U3518: High Voltage System Interlock Circuit C High

What it means

HVIL loop C 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 C is monitored as a safety circuit, and the control module checks its voltage state. This code sets when loop C voltage is above the expected range; an open loop, such as an unmated connector, is one possible cause. The fault can also interrupt communication with systems that rely on the affected module.

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 interlock and communication faults to see whether the problem is limited to loop C.
2. Check loop C live-data voltage and status against the manufacturer's specification.
3. With the high-voltage system safely disabled according to the vehicle's service procedure, inspect accessible loop C connectors for an unseated plug, damaged lock, corrosion, or broken wiring.
4. Have a qualified technician test loop C continuity and circuit behavior against the manufacturer's specification, looking for an open circuit.
5. If communication faults appear across multiple modules, check CAN termination and gateway routing using the vehicle's diagnostic procedure.

Common fixes

• Fully seat and secure a loose loop C connector after following the vehicle's safety procedure.
• Repair an open or damaged section of loop C wiring.
• Replace a damaged connector or terminal that prevents reliable contact.
• Correct a gateway or CAN termination fault if network testing identifies one.

Can I keep driving?

Do not continue driving if high-voltage warnings, reduced power, or shutdown symptoms appear; arrange safe diagnosis. An open loop C can disable high-voltage operation, and only qualified personnel should handle its connectors.