U3529: High Voltage System Interlock Circuit H

What it means

Generic fault on HVIL loop H. 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 H monitors the connection state of components and connectors in that section of the high-voltage system. This generic code identifies an electrical fault on loop H but does not specify whether its signal is high, low, or erratic; an HV connector that is not fully seated 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 other modules for related interlock and communication faults before clearing anything.
2. With the vehicle powered down according to manufacturer instructions, visually inspect accessible loop H connectors and wiring for incomplete seating, corrosion, or damage.
3. Compare live interlock status for loop H with the other loops, if available, to check whether the fault is isolated to H.
4. Have a qualified high-voltage technician inspect and test loop H connectors and circuit using the manufacturer's safety procedures and specifications.
5. If loop H checks out, investigate gateway message routing and CAN termination using vehicle-specific wiring information.

Common fixes

• Fully seat a loose loop H connector using the manufacturer's procedure.
• Repair damaged or corroded loop H wiring and connector terminals.
• Replace a loop H interlock component confirmed to be faulty.
• Correct gateway routing or CAN termination if network testing verifies the problem.

Can I keep driving?

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