Toggle Side Panel

Shopping Cart

No products in the basket.

Shopping Cart

No products in the basket.

Close search
Home / DTC / U0051 — Vehicle Communication Bus C (-) open

U0051 — Vehicle Communication Bus C (-) open

Detailed page for trouble code U0051.

35,102codes
59brands
11,925generic
23,177specific
All codes P-codes (Powertrain) B-codes (Body) C-codes (Chassis) U-codes (Network/User) Tesla Alerts
Reset
Code

U0051

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus C (-) open

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Broken/open CAN-L conductor on Bus C
  • Corroded, bent or poorly seated connector pins at a module or splice
  • Failed or unpowered control module on Bus C
  • Damaged or missing termination resistor(s) (open 120 Ω termination)
  • Short to battery or short to ground affecting CAN-L line
  • Water intrusion or body damage affecting harness or junctions

Symptoms

  • Multiple modules on Bus C show “no communication” or multiple U-codes
  • Warning lamps on dash (e.g., instrument cluster, traction, ABS)
  • Functions controlled by modules on Bus C are inoperative or intermittent
  • Modules report lost messages or timeouts in scan-tool data
  • Possible erratic module behavior or failed features tied to that network

What to check

  • Use a factory-level scan tool to read all stored U-codes and module status on Bus C
  • Visually inspect CAN wiring harnesses, grommets, junctions and control module connectors for damage, corrosion or water ingress
  • Backprobe CAN_H and CAN_L at multiple nodes to confirm presence/absence of expected voltages and differential signal
  • Measure termination resistance across CAN_H and CAN_L with ignition off (expected ≈60 Ω total for two 120 Ω terminators)
  • Check continuity of CAN-L to each module and to splice/junction points
  • Check for short to battery or ground on CAN-L using an ohmmeter (with power removed)

Signal parameters

  • Recessive (idle) voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both lines near battery mid-level)
  • Dominant (active) voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ~2 V)
  • Differential idle voltage ≈ 0 V, differential active ≈ 2 V
  • Expected bus termination: two 120 Ω resistors in parallel → measured ≈60 Ω across CAN_H to CAN_L with ignition off (verify manufacturer spec)

Diagnostic algorithm

  1. Connect a factory-capable scan tool; record all U-codes and note which modules on Bus C report communication loss.
  2. Visually inspect harnesses, connectors, and junctions on routes associated with Bus C for physical damage, corrosion or water entry; repair any damage found.
  3. With ignition off, measure resistance between CAN_H and CAN_L at convenient service connector — verify termination resistance ≈60 Ω (or factory spec). If open, locate splice/terminator with open circuit.
  4. With ignition on, backprobe CAN_H and CAN_L at a known good node and at suspected bad node. Verify recessive voltages and differential waveform with a multimeter and oscilloscope if available.
  5. If CAN_L is open at a location, trace wiring toward each module/junction. Use continuity checks to locate the open conductor; inspect and repair broken wire, corroded connector, or replace faulty splice.
  6. If a module is unresponsive, verify it has proper battery and ground power. Disconnect suspected faulty module (following manufacturer procedures) and re-check bus behavior; a shorted internal transceiver can be isolated by disconnection.
  7. After repairs, clear codes and perform a road or system test while monitoring bus messages to confirm proper restoration of communication.
  8. If wiring and terminations are good but fault persists, suspect failed module transceiver or control module; only replace module after confirming it is the root cause and following manufacturer reprogramming/configuration procedures.

Likely causes

  • Open circuit in CAN-L between modules or splice point
  • Disconnected or failed node (module) pulling the line open
  • Open/missing termination resistor causing open-circuit detection
  • Damaged connector or pin at a gateway or junction module

Fault status

⚠️ Status
U0051 — Vehicle Communication Bus C (−) open. CAN-L line on Bus C presents as open/invalid; modules report lost or invalid communication on Bus C.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0050 U0073 U0100 U0105 U0140
856

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

MERCEDES-BENZ

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

Report

Harassment or bullying behavior
Contains mature or sensitive content
Contains misleading or false information
Contains abusive or derogatory content
Contains spam, fake content or potential malware

Block Member?

Please confirm you want to block this member.

You will no longer be able to:

  • See blocked member's posts
  • Mention this member in posts
  • Invite this member to groups
  • Message this member
  • Add this member as a connection

Please note: This action will also remove this member from your connections and send a report to the site admin. Please allow a few minutes for this process to complete.

Report

You have already reported this .