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Home / DTC / U0028 — Vehicle Communication Bus A

U0028 — Vehicle Communication Bus A

Detailed page for trouble code U0028.

34,405codes
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22,491specific
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Code

U0028

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus A

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

Causes

  • Open, short or high resistance in CAN High or CAN Low wiring or connectors on Bus A
  • Failed or intermittent control module pulling the bus dominant
  • Missing or damaged termination resistor(s)
  • Corroded/wet connectors or poor ground at module(s)
  • Low battery/unstable supply voltage or poor battery connections
  • Aftermarket electronics or improperly installed modules interfering with the bus

Symptoms

  • Multiple control modules show ‘no communication’ or erroneous messages
  • Instrument cluster warnings and network-related dash messages
  • Loss of functions that depend on Bus A (e.g., ABS, ESP, transmission, gateway features)
  • Intermittent features working or vehicle entering limp mode
  • Inability to access certain modules with diagnostic tool

What to check

  • Read all stored and pending U-codes and manufacturer-specific freeze frame data
  • Verify battery voltage and charging system (stable 12–14.5 V) before tests
  • Visually inspect CAN connectors and harnesses for corrosion, damage or water
  • Measure resistance between CAN_H and CAN_L with ignition off (≈60 Ω expected)
  • Measure idle voltages on CAN_H and CAN_L with ignition on (approx. 2.4–2.7 V each)
  • Check for short to ground or battery on each bus wire (with power off)

Signal parameters

  • Idle (recessive) voltage: CAN_H ≈ 2.4–2.7 V, CAN_L ≈ 2.4–2.7 V (measured to chassis ground)
  • Dominant state during transmission: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
  • Termination resistance: total ≈ 60 Ω measured between CAN_H and CAN_L with power off
  • Short/low resistance to ground or battery on one line will pull idle voltages off nominal values
  • No valid CAN frames or heavy noise on scope indicates wiring/module fault

Diagnostic algorithm

  1. Capture all DTCs and freeze frame data; note when code set and any related U-codes.
  2. Verify battery voltage and ensure good ground and terminal connections.
  3. Visually inspect CAN bus wiring, connectors and modules related to Bus A (gateway, clusters, ECU locations) for damage or contamination.
  4. With ignition off, measure resistance between CAN_H and CAN_L at a convenient connector — expect ≈60 Ω. If open or out of range, isolate segments.
  5. With ignition on, measure idle voltages on CAN_H and CAN_L at a known-good connector. Deviations indicate shorted/dragging line or missing termination.
  6. Use a scanner/PC or CAN bus analyzer to monitor traffic; verify expected messages and check for error frames or noise.
  7. Perform module isolation: disconnect modules one at a time (following manufacturer safety) while observing bus voltage/resistance to identify the faulty node.
  8. If short to ground/battery suspected, perform continuity and short location techniques (backprobing, disconnecting branches) to find damaged harness or connector.
  9. Repair or replace damaged wiring, connectors, termination resistor(s) or the failed module as indicated by tests.
  10. Reprogram or update module software/configuration if communication failure was due to software mismatch; clear codes and verify repair with road test.
  11. After repair, clear DTCs and re-scan; confirm normal network operation and no reoccurrence.

Likely causes

  • Physical wiring damage (pinched/chafed harness) on CAN_H or CAN_L
  • Failed gateway or central communication module on Bus A
  • Bad connector with corrosion or water ingress at a network node
  • Missing/failed termination resistor (open or wrong value)
  • A module shorted to ground or battery dragging the bus low

Fault status

⚠️ Status
Communication fault on Vehicle Communication Bus A — messages missing, corrupted, or modules not responding on the CAN network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5 - 4.0 hours

Similar codes

U0073 U0100 U0115 U0121 U0140
856

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