U351C: High Voltage System Interlock Circuit D High

What it means

HVIL loop D voltage above 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 D is monitored as a safety circuit, and the control module checks its voltage state. This code sets when loop D voltage rises above the expected range; an open loop, such as an unmated connector, is one possible cause. Communication with dependent systems may also be affected.

Common causes

Most likely first.

1. Interlock loop open (connector unmated)
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. Capture freeze-frame data and scan all modules for loop D interlock and communication faults to see whether the issue is confined to this circuit.
2. Check loop D voltage and status in live data against the manufacturer's specification.
3. With the high-voltage system safely disabled according to the vehicle's service procedure, inspect accessible loop D connectors for an unmated plug, damaged lock, corrosion, or broken wiring.
4. Have a qualified technician test loop D continuity and circuit behavior against the manufacturer's specification, looking for an open circuit.
5. If several modules report communication faults, check CAN termination and gateway routing using the vehicle's diagnostic procedure.

Common fixes

• Fully seat and secure a loose loop D connector after following the vehicle's safety procedure.
• Repair open or damaged loop D wiring.
• Replace a damaged loop D connector or terminal that cannot maintain contact.
• Correct gateway routing or repair CAN termination if network testing confirms a fault.

Can I keep driving?

Avoid driving if high-voltage warnings, reduced power, or shutdown symptoms appear; arrange safe diagnosis. An open loop D can disable high-voltage functions, and its connectors should only be handled by qualified personnel.