What does it mean?
U1221 is a Network (CAN bus, module communications) fault code. It indicates: Invalid Data Received From DC-DC Converter Control Module .
Symptoms
- Charging system warning and hybrid or electric drive warning lamps on
- 12 V battery goes flat, especially after standing overnight
- Dim lights, slow window motors, infotainment and cluster resetting
- Vehicle refuses to enter ready mode, or shuts down while driving
- Power electronics cooling fan running loudly or continuously
- Several unrelated modules storing low-voltage and communication codes
Likely causes
- Failing DC-DC converter producing unstable output and reporting implausible values
- Loose or corroded 12 V output connection at the converter, or a poor converter ground
- Damaged bus wiring between the converter control module and the receiving module
- Weak or sulphated 12 V auxiliary battery dragging the converter output out of range
- Converter overheating because of blocked cooling or low coolant in the power electronics circuit
- A high-voltage system fault preventing the converter from being enabled
- Mismatched or corrupted converter software after a module replacement
How to fix it
- Load-test the 12 V auxiliary battery and charge it fully before any other measurement
- Measure system voltage with the vehicle in ready mode; the converter should hold a steady charging voltage
- Watch the converter live data for output voltage, current and temperature
- Inspect and retorque the 12 V output cable and the converter ground connection
- Check coolant level and flow in the power electronics cooling circuit
- Inspect the bus wiring and connectors at the converter for corrosion and chafe damage
- Following the high-voltage safety procedure, reprogram or replace the converter only once supply and wiring are proven