What it means
An open fault is detected on the negative conductor of vehicle communication bus B. The code identifies the conductor fault but not its cause.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
Bus B carries messages between its connected modules over a pair of conductors; the (−) conductor is the low-side line. U0042 indicates that this negative line is detected as open, so bus B communication can fail even when its (+) line remains intact.
Common causes
Most likely first.
- Bus-D high wire broken
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Record all codes and freeze-frame data, and identify which bus B modules stop responding when the fault is present.
- Inspect the bus B (−) wire and its connectors for a broken conductor, disconnected plug, pulled terminal, corrosion, or previous repair damage.
- Check scan-tool network status to map which modules or sections share the communication loss.
- With power isolated as directed by the manufacturer, test continuity of bus B (−) and check for an intermittent open while moving accessible harness sections.
- If the conductor tests sound, check gateway routing and bus termination against the manufacturer's specifications.
Common fixes
- Reconnect a disconnected bus B connector and secure it.
- Repair a broken bus B (−) conductor or damaged splice.
- Replace or repair corroded or loose terminals on the negative line.
- Repair gateway routing or replace a terminating resistor only if testing confirms the problem.
Can I keep driving?
If the affected bus B modules control steering, braking, stability, or another essential function, stop driving and arrange recovery. Otherwise, use caution and avoid unnecessary trips because connected functions may fail or enter limp mode.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.