What is U0077?
U0077 is a Diagnostic Trouble Code (DTC) in the Network Communication (U) category. It indicates control module communication bus d off. These codes relate to the vehicle's onboard communication network (CAN bus, LIN bus, etc.) and module-to-module data exchange.
When U0077 is set, it means one or more control modules have detected a communication fault, data integrity issue, or software incompatibility on the vehicle network.
Common Symptoms
- Check Engine Light on
- Communication errors on scan tool
- Multiple modules not responding
- Intermittent electrical issues
- Warning lights on dashboard
Possible Causes
- Damaged CAN bus wiring (open, short, high resistance)
- Corroded or loose connectors
- Failed CAN bus terminating resistor
- Faulty control module on bus
- Electromagnetic interference
Step-by-Step Diagnosis
- Connect a professional-grade OBD2 scanner and check for all active and pending U-codes.
- Identify which modules are communicating and which are not.
- Check power, ground, and communication wiring for the Control Module Communication Bus D Off using a wiring diagram.
- Perform a voltage drop test on power and ground circuits to the affected module.
- Check CAN bus resistance (should be ~60 ohms between CAN-High and CAN-Low).
- Use a scan tool to monitor live data from modules that are still communicating.
- Perform a wiggle test on wiring harnesses while monitoring for communication recovery.
- If a module replacement is suspected, verify correct part number and programming requirements.
- Clear codes and retest after repairs.
Recommended Repairs
- Repair any damaged wiring, connectors, or terminals. Ensure proper routing and secure mounting.
- Clean corroded connector pins with electrical contact cleaner.
- Replace blown fuses and verify correct amperage rating.
- If a module requires programming, ensure it is done with the correct software version.
- Update or reflash module software if a TSB applies.
- Replace failed control module if testing confirms internal failure.
- Verify all repairs by checking module communication with a scan tool.
Related Diagnostic Trouble Codes
External References and Forum Discussions
Disclaimer
The information provided is for educational purposes only. Always consult a qualified mechanic or dealership for accurate diagnosis and repair. U-codes often require specialized diagnostic tools and access to manufacturer-specific wiring diagrams and software.
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