U3516: High Voltage System Interlock Circuit C Performance

What it means

HVIL loop C timing/pattern does not match expected behavior. 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 monitors the continuity and behavior of safety interlocks around high-voltage components. The control module compares loop C's signal pattern over time with the expected pattern; this code sets when the pattern or timing is incorrect, often intermittently. A fault may also interrupt communication with systems that depend on the affected module.

Common causes

Most likely first.

1. Intermittent contact in HV connector
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 all modules for related interlock, gateway, and CAN communication faults to see whether loop C is the only issue.
2. Use live data to observe loop C status while the vehicle is safely stationary and note whether it changes or drops out.
3. With the high-voltage system safely disabled according to the vehicle's service procedure, inspect accessible loop C connectors and wiring for looseness, corrosion, damage, or signs of intermittent contact.
4. Have a qualified technician test loop C continuity and signal behavior against the manufacturer's specification, including while checking for an intermittent connection.
5. If several modules report communication faults, check gateway routing and CAN network termination against the manufacturer's diagnostic procedure.

Common fixes

• Secure or repair a loose, corroded, or damaged loop C connector.
• Repair damaged loop C wiring or an intermittent connection.
• Correct gateway configuration or routing if testing identifies a gateway fault.
• Repair or replace a CAN terminating resistor if it fails the manufacturer's specification.

Can I keep driving?

Treat this as a high-voltage safety fault: avoid driving if the vehicle shows reduced power, shutdown warnings, or unstable operation, and arrange safe diagnosis. A loop C fault can disable high-voltage functions or leave the vehicle in limp mode; do not handle high-voltage connectors yourself.