the DTC

C056D 33, C056D 34, C056D 35, C056D 36 or C056D 39

The TBCM detected a failure in its microprocessor.

C056E 41

The TBCM is not programmed.

Action Taken When the DTC Sets
    • The trailer brake control will be disabled at the end of the current brake apply for the remainder of the ignition cycle.
    • The driver information center (DIC) will display Service Trailer Brake System.

Conditions for Clearing the DTC
    • The condition for the DTC is no longer present.
    • The TBCM clears the history DTC when a current DTC is not detected in 40 consecutive drive cycles.

Reference Information

Schematic Reference

Trailer Brake Control Schematics

Connector End View Reference

Trailer Brake Control Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Output Controls
    •  Scan Tool Data List

Circuit/System Verification
  1. Verify that DTC C056E is not set.
    2. ⇒If the DTC is set, program the TBCM.
  2. Verify that DTC C056D is not set.
    3. ⇒If the DTC is set, replace the TBCM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for TBCM replacement, setup, and programming