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DTC codes / U35xx · Network communication / U3535

U3535High Voltage System Interlock Circuit L

U3535 indicates that the high-voltage interlock loop for circuit L has detected a voltage above the expected range, which the vehicle's control module interprets as a potential safety hazard. The interlock is a low-voltage safety circuit that runs through high-voltage connectors and components, and it must remain at a specific voltage to confirm that all high-voltage connections are secure before the system allows high-voltage operation.

SystemNetwork
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaNetwork communication

Symptoms you may notice

  • High-voltage system may be disabled, preventing the vehicle from starting or driving
  • Illumination of the high-voltage warning light or a master warning indicator
  • The vehicle may enter a reduced-power mode or fail to engage the high-voltage battery contactors
  • Often no symptoms besides the warning light if the fault is intermittent or stored without active disabling
  • Possible inability to charge the high-voltage battery on plug-in vehicles
  • Other high-voltage related codes may be stored simultaneously

Common causes, most likely first

  1. Damaged or corroded high-voltage interlock connector pins or terminals
  2. Faulty high-voltage interlock loop wiring, including opens or shorts to voltage
  3. Failed high-voltage component with an internal interlock switch fault
  4. Water intrusion or contamination in a high-voltage connector causing abnormal voltage
  5. Control module software error or internal fault in the interlock detection circuit
  6. Aftermarket accessories or improper previous repairs disturbing the interlock circuit

How to diagnose and fix it

  1. Perform a thorough visual inspection of all high-voltage connectors and wiring for damage, corrosion, or loose connections
  2. Use a scan tool to read and record all stored codes and freeze-frame data, then clear codes and verify if U3535 returns
  3. Check service information for the specific interlock circuit L voltage specifications and test the circuit with a multimeter
  4. Disconnect and inspect each high-voltage connector in the interlock loop, checking for proper pin fit and cleanliness
  5. Test the interlock switch within each high-voltage component if accessible, following manufacturer procedures
  6. Inspect for aftermarket installations or previous repair work that may have compromised the interlock circuit
  7. If all external checks pass, follow the manufacturer's diagnostic flow for internal module or component replacement

Interlock voltage checks before parts replacement

  • A high-voltage interlock circuit is not the same as the high-voltage power circuit; it carries a low-voltage signal, so a simple multimeter can be used for testing without high-voltage PPE.
  • Many technicians overlook that the interlock loop often passes through the charge port, service disconnect, and battery pack, so a fault in any of these areas can set U3535.
  • The code may set intermittently due to vibration or temperature changes, making it important to wiggle-test connectors while monitoring live data.
  • Replacing a high-voltage component without first verifying the interlock circuit integrity can lead to repeat failures and unnecessary expense.
  • Some vehicles require a specific sequence to reset the interlock system after repair, such as clearing codes and performing a battery pack learn procedure.

Can I keep driving with U3535?

Do not continue driving with an active U3535 code, as the high-voltage system may be disabled or operate unpredictably, posing a safety risk. Have the vehicle inspected by a qualified high-voltage technician before further use.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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