U1999 is a generic communication error code indicating a problem with the vehicle's communication network

Definition

Common causes

• Faulty wiring or connectors in the communication network
• Malfunctioning control modules
• Poor ground connections
• Interference from aftermarket devices
• Corroded or damaged pins in connectors
• Software issues or outdated firmware in control modules
• Low battery voltage or weak battery
• Faulty OBD-II port

Common misdiagnoses

• Faulty OBDII scanner
• Bad ground connections
• Defective wiring harness
• Malfunctioning ECU (Engine Control Unit)
• Issues with other control modules
• Corrupted software or firmware
• Faulty sensors
• Battery or power supply issues
• Intermittent connection problems
• Incorrectly installed aftermarket accessories

Troubleshooting steps

1. 1. Check for Other Codes

   Use an OBD-II scanner to check for any additional trouble codes. U1999 may be accompanied by other codes that can provide more context about the issue

2. 2. Inspect the OBD-II Connector

   Ensure that the OBD-II connector is clean and free of corrosion. Check for any bent or damaged pins that could affect communication

3. 3. Examine Wiring and Connectors

   Inspect the wiring and connectors associated with the communication network (CAN bus, LIN bus, etc.). Look for signs of damage, fraying, or loose connections

4. 4. Pay special attention to ground connections, as poor grounding can lead to communication issues

5. 5. Check for Short Circuits or Open Circuits

   Use a multimeter to check for continuity in the communication lines. Look for any short circuits or open circuits that could disrupt communication

6. 6. Inspect Modules

   Identify and inspect the modules that are part of the vehicle's communication network. This may include the Engine Control Module (ECM), Transmission Control Module (TCM), Body Control Module (BCM), etc

7. 7. Ensure that all modules are functioning properly and are powered on

8. 8. Test Communication

   Use a scan tool to test communication with each module. If certain modules are not responding, it may indicate a problem with that specific module or its wiring

9. 9. Check for Software Updates

   Verify if there are any software updates available for the vehicle’s modules. Sometimes, communication issues can be resolved with a software update

10. 10. Inspect for Interference

    Look for any aftermarket devices or modifications that may interfere with the vehicle’s communication network. This includes alarm systems, stereo systems, or other electronic devices

11. 11. Battery and Power Supply Check

    Ensure that the vehicle’s battery is in good condition and that the electrical system is functioning properly. Low voltage can cause communication errors

12. 12. Clear Codes and Test Drive

    After performing the above checks and repairs, clear the trouble codes using the OBD-II scanner and take the vehicle for a test drive to see if the code returns.1

13. 13. Consult Technical Service Bulletins (TSBs)

    Check for any TSBs related to the specific make and model of the vehicle. Manufacturers may have issued guidance on known communication issues.1

14. 14. 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 address the underlying issues causing the U1999 code