U353C: High Voltage System Interlock Circuit M High

What it means

HVIL loop M 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

HVIL loop M is a low-voltage safety circuit that checks the secure state of high-voltage connectors and covers. U353C sets when loop M voltage rises above the expected range, commonly when the loop is open. Check the loop M circuit first and compare any gateway or CAN communication codes before concluding the network is at fault.

Common causes

Most likely first.

1. Interlock loop open
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. Save freeze-frame data and scan all modules for U353C, related HVIL codes, and communication faults.
2. Check live data for loop M status or voltage, if available, and see whether the signal stays high while the fault is present.
3. With the vehicle powered down according to the manufacturer's high-voltage safety procedure, inspect loop M connectors and low-voltage wiring for an open, loose plug, or broken terminal.
4. Have a qualified technician test loop M continuity and signal voltage using the manufacturer's diagnostic procedure.
5. If loop M circuit checks pass, investigate gateway routing and CAN termination where other network evidence supports those checks.

Common fixes

• Reconnect a loose loop M interlock connector.
• Repair an open loop M wire or damaged terminal.
• Replace a loop M interlock connector or component confirmed to be faulty.
• Repair verified gateway or CAN termination faults if loop M wiring is sound.

Can I keep driving?

Avoid driving if a high-voltage warning appears, power is reduced, or the vehicle shuts down; this fault can cause the interlock to disable propulsion. Do not handle high-voltage connectors yourself.