Diagnostic trouble code U120100 — what it means, the common
causes, and how to confirm the real fault before replacing parts.
What does U120100 mean?
U120100 indicates: Data bus no communication (Unauthorized).
This relates to the Network / communication system.
Common causes of U120100
Interruption of the fiber-optic cable (ring break)
Faulty voltage supply of the transmitter or receiver control module
Faulty transmitter or receiver control module
Fatal errors detected in the affected control module causing it to disconnect from the bus (bus OFF) at least twice
Message time-out due to at least 10 messages not received
What to check (live data)
Gateway measuring value blocks (data blocks 125, 126) for active/passive state of control modules connected to CAN-bus (1 = active, 0 = passive)
Ring break diagnosis wire voltage signal (12-volt signal connected to earth for 10 seconds during ring break diagnosis first test cycle)
CAN-bus continuity check using ohmmeter (Drivetrain CAN)
Convenience/Infotainment CAN signal using VAS 5051 DSO function
Parts often involved in U120100
Fiber-optic cable
Transmitter control module
Receiver control module
Data Bus On Board Diagnostic Interface J533
Optical replacement control unit VAS 6778 (for MOST150 bus optical faults)
Don't replace parts until the root cause is confirmed — many of these codes are caused by wiring, a connector or an upstream fault, not the part itself.
Get the confirmed root cause for U120100 — on YOUR vehicle
A code is a symptom, not a diagnosis. AutoDX Pro cross-checks several
independent assessments, pinpoints the actual root cause, gives you the
exact tests to confirm it, and shows the parts — only after the cause is
confirmed. Reads your scan-tool photo and decodes your VIN with open
recalls.
U120100 is a diagnostic trouble code: Data bus no communication (Unauthorized).
What are the common causes of U120100?
Common causes include: Interruption of the fiber-optic cable (ring break), Faulty voltage supply of the transmitter or receiver control module, Faulty transmitter or receiver control module, Fatal errors detected in the affected control module causing it to disconnect from the bus (bus OFF) at least twice. The exact cause depends on the vehicle — confirm it before replacing parts.
Can I keep driving with U120100?
It depends on the fault and your symptoms. Some causes are minor, others can damage components or affect safety. Get the root cause confirmed before deciding.
How do I fix U120100?
Confirm the actual root cause with targeted tests first, then replace only the part that's at fault. AutoDX Pro gives you the root cause, the exact tests, and the parts for U120100 on your specific vehicle.