U1304 – Class 2 Communication Bus Off.

The ODBII code U1304 indicates a communication issue within the Class 2 data bus, typically meaning that the module is unable to communicate with other modules on the network, often due to a fault or disconnection.

The severity of ODBII code U1304 Class 2 Communication Bus Off is moderate.

Common Causes For: U1303 U1303: “Module Communication Failure”

• Faulty or damaged wiring in the Class 2 communication bus.
• Poor or corroded connections in the communication network.
• Malfunctioning control modules or electronic components.
• Short circuits or open circuits in the communication lines.
• Interference from aftermarket accessories or modifications.
• Low battery voltage or power supply issues.
• Faulty or failing Class 2 communication bus terminators.

Common misdiagnosis for U1304 Class 2 Communication Bus Off.

• Faulty Body Control Module (BCM)
• Defective Instrument Cluster
• Wiring issues or shorts in the communication lines
• Bad ground connections
• Malfunctioning Powertrain Control Module (PCM)
• Corrupted software or firmware in modules
• Faulty sensors or actuators on the communication bus
• Battery or charging system problems
• Intermittent connection issues
• Other communication bus-related codes misinterpretation

Common Troubleshooting Steps

The OBD-II code U1304 indicates a problem with the Class 2 communication bus, which is a network used in many General Motors (GM) vehicles for communication between various modules. When you encounter this code, it typically means that there is a communication issue between the modules on the Class 2 bus. Here are some common troubleshooting steps to help diagnose and resolve the issue:

• Check for Additional Codes: Use an OBD-II scanner to check for any other trouble codes that may be present. Other codes can provide additional context for the problem.
• Inspect Wiring and Connectors: Visually inspect the wiring and connectors associated with the Class 2 communication bus. Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the connectors on the modules that are part of the Class 2 network.
• Check for Short Circuits: Use a multimeter to check for short circuits in the wiring. Ensure that there are no shorts to ground or power in the communication lines.
• Test Modules: If possible, test the modules on the Class 2 bus to ensure they are functioning correctly. This may involve checking for proper power and ground at the modules.
• If you suspect a specific module is causing the issue, you may need to replace or reprogram it.
• Inspect for Intermittent Issues: Sometimes, communication issues can be intermittent. Pay attention to when the code appears (e.g., during certain driving conditions) and check for any patterns.
• Check for Software Updates: Check if there are any software updates available for the vehicle’s modules. Sometimes, communication issues can be resolved with a software update.
• Scan for Module Communication: Use a scan tool to check if all modules on the Class 2 bus are communicating properly. If some modules are not responding, it may indicate a problem with those specific modules.
• Inspect the Class 2 Bus Resistor: Some vehicles have a termination resistor on the Class 2 bus. Check to ensure that this resistor is present and functioning correctly.
• Clear Codes and Test Drive: After performing any repairs or inspections, 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 Class 2 communication bus. This can provide additional insights and steps tailored to your vehicle model.1
• Seek Professional Help: If you are unable to resolve the issue after performing these steps, it may be best to consult a professional mechanic or technician who has experience with GM vehicles and their communication systems. By following these troubleshooting steps, you should be able to identify and resolve the issue related to the U1304 code.