U3519: High Voltage System Interlock Circuit D

What it means

Generic fault on HVIL loop D. 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 D links safety interlocks around high-voltage components. The control module monitors loop D for a general fault, without this code specifying a low, high, or performance condition; a connector that is not fully mated is one possible cause. Communication faults may also affect dependent functions.

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. Record freeze-frame data and scan every module for interlock, gateway, and CAN faults to identify whether loop D is the common issue.
2. Check live data for loop D status and note whether the interlock state is reported as open or changes unexpectedly.
3. With the high-voltage system safely disabled according to the vehicle's service procedure, inspect accessible loop D connectors and wiring for incomplete seating, corrosion, damage, or loose terminals.
4. Have a qualified technician test loop D continuity and signal behavior against the manufacturer's specification.
5. If communication faults involve multiple modules, check gateway message routing and CAN termination using the vehicle's diagnostic procedure.

Common fixes

• Secure a loop D connector that is not fully seated.
• Repair damaged or corroded loop D wiring and terminals.
• Replace a faulty loop D connector if it cannot maintain a reliable connection.
• Correct gateway routing or repair CAN termination if network tests identify those faults.

Can I keep driving?

Avoid driving if the vehicle has high-voltage warnings, limp mode, or shutdown symptoms; arrange safe diagnosis. A loop D fault can disable high-voltage functions, and high-voltage connectors should only be handled by qualified personnel.