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Home / DTC / U0003 — High Speed CAN Communication Bus (+) open

U0003 — High Speed CAN Communication Bus (+) open

Detailed page for trouble code U0003.

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U0003

MERCEDES-BENZ U — Network/User

High Speed CAN Communication Bus (+) open

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

Causes

  • Open or broken CAN High (CAN+) conductor in the wiring harness
  • Corroded, loose, pushed-out or damaged connector pin at a module or junction
  • Damaged CAN transceiver (module internal hardware) pulling the line open
  • Missing or failed termination resistor(s)
  • Aftermarket device or splice interfering with the bus
  • Intermittent disconnect due to chafing, corrosion, or heat damage

Symptoms

  • Multiple control modules not responding to diagnostic scan tool or intermittent responses
  • Instrument cluster or warning lights showing multiple communication faults
  • Specific functions controlled over HS-CAN inoperative (e.g., ABS, ESP, transmission, gateway messages)
  • Stored or active communication-related fault codes
  • Possible limp-home mode or fail-safe operation of affected systems

What to check

  • Perform a full scan with a capable diagnostic tool to identify which modules are not responding and note timing of faults
  • Visual inspection of HS-CAN wiring, connectors, and ground points for damage or corrosion (follow harness routing diagrams if available)
  • Measure CANH and CANL voltages at a known-good connector with ignition ON, engine OFF
  • With vehicle power removed, measure resistance between CANH and CANL terminals across the network (expected ≈ 60 Ω with two 120 Ω terminators)
  • Check for unintended shorts to battery or ground on CANH conductor
  • Disconnect suspected module(s) one at a time to see if bus returns (to isolate defective transceiver)

Signal parameters

  • Recessive state: CANH ≈ 2.5 V, CANL ≈ 2.5 V (approximate)
  • Dominant state: CANH ≈ 3.5 V, CANL ≈ 1.5 V (approximate)
  • Idle differential voltage (CANH − CANL) ≈ 0 V (recessive) to ≈ 2.0 V (dominant)
  • Expected bus termination: two 120 Ω resistors in parallel → ≈ 60 Ω across CANH and CANL
  • Typical HS-CAN data rate: commonly 500 kbps (verify factory specification for vehicle)

Diagnostic algorithm

  1. Record all active and historical communication codes and identify which modules are not communicating using a factory-level scan tool.
  2. Visually inspect connectors and wiring along the HS-CAN trunk and splices for obvious damage, corrosion, or recent service areas.
  3. With ignition ON, engine OFF: measure voltages at the gateway or a central module connector — note CANH and CANL voltages. An open CANH will show no expected CANH voltage swing or stuck at open/near battery or ground.
  4. With ignition OFF and power removed, measure resistance between CANH and CANL at the same connector. Expect ≈ 60 Ω. Very high resistance or open indicates missing/failed termination or open conductor.
  5. If open is confirmed, perform continuity checks from affected connector back along harness to junction points and modules to locate break. Wiggle harness while monitoring resistance/voltage if intermittent.
  6. Disconnect suspect modules one at a time (with ignition off and proper safety) and re-check bus voltages/resistance to isolate a module pulling the line open or with failed transceiver.
  7. Use an oscilloscope at a known-good network location while the network is active to confirm absence of CANH waveform or presence of noise/flat line.
  8. Repair or replace damaged wiring, terminals or connectors. Replace failed module only after isolating and verifying the module is the fault.
  9. After repair, clear codes and perform a full system scan and road test to confirm communication is restored and no related codes return.

Likely causes

  • Broken wire at a flex point, door hinge, harness chafe, or behind dashboard
  • Corroded or de-pinned connector (battery disconnect, module harness connectors)
  • Failed module transceiver on one module causing bus to appear open
  • Disconnected or blown fusible link supplying a module that provides termination
  • Poor connector mating (water ingress) at gateway or network junction

Fault status

⚠️ Status
High Speed CAN Communication Bus (+) open — CAN High (CAN+) circuit is open or not meeting expected signal characteristics, causing loss of communication on the HS-CAN network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0001 U0002 U0073 U0100 U0101
856

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