U3523: High Voltage System Interlock Circuit F Low

What it means

HVIL loop F voltage below 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 F verifies connection continuity through monitored high-voltage components. This code sets when the control module sees loop F voltage below its expected range, consistent with a signal being pulled low, such as by a short to ground.

Common causes

Most likely first.

1. Interlock wire shorted to ground
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 check for other interlock, low-voltage, or communication codes that may help identify whether the fault is isolated to loop F.
2. With the vehicle safely powered down per manufacturer instructions, inspect accessible loop F wiring for crushed insulation, chafing, or contact with ground; do not handle high-voltage parts.
3. Compare loop F live status with other interlock loops, if available, to confirm the low condition is specific to F.
4. Have a qualified high-voltage technician test the loop F circuit for a short to ground and verify connectors and interlock components to manufacturer specifications.
5. If circuit tests are normal, check gateway routing and CAN termination using vehicle wiring information before considering module faults.

Common fixes

• Repair loop F wiring with damaged insulation or a confirmed short to ground.
• Clean or repair corroded loop F connector terminals.
• Replace a faulty loop F interlock component after testing confirms the failure.
• Correct a gateway or CAN termination issue only if network tests identify it.

Can I keep driving?

Do not continue driving if a high-voltage warning, reduced power, or shutdown is present; the vehicle may disable propulsion or charging. Arrange inspection by a qualified high-voltage technician.