U1257 – Communication Bus Off.

The ODBII code U1257 indicates a communication issue where the vehicle’s communication bus is offline or not functioning properly, preventing modules from exchanging data.

The severity of ODBII code U1257 is moderate to severe.

Common Causes For: U1256 U1256: “Control Module Communication Failure”

• Faulty wiring or connectors in the communication bus.
• Malfunctioning control modules.
• Short circuits or open circuits in the communication lines.
• Corroded or damaged connectors.
• Power supply issues to the control modules.
• Software or firmware issues in the control modules.
• Interference from aftermarket devices or modifications.

Common misdiagnosis for U1257 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 modules

Common Troubleshooting Steps

The OBD-II code U1257 indicates a communication issue on the vehicle’s communication bus, specifically that a module is not responding or is offline. This can be caused by various factors, including wiring issues, module failures, or problems with the communication protocols. Here are some common troubleshooting steps to diagnose and resolve the U1257 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, LIN bus, etc.). Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the connectors of the modules that are part of the communication network.
• Check Power and Ground: Ensure that the affected modules have proper power and ground connections. Use a multimeter to verify voltage and ground continuity.
• Test the Modules: If you suspect a specific module is causing the issue, you can test it by disconnecting it and checking if the communication bus returns to normal.
• If possible, swap the suspected module with a known good one to see if the problem persists.
• Inspect the Communication Bus: Use an oscilloscope or a CAN bus analyzer to check the communication signals on the bus. Look for any irregularities in the signal patterns.
• Check for short circuits or open circuits in the communication lines.
• Check for Interference: Ensure that there are no aftermarket devices or modifications that could be interfering with the communication bus.
• Reset the Codes: After performing checks and repairs, clear the trouble codes using the OBD-II scanner and take the vehicle for a test drive to see if the code returns.
• Consult Technical Service Bulletins (TSBs): Check for any manufacturer-specific TSBs related to the U1257 code or communication issues for your vehicle make and model.
• 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.
• Replace Faulty Components: If a specific module is determined to be faulty, replace it and recheck the system. By following these steps, you should be able to identify and resolve the underlying issue causing the U1257 code. Always refer to the vehicle’s service manual for specific procedures and specifications related to your vehicle.