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U1939 U1939: CAN Bus Communication Fault

The OBD2 code U1939: CAN Bus Communication Fault indicates a problem with the communication on the Controller Area Network (CAN) bus, which may affect the transmission of data between various vehicle modules

Definition

The OBD2 code U1939: CAN Bus Communication Fault indicates a problem with the communication on the Controller Area Network (CAN) bus, which may affect the transmission of data between various vehicle modules

Common causes

  • Faulty CAN bus wiring or connectors
  • Short circuit in the CAN bus
  • Open circuit in the CAN bus
  • Faulty control module
  • Interference from other electronic devices
  • Corroded or damaged connectors
  • Poor ground connections
  • Software issues or bugs in the control module
  • Incompatible or malfunctioning aftermarket accessories
  • Low battery voltage or power supply issues

Common misdiagnoses

  • Faulty CAN Bus wiring
  • Defective ECU (Electronic Control Unit)
  • Bad ground connections
  • Malfunctioning sensors
  • Issues with other communication protocols
  • Corrupted software or firmware in the ECU
  • Faulty connectors or terminals
  • Battery or power supply issues
  • Interference from aftermarket devices
  • Incorrectly configured or incompatible modules

Troubleshooting steps

  1. 1. Check for Other Codes

    Use an OBD-II scanner to check for any additional trouble codes. Sometimes, multiple codes can provide more context about the issue

  2. 2. Inspect the Wiring and Connectors

    Visual Inspection: Look for damaged, frayed, or corroded wires and connectors in the CAN bus network

  3. 3. Connector Integrity

    Ensure that all connectors are securely attached and free of corrosion or dirt

  4. 4. Test the CAN Bus Voltage

    Use a multimeter to check the voltage levels on the CAN High and CAN Low wires. The typical voltage levels should be around 2.5V when idle and should fluctuate between 0V and 5V when data is being transmitted

  5. 5. Check for Short Circuits

    Inspect the CAN bus wiring for any short circuits to ground or to other wires. This can cause communication faults

  6. 6. Look for Interference

    Ensure that there are no devices or components that could be causing electrical interference with the CAN bus signals

  7. 7. Test the Modules

    If possible, test the individual modules on the CAN bus to see if one of them is malfunctioning. This may involve disconnecting modules one at a time to isolate the problem

  8. 8. Check Termination Resistors

    Ensure that the CAN bus has the correct termination resistors (typically 120 ohms) at both ends of the bus. Missing or faulty resistors can lead to communication issues

  9. 9. Scan for Communication Issues

    Use a professional scan tool that can monitor live data and check for communication between modules. This can help identify which module is not communicating properly

  10. 10. Inspect for Water Damage

    Check for signs of water intrusion in the vehicle, especially around the connectors and modules, as moisture can cause communication faults

  11. 11. Reset the Codes

    After performing inspections and repairs, clear the codes and see if the U1939 code returns. If it does, further investigation may be needed.1

  12. 12. Consult Technical Service Bulletins (TSBs)

    Look for any TSBs related to your vehicle make and model that might address known issues with the CAN bus or specific modules.1

  13. 13. Seek Professional Help

    If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or technician who specializes in automotive electronics. By following these steps, you can systematically diagnose and address the U1939 CAN Bus communication fault