U1991 – Inter-Module Communication Failure

The ODBII code U1991 indicates an “Inter-Module Communication Failure,” meaning there is a problem with the communication between different electronic control modules in the vehicle.

The severity of ODBII code U1991 is generally considered moderate.

Common Causes For: U1990 CAN Communication Bus Fault.

• Faulty wiring or connectors
• Malfunctioning control modules
• Poor ground connections
• Corroded or damaged pins in connectors
• Software issues or incompatibility
• Low battery voltage or weak battery
• Interference from aftermarket devices
• Faulty CAN bus communication
• Electrical shorts or opens in the communication lines

Common misdiagnosis for U1991 U1991: “Inter-Module Communication Failure”

• Faulty wiring or connectors
• Defective control modules
• Software issues or updates needed
• Battery or power supply problems
• Grounding issues
• Corrupted data or communication protocols
• Sensor malfunctions
• Interference from aftermarket devices

Common Troubleshooting Steps

The OBD-II code U1991 indicates an “Inter-Module Communication Failure,” which typically means that there is a problem with the communication between different modules in the vehicle’s network. Here are some common troubleshooting steps to diagnose and resolve this issue:

• Check for Other Codes: Use an OBD-II scanner to check for any other trouble codes that may be present. Sometimes, U-codes can be related to other issues that may provide more context.
• Inspect Wiring and Connectors: Examine the wiring and connectors associated with the modules involved in the communication. Look for signs of damage, corrosion, or loose connections. Pay special attention to the ground connections.
• Check Module Power Supply: Ensure that the affected modules are receiving the proper voltage and ground. Use a multimeter to check the power supply to the modules.
• Verify Module Functionality: If possible, test the affected modules individually to ensure they are functioning correctly. This may involve using specialized diagnostic tools or software.
• Inspect the CAN Bus: If your vehicle uses a Controller Area Network (CAN) bus, check for issues with the bus itself. Look for short circuits, open circuits, or any other faults in the wiring.
• Check for Software Updates: Sometimes, communication issues can be resolved with a software update. Check with the vehicle manufacturer or a qualified technician to see if there are any updates available for the modules.
• Reset the Modules: Disconnect the battery for a few minutes to reset the modules. Reconnect the battery and see if the code reappears.
• Perform a Network Test: If you have access to advanced diagnostic tools, perform a network test to check the communication between modules. This can help identify which module is not communicating properly.
• Consult Technical Service Bulletins (TSBs): Check for any TSBs related to the specific make and model of your vehicle. Manufacturers may have issued guidance on common issues related to inter-module communication.
• Seek Professional Help: If the issue persists after performing the above steps, it may be necessary to consult a professional mechanic or dealership with experience in diagnosing and repairing communication issues in vehicles. By following these steps, you can systematically identify and address the underlying cause of the U1991 code.