U0039Vehicle Communication Bus B (+) Open
- Generic OBD-II code · Network
- How the car detects it
- U0039 reports a break in the (+) conductor of Vehicle Communication Bus B, which on a CAN-type network is CAN-High. Deutz assigns this code to the third CAN channel of its EMR L1 engine controller and lists it for main and secondary ECU simply as an open circuit on CAN 3, without naming the wire. On a classic high-speed CAN the interruption stays invisible to the transceiver; NXP notes that even diagnosing transceivers cannot flag a wire break, so the module infers it from partners falling silent. A fault-tolerant low-speed CAN behaves differently: NXP's TJA1055 keeps sending and receiving after a CAN-High break without spoiling frames already on the line. In practice, functions beyond the break drop out while others carry on. Look for unplugged connectors, backed-out pins and breaks near moving parts.
- What you may notice
- Functions of modules beyond the break unavailable
- Lost-communication codes for individual modules
- Likely causes
- Likely(+) wire of Bus B broken, often at a flex point
- PossibleTerminal backed out or spread in a connector
- PossibleConnector left open after repair work
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €700
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.