U352B: High Voltage System Interlock Circuit H Low

What it means

HVIL loop H 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 (HVIL) H monitors continuity through part of the high-voltage system. This code sets when the loop H signal falls below its expected range, which can indicate a short to ground or another fault pulling the monitored signal low.

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 scan all modules for related HVIL and communication faults before clearing anything, noting whether loop H is named.
2. Check live data for the HVIL H reading or status to confirm it remains low when the fault is active.
3. With the high-voltage system made safe using the manufacturer's procedure, inspect accessible loop H wiring for chafing, crushed insulation, or contact with ground, and check connectors for damage or corrosion.
4. Have a qualified technician isolate and test the loop H circuit for a short to ground using the manufacturer's procedure.
5. If the loop H circuit checks out, investigate gateway message routing and CAN termination against the manufacturer's specifications.

Common fixes

• Repair chafed loop H wiring that is shorting to ground.
• Repair damaged loop H connector terminals or secure a loose connection.
• Correct a confirmed gateway routing or configuration fault.
• Repair a CAN termination issue if measurements are outside the manufacturer's specification.

Can I keep driving?

A low HVIL H signal can trigger high-voltage shutdown or limp mode, so avoid driving if the vehicle shows reduced power or high-voltage warnings. If it remains driveable, limit the trip to reaching a safe location or repair facility; do not handle HV wiring yourself.