OBD-II code U3535 indicates a fault in the high-voltage interlock circuit 'L', a safety loop that monitors the integrity of high-voltage connections in hybrid or electric vehicles. When the circuit is open or out of specification, the system disables high-voltage operation to prevent shock hazards.
The most common cause is an open circuit, short, or high resistance in the low-voltage interlock wiring, often due to chafing, moisture intrusion, or loose connections.
A mechanically failed or electrically open interlock switch (e.g., in a high-voltage connector or service disconnect) can break the continuity of the safety loop.
A high-voltage connector that is not fully latched will open the interlock circuit, often after service or component replacement.
An internal fault in a high-voltage component that affects its interlock circuit continuity can trigger the code, though this is less common than external wiring issues.
A module that monitors the interlock circuit may misinterpret signals due to internal failure, but this is rare and should only be considered after all other causes are ruled out.
No. The high-voltage interlock system is a critical safety feature. A fault will typically disable the high-voltage system, preventing the vehicle from starting or operating normally. Do not attempt to bypass the interlock.
No. The PCM is rarely the cause. U3535 is almost always due to a wiring issue, loose connector, or faulty interlock switch. Replacing the PCM without thorough diagnosis is a common misstep and will not fix the underlying problem.
High-voltage systems are extremely dangerous. Only qualified technicians with proper training and equipment should diagnose or repair high-voltage interlock circuits. Incorrect handling can cause severe injury or death.
The 'L' designates a specific interlock circuit loop within the vehicle's high-voltage safety system. The exact location varies by manufacturer, so consult the service manual for the circuit's routing and components.