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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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Code

U0003

HYUNDAI U — Network/User

High Speed CAN Communication Bus (+) Open

Brand: HYUNDAI
Type: U — Network/User
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Page language: EN

Causes

  • Open or broken CAN High (CAN+) conductor in wiring harness
  • Corroded, loose or damaged connector at a module or splice
  • Failed CAN transceiver (ECU/module level)
  • Missing or faulty bus termination (open/missing resistor)
  • Aftermarket device or harness modification interrupting the bus
  • Water intrusion or physical damage to wiring harness

Symptoms

  • Malfunction Indicator Lamp (MIL) or network-related warning lights illuminated
  • Loss of communication with one or more control modules on HS-CAN
  • Multiple systems inoperative or limited (gauges, ABS, airbag, climate, etc.)
  • Intermittent or complete no-response to scanner commands on affected modules
  • Vehicle may not start if powertrain modules are isolated

What to check

  • Verify DTC U0003 is current; record freeze-frame and note any additional U-codes
  • Check battery voltage and ground integrity before network testing
  • Visually inspect CAN wiring, connectors, and splices along the HS-CAN trunk for damage and corrosion
  • Measure voltage at CAN High and CAN Low at a known good module connector with ignition ON
  • Measure resistance between CAN High and CAN Low (termination) with ignition OFF
  • Use a scan tool to monitor CAN traffic and see which modules are talking or missing

Signal parameters

  • Bus type: High Speed CAN (commonly 500 kbps; some vehicles use 250 kbps) — verify vehicle spec
  • Recessive state: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both centered at ~2.5 V)
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
  • Expected differential voltage during idle (recessive) ≈ 0 V; dominant differential ≈ 2.0 V
  • Bus termination: nominal ≈ 60 ohms across CAN_H and CAN_L (two 120 Ω terminators in parallel)

Diagnostic algorithm

  1. Confirm the code and check for additional network codes (U0xxx, U01xx). Note when the fault is present (key ON, engine running).
  2. Check battery and ground to ensure stable supply during tests.
  3. Perform visual inspection of HS-CAN trunk and module connectors (gateway, ECM, BCM, ABS, instrument cluster). Look for corrosion, bent pins, water, or recent repair areas.
  4. With ignition ON (engine off), measure voltage at a convenient CAN connector: CAN_H and CAN_L should sit near 2.5 V. If CAN_H is open it will be pulled to battery or module supply or float—record readings.
  5. With ignition OFF measure resistance across CAN_H and CAN_L at two locations. A healthy bus typically measures ~60 Ω. Very high/OL indicates open; very low indicates short to ground or Vbat.
  6. If open suspected, isolate sections: disconnect modules one at a time (start with modules at ends of trunk/terminators) and re-check continuity/voltage to find where the line goes open.
  7. Use an oscilloscope or CAN diagnostic tool to observe waveform and node activity while modules are connected. Look for absence of frames or abnormal wave shapes on CAN_H.
  8. Inspect and backprobe connectors/splices identified in step 6. Repair any damaged wiring, clean corroded pins, and reseat connectors.
  9. If wiring and connectors test OK, suspect faulty transceiver in the module that is last on the bus or shows abnormal behavior. Confirm by substituting a known-good module if available or following manufacturer module replacement procedure.
  10. After repair, clear codes and verify normal network communication and absence of U0003. Road test as required to ensure issue resolved.

Likely causes

  • Corroded or loose connector at a gateway/ECU or splice on CAN High
  • Broken wire due to chafing, repair error, or recent collision/stretch
  • Failed transceiver in one control module pulling CAN High open
  • Removed/loose termination resistor or connector (after servicing)
  • Moisture intrusion at a module connector

Fault status

⚠️ Status
High Speed CAN Communication Bus (+) Open — open or interrupted CAN High circuit detected causing loss of communication on HS-CAN.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 4.0 hours

Similar codes

U0001 U0002 U0073 U0100 U0077
371

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