What is U1166?
U1166 is a Diagnostic Trouble Code (DTC) in the Network Communication (U) category. It indicates lost communication with transmission fluid temperature module. These codes relate to the vehicle's onboard communication network (CAN bus, LIN bus, etc.) and module-to-module data exchange.
When U1166 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
- Module-specific function not working
- Warning lights related to affected system
- Reduced functionality
- Communication errors on scan tool
- Other modules setting related codes
Possible Causes
- Open or shorted communication wiring
- Faulty module power or ground
- Failed control module
- Corroded connector pins
- Network bus fault
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 Transmission Fluid Temperature Module 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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