U1939 – CAN Bus Communication Fault

The ODBII 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.

The severity of ODBII code U1939: CAN Bus Communication Fault is moderate to severe.

Common Causes For: U1938 U1938: “Module Communication Failure”

• 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 misdiagnosis for U1939 U1939: CAN Bus Communication Fault

• 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

Common Troubleshooting Steps

When dealing with OBD-II code U1939, which indicates a CAN Bus communication fault, there are several common troubleshooting steps you can take to diagnose and potentially resolve the issue. Here’s a structured approach:

• 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.
• Inspect the Wiring and Connectors: **Visual Inspection: Look for damaged, frayed, or corroded wires and connectors in the CAN bus network.
• **Connector Integrity: Ensure that all connectors are securely attached and free of corrosion or dirt.
• 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.
• Check for Short Circuits: Inspect the CAN bus wiring for any short circuits to ground or to other wires. This can cause communication faults.
• Look for Interference: Ensure that there are no devices or components that could be causing electrical interference with the CAN bus signals.
• 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.
• 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.
• 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.
• 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.
• 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
• 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
• 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.