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Home / DTC / U3100 — Lost Communication With Control Module

U3100 — Lost Communication With Control Module

Detailed page for trouble code U3100.

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U3100

Generic U — Network/User

Lost Communication With Control Module

Brand: Generic
Type: U — Network/User
Views: UK: 3 EN: 4 RU: 5
AI status
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Page language: EN

Causes

  • Module powered down (no B+ or ground)
  • Failed or crashed control module (internal fault)
  • Open/short in network wiring or connectors
  • Poor ground or corroded connector pins
  • CAN/LIN/FlexRay bus termination or wiring fault
  • Low battery or unreliable supply voltage

Symptoms

  • Warning lights or messages related to the affected system
  • Loss of function for systems controlled by the missing module
  • Intermittent or permanent faults reported on other modules
  • No communication messages from the module in a scan tool
  • Modules show 'NO COMMUNICATION' or timeouts
  • Vehicle may enter limp mode or disable certain features

What to check

  • Use a capable scan tool to confirm U3100 and view freeze frame / time stamps
  • Check for other network U-codes or related module codes
  • Verify battery voltage and key-on power rails (B+) at the suspected module
  • Check module ground(s) for continuity and low resistance to chassis
  • Inspect module connector for damage, corrosion, bent pins, water ingress
  • Measure CAN_H and CAN_L idle voltages with key on (approx. 2.5 V each)

Signal parameters

  • CAN idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (key on, ignition run)
  • CAN dominant level: CAN_H ≈ 3.5–4.0 V, CAN_L ≈ 1.0–1.5 V during transmission
  • Differential voltage: ~2.0–2.5 V when active
  • Bus termination: ~60 ohms between CAN_H and CAN_L with ignition on
  • Message frequency: expected messages per second for the module (use OEM network map)
  • Error frames/CAN bus error counters: elevated indicates bus faults

Diagnostic algorithm

  1. Read and record all DTCs and freeze-frame data from all modules with a scan tool.
  2. Identify which physical network and connector the reported module uses (CAN, LIN, MOST, FlexRay).
  3. Check battery voltage and main power fuses; restore if low or blown.
  4. Visually inspect the affected module connector and wiring harness for damage, corrosion, or moisture. Repair as needed.
  5. Verify module power and ground present at the connector with a DMM. Repair power/ground faults before further testing.
  6. With ignition on, measure CAN_H and CAN_L idle voltages at the module connector and at a known-good node. Compare values.
  7. Check termination resistance across CAN_H and CAN_L (approx. 60 ohms). Remove modules if necessary to isolate bus segments.
  8. Use an oscilloscope or CAN bus analyzer to check for activity, message IDs, differential waveforms, and error frames on the bus.
  9. If wiring and bus look correct, attempt to wake or reflash the module per manufacturer procedures; check for software updates or known module resets.
  10. If module still does not communicate and power/ground/wiring are good, consider replacing the module or sending it for bench repair/repair/reprogramming.
  11. After repair, clear codes and perform a full network scan; verify related systems operate and no new U-codes are present.

Likely causes

  • Lost power or ground to the affected module
  • Damaged or disconnected CAN/CAN-FD/LIN wiring or pins
  • Failed module electronics (crashed ECU)
  • Missing or incorrect termination resistor on the bus
  • Corroded connector causing high resistance
  • ECU asleep and not waking due to configuration or wake line fault

Fault status

⚠️ Status
Lost communication with control module — no valid messages received from the module on the vehicle network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3 hours

Similar codes

U0100 U0121 U0073 U0155 U3110
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