WOMS
U code network Medium severity

U0021 Low Speed CAN Communication Bus (+) Open

The OBD2 code U0021 indicates a fault in the low-speed CAN communication bus, specifically that the positive line of the bus is open or not functioning properly, which can disrupt communication between modules

Definition

The OBD2 code U0021 indicates a fault in the low-speed CAN communication bus, specifically that the positive line of the bus is open or not functioning properly, which can disrupt communication between modules

Common causes

  • Open circuit in the CAN bus wiring
  • Faulty CAN bus connector
  • Damaged or corroded wiring
  • Faulty CAN bus module
  • Poor ground connection
  • Short circuit to ground or power
  • Interference from other electrical components
  • Faulty or incompatible aftermarket accessories

Common misdiagnoses

  • Faulty Engine Control Module (ECM)
  • Faulty Body Control Module (BCM)
  • Faulty Transmission Control Module (TCM)
  • Wiring issues or shorts in the CAN bus
  • Faulty sensors connected to the CAN bus
  • Battery or charging system issues
  • Faulty connectors or terminals
  • Incorrectly installed aftermarket accessories
  • Faulty or damaged CAN bus termination resistors
  • Software or firmware issues in control modules

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other trouble codes that may be present. This can provide additional context for the problem

  2. 2. Inspect Wiring and Connectors

    Visual Inspection: Look for any visible damage to the wiring harness, connectors, or terminals associated with the Low Speed CAN bus. Check for frayed wires, corrosion, or loose connections

  3. 3. Connector Integrity

    Ensure that all connectors are securely plugged in and that there are no bent pins

  4. 4. Test the CAN Bus Voltage

    Use a multimeter to measure the voltage on the CAN bus wires. The typical voltage levels for a functioning CAN bus are around 2.5V to 3.5V. If the voltage is significantly lower or higher, there may be an issue

  5. 5. Check for Short Circuits

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

  6. 6. Test the CAN Bus Termination Resistors

    Ensure that the termination resistors (typically 120 ohms) are present and functioning at both ends of the CAN bus. A missing or faulty resistor can lead to communication issues

  7. 7. Scan for Module Communication

    Use a scan tool to check if other modules on the Low Speed CAN bus are communicating properly. If some modules are not responding, it may indicate a problem with that specific module or its connection

  8. 8. Check for Interference

    Look for sources of electrical interference that may be affecting the CAN bus communication. This can include aftermarket accessories or modifications

  9. 9. Inspect Ground Connections

    Ensure that all ground connections related to the CAN bus are secure and free of corrosion. Poor grounding can lead to communication issues

  10. 10. Perform a Wiggle Test

    With the vehicle running, gently wiggle the wiring harnesses associated with the CAN bus to see if the code returns or if communication is restored. This can help identify intermittent connections

  11. 11. Replace Faulty Components

    If a specific module is identified as faulty (e.g., a body control module or another component on the Low Speed CAN bus), consider replacing it.1

  12. 12. 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 returns.1

  13. 13. Seek Professional Help

    If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or automotive technician with experience in CAN bus systems. By following these steps, you should be able to identify and resolve the issue related to the U0021 code