U1266 – Communication Bus Off.

The ODBII code U1266 indicates a communication issue where the vehicle’s communication bus is offline or not functioning properly, often due to a fault in the network or a module.

The severity of ODBII code U1266 Communication Bus Off is moderate to severe.

Common Causes For: U1265 Invalid or Missing Data for Vehicle Speed.

• Faulty or damaged wiring in the communication bus.
• Poor or corroded connections in the wiring harness.
• Malfunctioning control modules.
• Short circuits in the communication lines.
• Open circuits in the communication lines.
• Interference from aftermarket devices.
• Low battery voltage or weak battery.
• Faulty or failing sensors on the communication bus.

Common misdiagnosis for U1266 Communication Bus Off.

• Faulty ECM (Engine Control Module)
• Bad wiring or connectors
• Defective sensors
• Malfunctioning ABS module
• Issues with the body control module (BCM)
• Powertrain control module (PCM) failure
• Battery or charging system problems
• Faulty communication lines or networks
• Incorrectly installed aftermarket accessories
• Software or firmware issues in control modules

Common Troubleshooting Steps

The OBD-II code U1266 indicates a “Communication Bus Off” condition, which typically means that there is a problem with the communication between the vehicle’s control modules. This can be caused by various issues, including wiring problems, module failures, or issues with the communication bus itself. Here are some common troubleshooting steps to diagnose and resolve the U1266 code:

• Check for Additional Codes: Use an OBD-II scanner to check for any other trouble codes that may be present. This can provide more context about the issue.
• Inspect Wiring and Connectors: Visually inspect the wiring and connectors related to the communication bus (CAN bus). Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the connectors at the control modules, as these are common failure points.
• Test the Communication Bus: Use a multimeter or an oscilloscope to check the voltage levels on the communication bus wires. The typical voltage levels for CAN bus should be around 2.5V to 3.5V.
• Check for short circuits or open circuits in the communication lines.
• Check for Ground Issues: Ensure that all control modules have a good ground connection. Poor grounding can lead to communication issues.
• Inspect Control Modules: If the wiring and connections are good, the issue may lie with one of the control modules. Check for any signs of damage or failure.
• If possible, swap the suspected module with a known good one to see if the problem persists.
• Look for Interference: Check for any aftermarket devices or modifications that may interfere with the communication bus. This includes alarms, stereo systems, or other electronic devices.
• Perform a Network Test: Some advanced scan tools can perform a network test to check the integrity of the communication bus. This can help identify which module is causing the issue.
• Clear Codes and Test Drive: After performing the above checks and repairs, clear the trouble codes and take the vehicle for a test drive to see if the code returns.
• Consult Service Manual: Refer to the vehicle’s service manual for specific diagnostic procedures related to the communication bus and the U1266 code.
• Seek Professional Help: If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or technician who has experience with automotive electronics and communication systems. By following these troubleshooting steps, you should be able to identify and resolve the underlying issue causing the U1266 code.