GM Service Manual Online
For 1990-2009 cars only

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 Descriptor

DTC C0845 03: Device Ignition 1 Circuit Voltage Below Threshold

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Ignition 1

C0845 03

C0845 03

--

--

TCC Brake Signal

C0845 03

--

--

--

Circuit/System Description

The trailer brake control module (TBCM) monitors the ignition circuit. This diagnostic compares ignition state with the engine running state received over serial data to determine if there is an ignition circuit fault.

Conditions for Running the DTC

Engine running.

Conditions for Setting the DTC

The TBCM detects that the IGN 1 voltage is less than 9.2 volts

Action Taken When the DTC Sets

    • The trailer brake control will be disabled 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

Description and Operation

Trailer Brake Controls Description and Operation

Connector End View Reference

    •  Trailer Brake Control Connector End Views
    •  Engine Control Module 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 Testing

  1. Verify that DTC B3445, C0277, C0297, or C0890 are not set.
  2. If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  3. Ignition OFF, disconnect the harness connector of the TBCM.
  4. Ignition ON, test for greater than 11 volts between the IGN 1 voltage circuit terminal 15 and ground.
  5. If less than the specified voltage, test the IGN 1 voltage circuit for a short to ground or an open/high resistance. If the circuit test normal and the MISC - IGN fuse listed below is open, test the signal circuit terminal 37 for a short to ground. If the circuit tests normal, test or replace the RUN/CRANK relay.
    • Fuse 43 (Gas)
    • Fuse 40 (Diesel)
  6. If all circuits test normal, replace the TBCM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for TBCM replacement, setup, and programming