U3517: High Voltage System Interlock Circuit C Low

What it means

HVIL loop C voltage below 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 a safety circuit whose voltage the control module monitors. This code sets when loop C voltage is below the expected range, which can happen if its wire is shorted to ground. The resulting interlock fault may also disrupt communication with dependent systems.

Common causes

Most likely first.

1. Interlock wire shorted to ground
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 interlock, gateway, and CAN faults to check whether the low-voltage report is isolated to loop C.
2. Read loop C voltage and status in live data and compare them with the manufacturer's specification.
3. With the high-voltage system safely disabled according to the vehicle's service procedure, visually inspect accessible loop C wiring and connectors for chafing, crushed insulation, corrosion, or loose terminals.
4. Have a qualified technician test the loop C wire for a short to ground and verify circuit continuity using the manufacturer's procedure.
5. If other modules also report communication problems, test gateway routing and CAN termination before attributing the issue to loop C alone.

Common fixes

• Repair loop C wiring that is shorted to ground.
• Clean, secure, or replace a damaged loop C connector or terminal.
• Correct gateway routing or configuration if diagnostic results point to the gateway.
• Repair or replace a CAN terminating resistor that does not meet the manufacturer's specification.

Can I keep driving?

Avoid driving if the vehicle reports high-voltage warnings, loses power, or shuts down; have the fault diagnosed safely. Low voltage in loop C can prevent the high-voltage system from operating, and exposed high-voltage parts must not be handled.