U3521: High Voltage System Interlock Circuit F

What it means

Generic fault on HVIL loop F. 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 F monitors whether the connectors and components in that part of the high-voltage system are properly connected. The control module detects an electrical fault in loop F but this generic code does not specify whether the signal is high, low, or erratic; an incompletely seated HV connector is one possible cause.

Common causes

Most likely first.

1. HV connector not fully mated
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. Save freeze-frame data and scan all modules for related interlock and communication codes before clearing anything.
2. With the vehicle safely powered down as specified by the manufacturer, visually check accessible loop F connectors and wiring for incomplete seating, damage, or corrosion; avoid contact with high-voltage components.
3. Compare live interlock status for loop F with other loops, if the scan tool supports it, to determine whether the issue is localized.
4. Have a qualified high-voltage technician inspect and electrically test loop F and its connectors using the manufacturer's procedures and specifications.
5. If loop F tests correctly, check gateway message routing and CAN termination against the vehicle wiring information and manufacturer's specifications.

Common fixes

• Fully seat a loose loop F connector using the manufacturer's procedure.
• Repair damaged or corroded loop F wiring or connector terminals.
• Replace a failed interlock component in loop F after confirming it is faulty.
• Correct a verified gateway routing or CAN termination problem.

Can I keep driving?

If high-voltage warnings, reduced power, or shutdown behavior occur, avoid driving because propulsion or charging may be disabled. Have the vehicle inspected by a qualified high-voltage technician.