U0042

Code

Generic U — Network/User

Vehicle Communication Bus B (-) Open

Brand: Generic

Type: U — Network/User

Views: UK: 22 EN: 32 RU: 24

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Broken or disconnected wiring on Bus B negative (CAN‑L) conductor
Corroded, loose, or damaged connector at a module or splice
Failed module with an open bus transceiver
Blown fuse or power/ground fault to a gateway/module that terminates Bus B
Aftermarket equipment or repairs that cut or disturbed the bus wiring

Symptoms

Loss of communication with one or more modules on Bus B (scan tool shows module not responding)
Multiple communication-related DTCs for modules on the same network
Warning lights or message center alerts about system faults
Functions controlled by modules on Bus B inoperative or intermittent
No obvious engine mechanical effect unless Bus B controls drivability modules

What to check

Use a scan tool to confirm U0042 and list other communication DTCs and affected modules
Visually inspect Bus B wiring harness, connectors, and terminal pins for damage or corrosion
Check for recently serviced or aftermarket equipment near the affected harness
Backprobe suspected connectors and measure bus voltages with ignition ON (no cranking)
If available, inspect bus waveforms with an oscilloscope on CAN‑H and CAN‑L

Signal parameters

Idle/common‑mode voltage on CAN‑H and CAN‑L (approx. same ~2.5 V on many systems)
Differential voltage during activity (dominant vs recessive waveform on oscilloscope)
Continuity/ohms of CAN‑L conductor between modules and to splice points
Resistance between CAN‑L and battery negative (should not be open)
Presence/absence of expected CAN traffic frames on Bus B via scan tool/oscilloscope

Diagnostic algorithm

Read and record all U‑codes and module statuses with a scan tool to identify which modules are not communicating on Bus B.
Visually inspect wiring, connectors, ground points and module mounting areas along the Bus B route for damage, corrosion, or loose connectors.
With ignition ON (engine off), backprobe CAN‑L and CAN‑H at the gateway or junction and at one or more module connectors. Verify expected idle voltages and presence of signals. Use an oscilloscope if available to confirm waveform activity.
If CAN‑L is open at one end but present at another, isolate by disconnecting connectors or modules one at a time to find the segment causing the open condition.
Check continuity of the CAN‑L conductor between modules and to splices. Repair broken wires, pinned connectors, or corroded terminals as found.
If wiring and connectors test good, replace or bench‑test suspect module(s) that may present an open transceiver on Bus B.
After repair, clear codes and verify successful communication restoration and that related modules return to normal operation. Monitor bus for intermittent faults during road test where applicable.

Likely causes

Physical open in CAN‑L between modules (chafing, pinched harness, disconnected connector)
Faulty module transceiver presenting an open on Bus B when powered
Damaged splice/junction or poor ground affecting bus return
Connector corrosion or bent pins at a module on Bus B

Fault status

⚠️ Status

Vehicle Communication Bus B (-) Open — The Bus B negative circuit is open or not providing expected signal levels. Modules on Bus B may be unable to communicate. Inspect wiring, connectors and related modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours