U1721 – Transmission Control Module Communication Error

The ODBII code U1721 indicates a communication error between the Transmission Control Module (TCM) and other control modules in the vehicle, suggesting a possible issue with wiring, connectors, or the TCM itself.

The severity of ODBII code U1721 is moderate to severe.

Common Causes For: U1720 U1720: “Module Communication Failure”

• Faulty Transmission Control Module (TCM)
• Damaged wiring or connectors
• Poor electrical connections
• Faulty Engine Control Module (ECM)
• Software issues or programming errors
• Corroded terminals
• Short circuits in the wiring harness
• Faulty sensors related to transmission operation
• Issues with the vehicle’s CAN bus system
• Low battery voltage or power supply issues

Common misdiagnosis for U1721 U1721: Transmission Control Module Communication Error

• Faulty Transmission Control Module (TCM)
• Wiring issues or damaged connectors
• Faulty Engine Control Module (ECM)
• Low transmission fluid levels
• Faulty sensors (e.g., speed sensors)
• Software issues or outdated firmware
• Battery or charging system problems
• Grounding issues
• Faulty transmission solenoids
• Other communication errors in the vehicle’s network

Common Troubleshooting Steps

The OBD-II code U1721 indicates a communication error between the Transmission Control Module (TCM) and other control modules in the vehicle. Here are some common troubleshooting steps to diagnose and resolve this issue:

• Check for Other Codes: Use an OBD-II scanner to check for any additional trouble codes. Sometimes, related codes can provide more context for the issue.
• Inspect Wiring and Connectors:
• Visually inspect the wiring harness and connectors associated with the TCM. Look for signs of damage, corrosion, or loose connections.
• Ensure that all connectors are securely plugged in and that there are no bent pins.
• Check Power and Ground:
• Verify that the TCM is receiving the correct power supply. Use a multimeter to check the voltage at the TCM connector.
• Check the ground connections to ensure they are secure and free of corrosion.
• Inspect the CAN Bus:
• The TCM communicates with other modules via the Controller Area Network (CAN) bus. Inspect the CAN bus wiring for damage or shorts.
• Use a scan tool to monitor the CAN bus for any irregularities in communication.
• Test the TCM:
• If the wiring and connections are intact, the TCM itself may be faulty. If possible, test the TCM in another vehicle or replace it with a known good unit to see if the problem persists.
• Check for Software Updates:
• Sometimes, a software glitch can cause communication errors. Check with the manufacturer for any available software updates for the TCM or other related modules.
• Clear Codes and Test Drive:
• After performing any repairs or inspections, clear the trouble codes and take the vehicle for a test drive to see if the code returns.
• Consult Service Manual:
• Refer to the vehicle’s service manual for specific diagnostic procedures related to the TCM and communication errors. This can provide additional insights and steps tailored to your vehicle model.
• Professional Diagnosis:
• If the issue persists after performing the above steps, consider seeking help from a professional mechanic or dealership. They may have specialized tools and knowledge to diagnose complex communication issues. By following these steps, you can systematically identify and resolve the underlying cause of the U1721 code.