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Home / DTC / U1601 — CAN bus communication (C-NCM)

U1601 — CAN bus communication (C-NCM)

Detailed page for trouble code U1601.

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U1601

ALFA ROMEO U — Network/User

CAN bus communication (C-NCM)

Brand: ALFA ROMEO
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Intermittent or open/shorted CAN_H or CAN_L wiring
  • Poor connector contact or corrosion at CAN connectors or NCM connector
  • Failed or malfunctioning C‑NCM module
  • Incorrect or missing CAN bus termination resistors
  • Low battery voltage or poor ground at NCM affecting CAN transceiver
  • Software/firmware mismatch or module corruption after update

Symptoms

  • MIL or network warning indicator illuminated
  • One or more control modules not responding to diagnostic tool
  • Loss of functionality for systems managed by C‑NCM (lighting, central locking, body functions)
  • Intermittent faults or modules disappearing/reappearing on the scan tool
  • Starting or immobilizer-related faults if gateway communication is lost

What to check

  • Record freeze frame and all stored network-related DTCs with a capable scan tool
  • Visually inspect CAN wiring and connectors for damage, corrosion, and proper routing
  • Check battery voltage and ground integrity at NCM and vehicle chassis (engine off and cranking)
  • Scan for presence of other modules on the same CAN network and note any missing devices
  • Measure resistance across CAN_H and CAN_L with ignition off — expect roughly 60 ohms (two 120 Ω terminators in parallel)
  • Measure idle voltages on CAN_H and CAN_L with key on (no start) using a DVM or oscilloscope

Signal parameters

  • Idle (recessive) voltage CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (voltage swing about ±1 V from recessive)
  • Termination resistance across CAN_H–CAN_L ≈ 60 Ω (two 120 Ω in parallel)
  • Typical CAN data rates: 500 kbps or 250 kbps depending on vehicle model — verify correct network rate from OEM data

Diagnostic algorithm

  1. Connect OEM-level diagnostic tool; read and record U1601 and all related network DTCs and module presence.
  2. Note when the fault occurs (ignition on, during drive, constant) and any freeze-frame data.
  3. Verify battery voltage and ground integrity; charge battery or repair ground if low.
  4. With ignition off, disconnect relevant connectors and inspect pins for corrosion, bent pins, or water ingress. Repair connectors as needed.
  5. Measure resistance across CAN_H–CAN_L at multiple points (near NCM, gateway, and other modules). Confirm ~60 Ω; if open or shorted, isolate by disconnecting segments.
  6. With key on, measure CAN_H and CAN_L voltages at the NCM and at other modules. Compare to expected values and look for abnormal bias or stuck dominant state.
  7. Use an oscilloscope (or scope capable scan tool) to view CAN waveform for noise, missing edges, or reflections. Look for nodes pulling bus low or excessive bit errors.
  8. If a single module is missing from the bus, disconnect that module to see if bus restores. Replace or repair the module or its harness if it causes bus failure.
  9. If wiring and connectors are good, consider reprogramming/flash or replacing the C‑NCM per OEM instructions; follow security/immobilizer procedures.
  10. Clear DTCs and perform a road test to verify repair. If issue recurs, escalate with wiring diagrams, manufacturer bulletins, or technical support.

Likely causes

  • Damaged CAN wiring harness (abrasion, pinch, rodent chew) near left fender, dash, or NCM connector
  • Corroded or loose pin at NCM or a gateway/relay module on the same CAN segment
  • Failed CAN transceiver inside the C‑NCM
  • Missing/shorted termination resistor (open or short to ground/power)

Fault status

⚠️ Status
Communication fault detected on CAN bus by C‑NCM (loss/corruption of network messages).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-3 hours

Similar codes

U0100 U0073 U0077 U0140 U0121
89

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