Applicable Systems
OBD-II
Symptoms
The rear climate control may not function. Communication with the rear HVAC module is lost. DTC U0621 is stored by a control module that lost communication with the Rear HVAC module.
Possible Causes
- CAN bus wiring to rear HVAC module fault
- Rear HVAC module power supply fault
- Rear HVAC module internal failure
- Rear HVAC connector corroded
Troubleshooting Steps
- Step 1: Connect a diagnostic scanner and read all module DTCs to confirm U0621
DTC U0621 is present; note which module logged the code - Step 2: Check CAN bus wiring between the affected modules for continuity, shorts, and termination resistor values
CAN high and low lines intact, 60 ohms across the bus - Step 3: Verify power and ground supplies to the affected module
Battery voltage at module power pin, less than 0.1V drop on ground side - Step 4: If wiring is intact, the affected module may need replacement and programming
Communication restored after module replacement and programming
How to Reset
Clear the DTC after repair using a diagnostic scanner. The code will clear automatically if communication is restored and no fault is detected for 3 consecutive drive cycles.
Common Replacement Parts
- varies: Rear HVAC module (if confirmed faulty) (Confirm part number via VIN before ordering; programming typically required)
Frequently Asked Questions
- Q: What does code U0621 mean?
This is a network communication DTC indicating lost communication with the Rear HVAC module. The CAN bus wiring, module power supply, and the module itself should be checked.
- Q: Can I drive with code U0621?
Driving may be possible but the affected system may not function. If the code relates to ECM, transmission, or brake communication, have it diagnosed promptly for safety.
Source: SAE J2012 Standard OBD-II Diagnostic Trouble Codes
Category: OBD-II