U0039
Vehicle Communication Bus B (+) Open
Communication network (vehicle bus)
Generic OBD-II
Medium
What it means
U0039 means the module detected an open circuit on the plus (+) side of vehicle communication bus "B". With the bus wire broken, modules on that bus can't exchange data, so one or more of them go silent and set lost-communication codes. The impact depends on which modules share bus B; the fault itself is almost always a broken wire, a backed-out connector pin, or corrosion.
Medium
Can I keep driving? Depends on what shares bus B — the car may drive normally or may lose features and light warnings; if a powertrain module is on this bus you could see limp mode.
Common symptoms
What you'll notice
- Warning lights on with multiple lost-communication codes
- One or more modules unresponsive to a scan tool
- Features handled by bus-B modules stop working
- Cluster warnings or gauges affected
Common causes
Most likely first
- Open or broken (+) bus wire, or a backed-out connector pinLikely
- Corroded or damaged connector on the busCommon
- A failed module leaving the bus circuit openCommon
- Poor splice or previous harness repair on the busLess often
How to diagnose & fix
Work through it in order
- Scan all modules and identify which ones went silent to locate bus B.
- Inspect the (+) bus wire and connectors for an open/break, backed-out pins, or corrosion.
- Check bus continuity and terminating resistance with a meter.
- Repair the open wire or connector; if a module caused it, replace that module.
- Clear codes and confirm all modules communicate again.
FAQ
Quick answers
Is it safe to drive with U0039?+
Only if drivability and safety systems are unaffected. An open bus can knock several modules offline, so have it diagnosed to know what's lost before relying on the car.
Can I just clear U0039?+
An open circuit won't clear itself — the code returns immediately until the broken wire, pin, or module is repaired.
Related codes
Often seen with U0039
Read & clear U0039 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX