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Home / DTC / U0004 — High Speed CAN Communication Bus (+) low

U0004 — High Speed CAN Communication Bus (+) low

Detailed page for trouble code U0004.

34,405codes
59brands
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22,491specific
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Code

U0004

MERCEDES-BENZ U — Network/User

High Speed CAN Communication Bus (+) low

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

Causes

  • Open or intermittent wiring in CAN_H conductor
  • Short to ground on CAN_H
  • Poor or corroded connector pins at a module or splice
  • Failed CAN transceiver inside a module
  • Missing or damaged bus termination resistor(s)
  • Low or missing module supply/ground affecting transceiver operation

Symptoms

  • MIL/vehicle warning lamp illuminated
  • Loss of communication with one or more modules (scantool shows modules offline)
  • Multiple network communication DTCs present
  • Intermittent or permanent loss of functions controlled over HS-CAN (e.g., instrument cluster, ABS, transmission)
  • No or low CAN bus traffic on a scanner or oscilloscope

What to check

  • Read all U-/module-specific codes and record module list/addresses from scan tool
  • Visually inspect CAN bus wiring, splices and connectors for damage, corrosion, or water entry
  • With ignition ON, measure voltage between CAN_H and vehicle ground and between CAN_L and ground at multiple harness connectors
  • Measure resistance across CAN_H to CAN_L with ignition OFF — expected ~60 Ω (two 120 Ω terminators in parallel)
  • Check for short to ground or battery on CAN_H and CAN_L (ohmmeter or back-probing with power off)
  • Disconnect individual modules one at a time to isolate a faulting node and see if bus returns to normal

Signal parameters

  • Recessive voltage (idle) on CAN_H ≈ 2.5 V (both CAN_H and CAN_L ≈2.5 V)
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
  • Termination resistance across CAN_H–CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)
  • Typical HS-CAN baud rate commonly 500 kbps (confirm vehicle-specific rate)
  • Voltage tolerance: CAN_H low or stuck near ground (< ~1.5 V) indicates fault

Diagnostic algorithm

  1. Connect a capable scan tool and capture U0004 plus all related stored U-codes and module list
  2. Perform a careful visual inspection of the CAN network harness, connectors, and module grounds (repair obvious damage before electrical testing)
  3. With ignition OFF, measure resistance across CAN_H–CAN_L at a central junction; expect ≈60 Ω. If open, check terminator circuits and connector continuity
  4. With ignition ON (engine off), measure CAN_H and CAN_L vs. chassis ground at several points. Note any location where CAN_H is significantly lower than expected
  5. Check for shorts: with power removed, measure continuity from CAN_H to ground and to battery positive to find inadvertent shorts
  6. Disconnect suspect modules one at a time (starting with recently serviced or wet/damaged modules) and re-check bus voltages/communications to isolate a failing node
  7. Use an oscilloscope to view CAN_H/CAN_L while the network is active; verify recessive and dominant voltages and clean differential waveform
  8. If a module transceiver is found faulty or wiring/connector damaged, repair or replace the faulty component and re-verify network operation
  9. Clear codes and road/test-cycle to confirm repair. If fault returns, repeat isolation focusing on the module that reappears in fault history

Likely causes

  • Damaged CAN_H wire at harness routing (pinches, chafing, water ingress)
  • Corroded connector at gateway/module causing high resistance on CAN_H
  • CAN transceiver failed in gateway, ECU, or ABS/BCM module
  • One module shorting CAN_H to ground when powered
  • Loss of termination (open 120 Ω resistor or connector) reducing recessive voltage level

Fault status

⚠️ Status
High-Speed CAN bus positive (CAN_H) voltage low — network communication degraded or lost.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5–4.0 hours

Similar codes

U0001 U0073 U0100 U0121 U0140 U0105
856

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