U3520: High Voltage System Interlock Circuit E High

What it means

HVIL loop E voltage above expected range. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

The high-voltage interlock loop E is a safety monitoring circuit that lets the vehicle detect whether covered high-voltage connectors and components are securely connected. The control module monitors the loop's electrical state; this code sets when loop E reads higher than its expected range, which can happen if the loop is open.

Common causes

Most likely first.

1. Interlock loop open
2. Gateway module routing CAN messages incorrectly
3. CAN terminating resistor (120 Ω) drifted out of spec

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Use a scan tool to save freeze-frame data and check whether U3520 is current or intermittent, alongside any other high-voltage or communication codes.
2. With the vehicle powered down according to its owner or service instructions, visually inspect accessible loop E connectors and wiring for a loose connection, damage, or corrosion; do not touch exposed high-voltage parts.
3. Use scan-tool data, if available, to compare loop E status with the other interlock loops and see whether the fault is isolated to E.
4. Have a qualified high-voltage technician test loop E continuity and connector seating using the manufacturer's safety procedure and specifications.
5. If loop E checks out, investigate gateway routing and CAN network integrity, including termination, against the vehicle's wiring information and manufacturer's specifications.

Common fixes

• Secure a loose loop E connector using the manufacturer's procedure.
• Repair damaged or corroded accessible loop E wiring or connector terminals.
• Replace a confirmed faulty interlock component in loop E.
• Repair gateway routing or CAN termination faults only after circuit testing confirms them.

Can I keep driving?

Avoid driving if the vehicle shows high-voltage warnings, reduced power, or shutdown behavior; the system may disable propulsion or charging. If the warning is present, arrange inspection by a qualified high-voltage technician.