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C code chassis Medium severity

C1749 Trailer Input Circuit Failure

The OBD2 code C1749 indicates a malfunction in the trailer input circuit, which may involve issues with the wiring, connectors, or the trailer brake controller

Definition

The OBD2 code C1749 indicates a malfunction in the trailer input circuit, which may involve issues with the wiring, connectors, or the trailer brake controller

Common causes

  • Faulty trailer connector
  • Damaged wiring or harness
  • Corroded terminals
  • Poor ground connection
  • Malfunctioning trailer brake controller
  • Defective vehicle control module
  • Improperly connected trailer
  • Short circuit in the trailer wiring

Common misdiagnoses

  • Faulty trailer wiring
  • Bad trailer connector
  • Defective brake controller
  • Malfunctioning ABS module
  • Faulty vehicle speed sensor
  • Wiring harness issues
  • Defective powertrain control module (PCM)
  • Incorrectly installed or damaged trailer brake system

Troubleshooting steps

  1. 1. Check the Trailer Connection:

    Inspect the trailer connector for any signs of damage, corrosion, or dirt. Clean the connectors if necessary

  2. 2. Ensure that the trailer is properly connected to the vehicle and that all pins are making good contact

  3. 3. Inspect Wiring and Harness:

    Examine the wiring harness leading to the trailer connector for any signs of wear, fraying, or damage

  4. 4. Look for any loose or corroded connections in the wiring harness

  5. 5. Test the Trailer Lights:

    If possible, connect a different trailer to the vehicle to see if the issue persists. This can help determine if the problem lies with the trailer or the vehicle

  6. 6. Check the functionality of the trailer lights, brakes, and turn signals to ensure they are working correctly

  7. 7. Check Fuses and Relays:

    Inspect the vehicle’s fuse box for any blown fuses related to the trailer towing circuit. Replace any blown fuses as needed

  8. 8. Check for any relays associated with the trailer wiring and ensure they are functioning properly

  9. 9. Scan for Additional Codes:

    Use an OBD-II scanner to check for any additional trouble codes that may provide more context or indicate related issues

  10. 10. Inspect the Vehicle’s Trailer Module:

    If your vehicle is equipped with a trailer control module, check for any faults or issues with the module itself. This may require specialized diagnostic equipment

  11. 11. Check for Ground Issues:

    Ensure that the ground connections for the trailer wiring are secure and free of corrosion. Poor grounding can lead to various electrical issues

  12. 12. Consult the Vehicle’s Service Manual:

    Refer to the vehicle’s service manual for specific wiring diagrams and troubleshooting procedures related to the trailer input circuit

  13. 13. Perform a Visual Inspection:

    Look for any signs of water intrusion or damage in the areas where the wiring is routed, especially if the vehicle has been exposed to harsh weather conditions

  14. 14. Reset the Code

  15. 15. After performing any repairs or adjustments, clear the code using an OBD-II scanner and take the vehicle for a test drive to see if the code returns. If the issue persists after following these steps, it may be necessary to consult a professional mechanic or technician who specializes in automotive electrical systems for further diagnosis and repair