U1821 – Network Communication Line Fault

The ODBII code U1821 indicates a “Network Communication Line Fault,” which means there is a problem with the communication between modules in the vehicle’s network, potentially due to a wiring issue, a faulty module, or a poor connection.

The severity of ODBII code U1821 is moderate.

Common Causes For: U1820 U1820: Audio System Communication Failure

• Damaged wiring or connectors
• Faulty control modules
• Poor ground connections
• Short circuits in the communication line
• Open circuits in the communication line
• Corroded connectors
• Interference from aftermarket devices
• Software issues or bugs in modules
• Faulty or incompatible network components

Common misdiagnosis for U1821 U1821: “Network Communication Line Fault”

• Faulty wiring or connectors
• Defective control modules
• Communication protocol issues
• Power supply problems
• Grounding issues
• Intermittent faults in the network
• Software glitches or outdated firmware
• Incorrectly configured settings or parameters

Common Troubleshooting Steps

The OBD-II code U1821 indicates a “Network Communication Line Fault,” which typically points to 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. Other codes can provide context and help identify the root cause of the issue.
• Inspect Wiring and Connectors: Visually inspect the wiring and connectors associated with the communication network (CAN bus). Look for signs of damage, corrosion, or loose connections.
• Pay special attention to the connectors at the modules involved in the communication.
• Test the Communication Network: Use a multimeter or an oscilloscope to test the voltage levels on the communication lines. The typical voltage levels for CAN bus should be around 2.5V to 3.5V.
• Check for short circuits or open circuits in the wiring.
• Check for Module Malfunction: Identify the modules that are part of the communication network. If one module is malfunctioning, it can disrupt communication.
• Disconnect each module one at a time and see if the code clears or if communication is restored.
• Inspect for Ground Issues: Ensure that all ground connections related to the communication network are secure and free of corrosion. Poor grounding can lead to communication faults.
• Check for Interference: Look for any aftermarket devices or modifications that may interfere with the vehicle’s communication network. This includes alarms, stereo systems, or other electronic devices.
• Perform a Network Scan: Use a professional-grade scan tool to perform a network scan. This can help identify which modules are not communicating properly.
• Update or Reprogram Modules: In some cases, a software update or reprogramming of the affected modules may be necessary. Check with the manufacturer for any available updates.
• Consult Technical Service Bulletins (TSBs): Check for any TSBs related to the U1821 code for your specific vehicle make and model. Manufacturers may have specific fixes or recommendations.
• Clear Codes and Test Drive: After performing the necessary repairs, clear the codes and take the vehicle for a test drive to see if the code returns.1
• Seek Professional Help: If the issue persists after following these steps, it may be necessary to consult a professional mechanic or technician who specializes in automotive electronics. By following these troubleshooting steps, you can systematically identify and resolve the underlying issue causing the U1821 code.