U0042
Vehicle Communication Bus B (-) Open
CAN communication bus
Generic OBD-II
High
What it means
U0042 is a physical-layer fault on the vehicle communication bus B — the CAN-Low (-) wire is open or broken. The bus is the shared wiring that lets control modules talk to each other, so a wiring fault here can silence one or many modules at once. What you notice depends on which modules ride this bus.
High
Can I keep driving? Have it checked before relying on the car — this is a manufacturer-defined bus, so effects range from a few dead features to limp mode or a no-start depending on which modules share it.
Common symptoms
What you'll notice
- Check-engine and/or several other warning lights on at once (ABS, airbag, traction)
- Multiple unrelated features stop working or behave erratically
- Communication or 'lost communication' codes stored in several modules
- Possible no-communication with a scan tool on the affected bus
- In some cases reduced power / limp mode or a no-start if powertrain modules share the bus
Common causes
Most likely first
- Damaged, chafed, or broken bus wiring (the most common cause)Likely
- Corroded, spread, or backed-out terminals at a connector or moduleCommon
- Water intrusion into a connector or harnessCommon
- A failed module pulling the bus voltage out of rangeLess often
- Wiring or terminating-resistor fault at the affected end of the busLess often
How to diagnose & fix
Work through it in order
- Scan every module and note which ones report communication faults — the pattern points to the break in the bus.
- Check battery voltage and grounds first; low voltage alone can trigger bus faults.
- Inspect the bus harness and connectors along the affected run for damage, corrosion, or water.
- Back-probe the bus wires and measure resistance/voltage against spec to locate the open or short.
- Repair the wiring or connector; if a single module is dragging the bus down, isolate and replace it. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with U0042?+
Only cautiously. If safety or powertrain modules share this bus you may see warning lights, limp mode, or a no-start, and driver aids like ABS or stability control may be offline. Have it diagnosed before relying on the vehicle.
Can I just clear U0042?+
Clearing hides it briefly, but a wiring open or short on the bus will set it again almost immediately. The physical fault has to be repaired.
Related codes
Often seen with U0042
Read & clear U0042 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