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Home / DTC / U0042 — Vehicle Communication Bus B (-) open

U0042 — Vehicle Communication Bus B (-) open

Detailed page for trouble code U0042.

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Code

U0042

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus B (-) open

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
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ready
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Page language: EN

Causes

  • Broken or damaged wiring on Bus B negative conductor (open circuit)
  • Corroded, loose or pushed-out connector pin at a module or splice
  • Failed control module or gateway driving/terminating Bus B
  • Aftermarket device or recent repair that interrupted the bus wiring
  • Water intrusion or contamination in connectors causing intermittent opens
  • High-resistance connection or poor crimp at splice or terminal

Symptoms

  • One or more control modules on Bus B show 'no communication' or appear offline
  • Multiple U-codes or communication warnings on instrument cluster
  • Loss of vehicle features controlled by modules on Bus B (e.g. ABS, airbags, telematics depending on vehicle)
  • Intermittent or permanent MIL/warning lamps
  • Vehicle may enter limp mode or exhibit degraded functionality
  • Unable to reprogram or communicate with specific modules on Bus B

What to check

  • Retrieve full DTC list and note any additional U-codes or module-specific codes
  • Visually inspect wiring harnesses, connectors and splices associated with Bus B for damage, corrosion, or water intrusion
  • Verify battery voltage and ground integrity before performing bus tests
  • Measure DC resistance between CAN/BUS B+ and BUS B- with ignition off — expected ≈ 60 Ω (two 120 Ω terminators in parallel) across the bus (H to L)
  • Check continuity of the Bus B negative conductor end-to-end and to each module on that bus
  • Probe idle voltages at CAN_H and CAN_L (or Bus B +/−) with ignition in ON (do not crank) — verify recessive voltages and compare against expected values

Signal parameters

  • Recessive (idle) voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both near mid-supply)
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active bits
  • Differential voltage: typically up to ≈ 2 V when dominant
  • Bus resistance between CAN_H and CAN_L (ignition ON or OFF depending on vehicle) ≈ 60 Ω (two 120 Ω terminators in parallel)
  • Open-circuit symptom: CAN_L (Bus B −) may show no valid recessive voltage, erratic readings, or very high/ infinite resistance to expected nodes

Diagnostic algorithm

  1. Read and record all DTCs and freeze frame data from all modules. Note which modules report lost communication.
  2. Confirm code is current (not historic) and attempt a key cycle to see if fault is persistent.
  3. Visually inspect the Bus B wiring route, connectors and termination resistors (look for corrosion, broken wires, rodent damage).
  4. With ignition OFF, measure resistance between Bus B + and Bus B − at an accessible connector — expect ≈ 60 Ω. If open/infinite, suspect an open conductor or missing termination.
  5. Check continuity of Bus B − conductor from the gateway/module connector to individual modules on that bus. Repair or reconnect any open circuits.
  6. With ignition ON (engine not running), measure idle voltages on Bus B + and Bus B − relative to chassis ground. Compare to expected recessive voltages. An open Bus B − will show abnormal/floating voltages.
  7. Disconnect suspect modules on Bus B one at a time (or remove aftermarket devices) and observe if the code clears or communication is restored to isolate a module short/open.
  8. Use an oscilloscope or CAN analyzer to inspect waveform integrity. Look for absent or severely distorted activity on Bus B −.
  9. Repair damaged wiring, corroded pins, poor splices or replace faulty module/connectors. Ensure correct crimping, sealing and routing.
  10. After repair, clear DTCs, cycle ignition and verify full communication to all previously affected modules. Road test and re-scan to confirm fault does not return.
  11. Safety note: when performing wiring repairs disconnect the battery or follow vehicle manufacturer procedures for live-bus work; avoid shorting CAN conductors to power or ground.

Likely causes

  • Physical break in CAN/Bus B negative wire where harness flexes (door hinge, subframe)
  • Connector pin backed out or corroded at a module on Bus B
  • Failed gateway/module that interfaces Bus B to main network
  • Incorrect wiring following prior repair or installation of accessories
  • Intermittent open caused by corrosion/water in a connector

Fault status

⚠️ Status
Communication fault: Bus B negative conductor open (loss of continuity) detected. Modules on Bus B may be offline or unresponsive.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0041 U0073 U0100 U0121 U0140
856

Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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