U1601 — CAN bus communication (C-NCM)

Causes

• Open or shorted CAN_H or CAN_L wiring
• Poor connector/terminal contact or corrosion at C‑NCM or other nodes
• Failed C‑NCM (internal fault)
• Missing or incorrect CAN termination (open/shorted resistor)
• Low battery voltage or intermittent power/ground to C‑NCM
• Software/configuration mismatch after replacement or update

Symptoms

• Multiple modules not responding to scanner or intermittent responses
• Warning lights or messages about network faults on the dash
• Loss of features controlled across the CAN network (doors, lighting, HVAC, instrument cluster)
• Engine or drivability faults if powertrain modules affected
• Intermittent electrical behavior or random faults that clear/reappear
• No communication to C‑NCM on scan tool or inconsistent node list

What to check

• Read all network U‑codes and freeze frame data with a capable scan tool
• Verify battery voltage (key on and engine running) and check C‑NCM fused power/ground circuits
• Visual inspection of C‑NCM connectors and CAN wiring for damage, corrosion or repairs
• Check for other active network codes that identify affected subnet or node
• Measure CAN bus differential and individual wire voltages with key ON (see signal params)
• With vehicle power OFF, measure resistance between CAN_H and CAN_L to check termination (~60 ohms with two 120Ω resistors)

Signal parameters

• Recessive (idle) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ~2.5 V relative to ground)
• Dominant frame voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
• Bus differential: ~0 V when idle? (recessive) and ~2.0 V during dominant bits
• Termination resistance (power OFF): ~60 Ω across CAN_H and CAN_L (two 120 Ω resistors in parallel)
• Bus bit rate: depends on vehicle (commonly 500 kbps or 250 kbps); mismatch can prevent communication

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