Diagnostic trouble code U16A400 on Audi vehicles — what it means, the common
causes, and how to confirm the real fault before replacing parts.
What does U16A400 mean?
U16A400 indicates: CAN data bus 3 No communication.
This relates to the Network / communication system.
Common causes of U16A400
Open circuit in CAN-High or CAN-Low wire
Short circuit between CAN-High and CAN-Low
Short circuit between CAN-High and positive voltage
Short circuit between CAN-High and Ground
Short circuit between CAN-Low and Ground
Short circuit between CAN-Low and positive voltage
Communication partner not responding (no voltage supply or cable fault)
Control module disconnected from bus at least twice (bus OFF)
Message timeout due to at least 10 messages not received
What to check (live data)
CAN-High wire voltage (Channel A on DSO)
CAN-Low wire voltage (Channel B on DSO)
Gateway measuring value blocks for data bus fault messages
Parts often involved in U16A400
CAN-High wire
CAN-Low wire
Voltage supply to communication partner
Cable/wiring harness
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 U16A400 — 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.
U16A400 is a diagnostic trouble code: CAN data bus 3 No communication.
What are the common causes of U16A400 on an Audi?
Common causes include: Open circuit in CAN-High or CAN-Low wire, Short circuit between CAN-High and CAN-Low, Short circuit between CAN-High and positive voltage, Short circuit between CAN-High and Ground. The exact cause depends on the vehicle — confirm it before replacing parts.
Can I keep driving an Audi with U16A400?
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 U16A400 on an Audi?
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 U16A400 on your specific Audi.