U1977 – Module Communication Fault

The ODBII code U1977: Module Communication Fault indicates a communication issue between vehicle control modules, suggesting that one or more modules are not properly exchanging data.

The severity of ODBII code **U1977: Module Communication Fault** is generally considered moderate.

Common Causes For: U1971: Vehicle Communication Bus Malfunction

• Faulty wiring or connectors
• Malfunctioning control modules
• Poor ground connections
• Communication network issues (e.g., CAN bus problems)
• Software or firmware issues in modules
• Corroded or damaged pins in connectors
• Intermittent electrical connections
• Low battery voltage or power supply issues

Common misdiagnosis for U1977: Module Communication Fault

• Faulty wiring or connectors
• Defective control module
• Software issues or outdated firmware
• Battery or power supply problems
• Grounding issues
• Intermittent communication issues
• Incorrectly configured or programmed modules
• Faulty sensors or actuators
• Corrupted data or signals
• Environmental interference (e.g., electromagnetic interference)

Common Troubleshooting Steps

The OBD-II code **U1977: Module Communication Fault** indicates a communication issue between modules in the vehicle’s network. This can be caused by various factors, including wiring issues, faulty modules, or problems with the vehicle’s communication 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 additional trouble codes. Other codes may provide more context about the issue.
• Inspect Wiring and Connectors:
• Visually inspect the wiring and connectors associated with the affected modules. Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the CAN (Controller Area Network) bus wiring, as this is often where communication faults occur.
• Check Module Power and Ground:
• Ensure that the affected modules are receiving proper power and ground. Use a multimeter to check voltage and ground continuity.
• Test Communication Lines:
• Use a scan tool to monitor the communication lines. Check for proper voltage levels on the CAN bus wires (typically around 2.5V when idle).
• If you have access to an oscilloscope, you can check the signal waveforms for abnormalities.
• Disconnect Modules:
• If multiple modules are connected to the same communication line, try disconnecting them one at a time to isolate the faulty module.
• After disconnecting a module, clear the codes and see if the U1977 code returns.
• Check for Short Circuits:
• Inspect the wiring for any short circuits or open circuits that could be affecting communication. This may require a more in-depth examination of the wiring harness.
• Update or Reprogram Modules:
• In some cases, a software update or reprogramming of the affected modules may resolve communication issues. Check with the manufacturer for any available updates.
• Replace Faulty Modules:
• If a specific module is identified as faulty and is not responding to communication, it may need to be replaced.
• Consult Service Manual:
• Refer to the vehicle’s service manual for specific diagnostic procedures related to the U1977 code and the modules involved.
• Professional Diagnosis:
• If the issue persists after performing the above steps, consider seeking help from a professional technician or dealership with experience in diagnosing communication faults. By following these steps, you should be able to identify and resolve the underlying cause of the U1977 code.