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Home / DTC / U3537 — High Voltage System Interlock Circuit L Low

U3537 — High Voltage System Interlock Circuit L Low

Detailed page for trouble code U3537.

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Code

U3537

Generic U — Network/User

High Voltage System Interlock Circuit L Low

Brand: Generic
Type: U — Network/User
Views: UK: 13 EN: 20 RU: 18
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Page language: EN

Causes

  • Damaged or corroded interlock harness or connector (left side)
  • Open circuit (broken wire) in interlock loop
  • Short to ground on the interlock signal wire
  • Faulty interlock switch or set of interlock contacts (service plug, HV disconnect, charge port interlock, etc.)
  • Blown fuse or fault in related relay/power supply
  • Poor ground or low 12 V battery-voltage affecting circuit reference

Symptoms

  • MIL (check engine) or warning indicator illuminated
  • High-voltage system disabled or inhibited (vehicle may not start or propel)
  • Loss of EV/HEV drive or reduction to limp mode
  • Charging disabled or charge authorisation failure
  • HV system interlock or safety warnings displayed on dash

What to check

  • Read and record freeze-frame data and related active/inactive codes
  • Check vehicle battery voltage and charging system prior to testing (unstable supply can affect readings)
  • Visually inspect interlock wiring harness, connectors, and visible HV interlock devices for damage, corrosion, or poor seating
  • Check fuses and relays related to interlock circuit and HV system power
  • Backprobe the interlock signal pin at the control module and at interlock devices to measure voltage with key ON/engine OFF
  • Perform continuity and resistance checks of the interlock loop (module pin to each interlock device)

Signal parameters

  • Expected reference voltage: typically battery/ignition supply (approx. 9–14 V) or a 5 V reference depending on vehicle — consult specific wiring diagram
  • Expected signal HIGH: near supply voltage (e.g., >9 V if 12 V system; >4.5 V if 5 V system)
  • Expected signal LOW: below threshold (e.g.,
  • Open-circuit indication: very high resistance or infinite (continuity >10 kΩ)
  • Short-to-ground: continuity to chassis ground

Diagnostic algorithm

  1. Confirm DTC and note freeze-frame/occurrence history; check for related HV or communication codes
  2. Verify stable 12 V battery supply and proper vehicle grounds before further testing
  3. Visually inspect left-side interlock harness, connectors, and related HV interlock devices (service disconnect, charge inlet interlock, interlock switches) for damage or poor seating
  4. Check fuses/relays supplying the interlock circuit and replace if blown/faulty
  5. Backprobe interlock signal at the control module connector with a digital multimeter: record voltage with interlock engaged and disengaged; compare to expected signal parameters
  6. Measure continuity from module pin to each interlock device and to chassis ground. Look for open circuits (high resistance) or low-resistance shorts to ground
  7. If intermittent, use a lab scope or data logger to monitor the signal during operation or while wiggling harnesses to reproduce the fault
  8. If harness/connectors are damaged, repair or replace wiring and connectors per manufacturer repair methods (avoid splicing in high-voltage areas; follow safety procedures)
  9. If interlock device (service plug/switch) fails continuity checks, replace the device
  10. After repairs clear DTCs, perform system relearn or HV enable procedures if required, and confirm fault does not return with functional test and short road/operational test

Likely causes

  • Disconnected or loose interlock connector at HV service disconnect
  • Damaged insulation with wire chafing to chassis (short to ground)
  • Corroded terminals at interlock switch or HV pack connector
  • Open in the loop caused by service plug not fully engaged

Fault status

⚠️ Status
High Voltage System Interlock Circuit L - signal voltage lower than expected (circuit low). May inhibit high-voltage system and set MIL; requires inspection of interlock loop, connectors, fuses, and related components.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.5 hours

Similar codes

U3536 U3538 U3540 U0100
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