U1691 – Communication Bus Off.

The ODBII code U1691 indicates a communication issue where a module on the vehicle’s communication bus is not responding or is offline, potentially due to a fault in the wiring, connectors, or the module itself.

The severity of ODBII code U1691 Communication Bus Off is moderate to severe.

Common Causes For: U1690 U1690: “Module Communication Failure”

• Faulty wiring or connectors in the communication bus.
• Malfunctioning control modules.
• Short circuits or open circuits in the communication lines.
• Corrupted software or firmware in the control modules.
• Electrical interference from other components.
• Low battery voltage or poor power supply.
• Faulty or damaged sensors connected to the bus.

Common misdiagnosis for U1691 Communication Bus Off.

• Faulty ABS module
• Defective engine control module (ECM)
• Malfunctioning transmission control module (TCM)
• Bad wiring or connectors
• Faulty body control module (BCM)
• Incorrectly diagnosed sensor issues
• Power supply problems
• Communication issues with other modules

Common Troubleshooting Steps

The OBD-II code U1691 indicates a communication issue on the vehicle’s communication bus, specifically that a module is reporting a “Bus Off” condition. This typically means that the module is unable to communicate properly due to a fault in the communication network. Here are some common troubleshooting steps to diagnose and resolve the issue:

• Check for Other Codes:: Use an OBD-II scanner to check for any other diagnostic trouble codes (DTCs) that may provide additional context or indicate which module is having issues.
• Inspect Wiring and Connectors:: Visually inspect the wiring and connectors associated with the communication bus (CAN bus, LIN bus, etc.) for any signs of damage, corrosion, or loose connections.
• Pay special attention to the connectors of the modules that are part of the communication network.
• Check for Short Circuits:: Use a multimeter to check for short circuits in the wiring. Look for continuity between the communication wires and ground or power.
• Test the Modules:: Identify which module is reporting the U1691 code. If possible, test the module for proper operation. This may involve checking power and ground to the module and ensuring it is functioning correctly.
• Inspect the Termination Resistors:: Ensure that the termination resistors at the ends of the communication bus are present and functioning correctly. A missing or faulty termination resistor can cause communication issues.
• Check for Interference:: Look for any aftermarket devices or modifications that may be interfering with the communication bus. This includes alarm systems, stereo systems, or other electronic devices.
• Scan Tool Communication:: Use a scan tool to check if you can communicate with all modules on the bus. If some modules are not responding, it may indicate a problem with those specific modules or their connections.
• Battery Voltage:: Check the vehicle’s battery voltage. Low voltage can cause communication issues. Ensure the battery is fully charged and in good condition.
• Clear Codes and Test Drive:: After performing any repairs or adjustments, clear the codes and take the vehicle for a test drive to see if the code reappears.
• Consult Service Manual:: Refer to the vehicle’s service manual for specific troubleshooting procedures related to the communication bus and the U1691 code.1
• Professional Diagnosis:: If the issue persists after performing the above steps, consider seeking help from a professional mechanic or dealership with advanced diagnostic tools. By following these steps, you should be able to identify and resolve the underlying issue causing the U1691 code.