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U code network Medium severity

U1638 U1638: Vehicle Communication Bus Malfunction

The OBD2 code U1638: Vehicle Communication Bus Malfunction indicates a problem with the communication between the vehicle's control modules, which may affect their ability to exchange data properly

Definition

The OBD2 code U1638: Vehicle Communication Bus Malfunction indicates a problem with the communication between the vehicle's control modules, which may affect their ability to exchange data properly

Common causes

  • Faulty OBD-II connector
  • Damaged wiring or connectors in the communication bus
  • Malfunctioning control modules
  • Poor ground connections
  • Corroded or loose terminals
  • Software issues or bugs in the control modules
  • Interference from aftermarket devices
  • Low battery voltage or power supply issues

Common misdiagnoses

  • Faulty OBDII connector
  • Bad ground connections
  • Defective ECM (Engine Control Module)
  • Faulty wiring or harness issues
  • Malfunctioning CAN bus modules
  • Corrupted software or firmware in control modules
  • Issues with other communication protocols (e.g., LIN, K-Line)
  • Battery or power supply problems
  • Faulty sensors or actuators
  • Intermittent electrical connections

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other trouble codes that may be present. Addressing related codes can help pinpoint the issue

  2. 2. Inspect the Wiring and Connectors

    Visual Inspection: Check the wiring harnesses and connectors associated with the communication bus for any signs of damage, corrosion, or loose connections

  3. 3. Repair or Replace

    Repair any damaged wires or connectors and ensure all connections are secure

  4. 4. Check the Communication Bus

    CAN Bus Integrity: If your vehicle uses a CAN (Controller Area Network) bus, check the integrity of the bus. Use a multimeter to measure the voltage levels on the CAN high and CAN low wires

  5. 5. Termination Resistors

    Ensure that the termination resistors (typically 120 ohms) are present and functioning at both ends of the bus

  6. 6. Inspect Control Modules

    Module Functionality: Check if any control modules are malfunctioning. This can include modules like the Engine Control Module (ECM), Transmission Control Module (TCM), or Body Control Module (BCM)

  7. 7. Reprogram or Replace

    If a specific module is found to be faulty, it may need to be reprogrammed or replaced

  8. 8. Check for Short Circuits

    Use a multimeter to check for short circuits in the wiring harnesses. A short can disrupt communication between modules

  9. 9. Battery and Ground Connections

    Battery Voltage: Ensure the vehicle's battery is fully charged and in good condition

  10. 10. Ground Connections

    Check all ground connections for corrosion or looseness, as poor grounding can affect communication

  11. 11. Scan Tool Communication

    Use a professional scan tool to check if you can communicate with all modules. If some modules are unresponsive, it may indicate a communication issue

  12. 12. Software Updates

    Check if there are any software updates available for the vehicle's control modules. Sometimes, manufacturers release updates to fix communication issues

  13. 13. Clear Codes and Test Drive

    After performing repairs, clear the trouble codes and take the vehicle for a test drive to see if the code reappears

  14. 14. Consult Technical Service Bulletins (TSBs)

    Look for any TSBs related to the U1638 code for your specific vehicle make and model. Manufacturers may have specific recommendations for addressing this issue.1

  15. 15. Professional Diagnosis

    If the issue persists after performing the above steps, consider seeking help from a professional mechanic or dealership with experience in diagnosing communication issues. By following these steps, you should be able to identify and resolve the underlying cause of the U1638 code