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Home / DTC / U0037 — Vehicle Communication Bus B

U0037 — Vehicle Communication Bus B

Detailed page for trouble code U0037.

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59brands
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23,694specific
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Code

U0037

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus B

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

Causes

  • Broken or shorted CAN wiring (CAN High / CAN Low) on Bus B
  • Open or poor ground or supply to gateway or control module on Bus B
  • Failed gateway / bus controller or other control module driving Bus B
  • Missing or failed termination resistor (open or shorted)
  • Corroded/loose connector or water intrusion in harness
  • Intermittent connector contact or chafed wiring contacting chassis

Symptoms

  • Multiple modules unavailable or 'communication' faults in scan tool
  • Warning lamps or messages related to vehicle systems (depending on which modules on Bus B)
  • Features controlled by modules on Bus B not working (e.g., body, chassis, comfort functions)
  • Intermittent operation of affected systems or erratic behavior
  • No CAN traffic for Bus B visible on diagnostic tool or oscilloscope

What to check

  • Read all U- and manufacturer codes with Mercedes diagnostic tool (Xentry/DAS) and note which modules report loss of comm
  • Check battery voltage and charging system; ensure stable 12V supply
  • Inspect fuses and relays that power the gateway and Bus B modules
  • Visually inspect CAN wiring, connector pins and harnesses along Bus B route for damage or corrosion
  • Measure resistance across CAN H and CAN L with ignition off (expect ~60 Ω if two 120 Ω terminators present)
  • Using a DVOM in volts mode or oscilloscope measure idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (recessive); dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V

Signal parameters

  • Recessive (idle) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both near mid-supply)
  • Dominant state voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
  • Termination resistance across CAN_H–CAN_L (ignition off): ≈ 60 Ω (two 120 Ω terminators in parallel)
  • Expected CAN bus message activity: depends on vehicle, typically periodic messages from gateway / ECUs (use manufacturer tool to view)
  • No continuous low or high voltage on either line (constant dominant indicates short or stuck driver)

Diagnostic algorithm

  1. Record all stored codes, freeze frame and module list using Mercedes diagnostic tool. Note which modules are not responding.
  2. Verify battery/charging voltage and ground integrity. Repair weak battery, loose terminals, or poor grounds first.
  3. Inspect power/ground/fuse circuits for gateway and BUS B modules; repair any open circuits.
  4. Visually inspect connectors and harness along Bus B for damage, corrosion or moisture; repair as needed.
  5. With ignition off, measure resistance across CAN_H and CAN_L at a convenient connector; expect ~60 Ω. If not, trace missing terminator or short.
  6. With ignition on, measure DC voltages on CAN_H and CAN_L (approx. 2.5 V idle). If one line is stuck high/low, isolate the shorted module or wiring.
  7. Use an oscilloscope to check CAN waveforms for distortion, missing edges, or bus contention. Look for dominant stuck states or excessive noise.
  8. If a single module is suspected, disconnect that module and recheck the bus. If bus returns to normal, repair/replace the faulty module or its harness.
  9. If wiring fault suspected, perform continuity and short-to-ground/Vbatt checks and repair wiring harness sections as indicated by topology.
  10. Update or reflash gateway / affected module software if recommended by manufacturer after hardware faults are corrected.
  11. Clear codes, perform a full network scan, and road test to confirm no recurrence.

Likely causes

  • Open/shorted CAN H or CAN L wiring on Bus B (harness damage, pin pushout)
  • Failed gateway or a node on Bus B pulling the bus to dominant state
  • Loss of power/ground to gateway or Bus B modules (fuse, ground strap)
  • Missing or damaged termination (two 120 Ω in parallel ≈ 60 Ω expected)

Fault status

⚠️ Status
Vehicle Communication Bus B - communication fault detected. One or more modules on Bus B are not communicating or the bus is inactive/shorted.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5-3.0 hours

Similar codes

U0073 U0100 U0101 U0121 U0140 U0155
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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