U0039 code meaning
Vehicle Communication Bus B (+) Open
What U0039 means
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.
U0039 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyHard
- Typical labor0.5 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0039
- Functions of modules beyond the break unavailable
- Lost-communication codes for individual modules
Common causes of U0039
- Most 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
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Bus b wiringModule connector
What each part of U0039 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 39 | The exact fault in that area. |
What to do next
- Note U0039 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0039
What does U0039 mean?
U0039 means "Vehicle Communication Bus B (+) Open". U0039 reports a break in the (+) conductor of Vehicle Communication Bus B, which on a CAN-type network is CAN-High. It is a generic code, so it means the same on every make.
Is it safe to drive with U0039?
U0039 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0039 fail an emissions test?
U0039 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0039 to fix?
Difficulty: hard. A typical repair takes 0.5 to 5 hours of work. This is a job for a workshop.
Will U0039 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.