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Home / DTC / U3518 — High Voltage System Interlock Circuit C High

U3518 — High Voltage System Interlock Circuit C High

Detailed page for trouble code U3518.

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U3518

Generic U — Network/User

High Voltage System Interlock Circuit C High

Brand: Generic
Type: U — Network/User
Views: UK: 15 EN: 30 RU: 15
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Page language: EN

Causes

  • Open or intermittent connection in interlock C wiring
  • Short to battery positive on interlock C
  • Corroded or damaged interlock connector or pin
  • Faulty interlock switch or service disconnect (service plug) associated with circuit C
  • Faulty body/vehicle control module or HV system controller input
  • Water ingress or contamination at connector or switch

Symptoms

  • HV system disabled or prevented from enabling
  • Warning lights and messages related to high-voltage system or safety interlock
  • No EV drive or limited propulsion available
  • Charging function may be inhibited
  • DTC stored and possibly present with freeze-frame data

What to check

  • Verify safety: follow manufacturer high-voltage isolation procedures before inspecting HV components
  • Retrieve DTCs and freeze-frame data with a capable scan tool; note other HV or communication codes
  • Visually inspect interlock C connector(s), routing and harness for damage, chafe, pin corrosion, or water intrusion
  • Check service disconnect/insulation monitor and service plug seating (if applicable) for correct position and mechanical condition
  • With appropriate safety precautions and HV system disabled, measure continuity of the interlock C circuit to the expected reference (ground or controller input)
  • Measure voltage at the interlock C terminal with the system in the states specified by the manufacturer (key off/key on, service plug inserted/removed)

Signal parameters

  • Normal (interlock satisfied/closed): low/near 0 V at the ECU input (typically
  • Fault/High condition: elevated voltage near battery voltage or system supply (approx. 9–16 V) indicating open or short to B+
  • Expected continuity when closed: low resistance (
  • No unexpected pulsing or noise on the interlock line under steady-state conditions

Diagnostic algorithm

  1. Step 1 — Confirm and record: Read and record all related DTCs and freeze-frame values. Note if fault is current, intermittent, or historical.
  2. Step 2 — Visual inspection: With HV system made safe, inspect interlock C connectors, pins, wiring harness along its entire routing, service plug and associated switches for damage, corrosion or contamination.
  3. Step 3 — Connector and pin check: Disconnect the interlock connector(s). Inspect pins for corrosion, bent pins or poor contact. Clean or repair as required.
  4. Step 4 — Signal verification: With ignition and vehicle power state as specified by manufacturer, measure voltage at the interlock C input to the controller. Compare to expected values (low when closed, high when open).
  5. Step 5 — Continuity and short checks: With battery HV isolated per procedures, measure continuity of the interlock conductor to the interlock switch and to ground or reference. Check for short to battery positive using resistance and isolation tests.
  6. Step 6 — Actuate interlock: Operate the interlock switch/service plug while monitoring the input voltage to confirm correct state changes. If the signal does not change, suspect switch or wiring failure.
  7. Step 7 — Isolate components: If wiring appears intact, substitute or bench-test the interlock switch or module if possible, or disconnect the harness near the controller to determine if the controller input or upstream wiring is at fault.
  8. Step 8 — Repair and verify: Repair or replace damaged wiring, connectors or the interlock switch. Reconnect, clear codes, and perform full system enable and functional test per manufacturer procedures. Verify code does not return and that normal HV operation and charging are restored.
  9. Safety note — High-voltage systems are dangerous. Only trained technicians with appropriate PPE and following vehicle-specific HV safety procedures should perform these tests and repairs.

Likely causes

  • Open connector or broken conductor in interlock C harness
  • Interlock switch stuck or mechanically damaged (service plug not seated)
  • Short to 12 V from adjacent wiring harness damage
  • Corrosion or moisture causing high resistance and resulting voltage rise at the ECU input

Fault status

⚠️ Status
High-voltage system interlock C shows high voltage (open or short). High-voltage system may be disabled; service by qualified technician required.
🔴 Repair difficulty: Hard
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U3505 U3517 U3519 U0100 U1101
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