U1861 – Module Communication Line Fault

The ODBII code U1861 indicates a “Module Communication Line Fault,” which means there is a problem with the communication between vehicle modules, often due to a wiring issue, a faulty module, or a poor connection.

The severity of ODBII code U1861 is moderate.

Common Causes For: U1859 U1859: “Module Communication Failure”

• Faulty wiring or connectors
• Short circuits in the communication line
• Open circuits in the communication line
• Malfunctioning control modules
• Corroded or damaged connectors
• Poor ground connections
• Interference from aftermarket devices
• Software issues or bugs in the modules
• Low battery voltage or power supply issues
• Faulty CAN bus transceiver

Common misdiagnosis for U1861 U1861: “Module Communication Line Fault”

• Faulty wiring or connectors
• Defective control modules
• Battery or power supply issues
• Grounding problems
• Intermittent communication issues
• Software or firmware glitches
• Incorrectly configured or programmed modules

Common Troubleshooting Steps

The OBD-II code U1861 indicates a “Module Communication Line Fault,” which typically means there is a problem with the communication between various modules in the vehicle’s network. Here are some common troubleshooting steps to diagnose and resolve this issue:

• Check for Additional Codes: Use an OBD-II scanner to check for any other trouble codes that may be present. Sometimes, multiple codes can provide more context for the issue.
• Inspect Wiring and Connectors:
• Visually inspect the wiring and connectors associated with the modules in question. Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the communication lines (CAN bus lines) between modules.
• 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.
• If a module is not powered correctly, it may not communicate with other modules.
• Test Communication Lines:
• Use a scan tool to monitor the communication between modules. Check for any interruptions or irregularities in the data stream.
• If possible, use an oscilloscope to analyze the CAN bus signals for any abnormalities.
• Disconnect and Reconnect Modules:
• Disconnect the affected module(s) and reconnect them to ensure a good connection. Sometimes, reseating the connectors can resolve communication issues.
• Check for Short Circuits:
• Inspect the communication lines for any short circuits to ground or to each other. This can cause communication failures.
• Look for Intermittent Issues:
• If the code is intermittent, try to replicate the conditions under which the code appears. This may help identify the root cause.
• Update or Reprogram Modules:
• In some cases, a software update or reprogramming of the affected module(s) may be necessary. Check with the manufacturer for any available updates.
• Replace Faulty Modules:
• If all other troubleshooting steps fail and a specific module is identified as faulty, it may need to be replaced.
• Consult Technical Service Bulletins (TSBs):
• Check for any TSBs related to the U1861 code for your specific vehicle make and model. Manufacturers may have specific recommendations or known issues. 1
• Professional Diagnosis:
• If you are unable to resolve the issue, consider seeking help from a professional mechanic or dealership with experience in diagnosing communication issues. By following these steps, you should be able to identify and resolve the underlying cause of the U1861 code.