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Home / DTC / U3520 — High Voltage System Interlock Circuit E High

U3520 — High Voltage System Interlock Circuit E High

Detailed page for trouble code U3520.

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Code

U3520

Generic U — Network/User

High Voltage System Interlock Circuit E High

Brand: Generic
Type: U — Network/User
Views: UK: 15 EN: 27 RU: 16
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Page language: EN

Causes

  • Open or short in HV interlock harness or connector
  • Faulty interlock switch, service plug sensor, or charge-port interlock
  • Corroded or loose connector pins at interlock components or control module
  • Blown fuse or failed relay in interlock supply/monitoring circuit
  • Faulty body or powertrain control module that monitors the interlock
  • Water intrusion or mechanical damage to interlock components

Symptoms

  • HV system disabled or inhibited (vehicle may refuse to start or enable EV drive)
  • HV or READY warning lamp on instrument cluster
  • Reduced power or limp-home mode
  • Charging disabled or charge port fault indicated
  • DTC stored and possibly other related U or P codes present

What to check

  • Read freeze frame/live data and confirm U3520 present; note any related codes
  • Visual inspection of interlock components (service plug, charge port, HV disconnect switches) and surrounding area for damage or water
  • Inspect connectors and wiring for corrosion, damage, loose pins or melted insulation
  • Check for blown fuses and proper operation of relays in the HV interlock supply/monitoring circuit
  • Measure interlock circuit voltage and continuity at the module connector and at the interlock device
  • Perform wiggle tests while monitoring signal to detect intermittent faults

Signal parameters

  • Interlock circuit is a low-voltage monitoring/switch signal (manufacturer-specific). Typical nominal ranges: 0–12 V.
  • Normal 'closed' (interlock satisfied) often reads near 0–1 V or near ground; 'open' or fault condition may read high (example: >8–12 V) or show infinite resistance.
  • Expected continuity across a closed mechanical interlock device: near 0 ohms to low single-digit ohms (manufacturer-specified).
  • Open circuit measured as infinite/OL; short to supply measured as near battery voltage on the interlock sensor line.
  • Always confirm exact expected voltages/resistances with OEM data before concluding.

Diagnostic algorithm

  1. WARNINGS: Only qualified technicians with appropriate high-voltage training and PPE should work on HV systems. Disable the HV system following OEM procedures (remove service plug, isolate battery, etc.) before performing hands-on diagnostics.
  2. Connect a scan tool, record freeze-frame and any related DTCs. Note module reporting the code and live interlock signal values with ignition/READY states as required by vehicle.
  3. Visually inspect interlock components (service plug, charge inlet interlock, HV disconnect switches), wiring harnesses, and connectors for damage, corrosion, or water intrusion.
  4. With HV system disabled per OEM procedure, disconnect the interlock connector at the module and at the interlock device. Check for continuity and proper resistance across the interlock device (compare to OEM spec).
  5. Inspect connector pins for corrosion, bent pins, or pushed-out terminals. Repair or replace damaged connectors or terminals and re-seat securely.
  6. Re-enable vehicle control power (following safety procedures). Backprobe the interlock signal at the module harness and at the interlock device while observing live data and using a multimeter/oscilloscope. Verify expected voltages and look for high voltage or spikes.
  7. If open/high signal is present at module but correct at interlock device, suspect wiring fault between them (inspect, repair, or replace wiring harness). If signal is incorrect at the device, replace or repair the interlock device.
  8. Check related fuses and relays feeding the interlock circuit; replace if faulty and re-test.
  9. After repairs, clear codes, cycle HV enable per OEM procedure, and confirm code does not return. Road test if required and re-scan for intermittent faults.
  10. If wiring and components check OK but fault persists, evaluate the monitoring/control module for internal fault or program/error (refer to OEM advanced diagnostics).

Likely causes

  • Failed interlock device (service plug/charge-port switch) or its internal sensor
  • Open circuit or high-resistance connection in the interlock wiring between the interlock device and the monitoring module
  • Connector corrosion or bent/broken pin causing intermittent/high voltage reading
  • Short to battery positive or another high-voltage source in the interlock wiring
  • Blown inline fuse or faulty relay supplying the interlock sensor circuit

Fault status

⚠️ Status
High Voltage System Interlock Circuit E — signal high or open. HV system may be disabled until interlock fault is cleared and repaired.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U3500 U3505 U3510 U3521 U3530
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