U353B: High Voltage System Interlock Circuit M Low

What it means

HVIL loop M 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

HVIL loop M is a low-voltage safety circuit that verifies high-voltage connectors and covers are secure. The control module sets U353B when loop M voltage is below its expected range, which can happen if its signal wire is shorted to ground. A gateway or CAN issue may affect related communication, but the low loop voltage should be checked directly.

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. Capture freeze-frame data and scan all modules for U353B and related communication faults before clearing codes.
2. Review live data for loop M voltage or state, if supported, and confirm whether it reads low while the fault is active.
3. With the vehicle safely powered down under the manufacturer's high-voltage procedure, inspect loop M low-voltage wiring and connectors for grounded, crushed, corroded, or damaged sections.
4. Have a qualified technician test the loop M signal circuit for a short to ground and check continuity against the manufacturer's procedure.
5. If the loop M circuit passes, investigate gateway routing or CAN termination only if other codes indicate a network fault.

Common fixes

• Repair loop M signal wiring that is shorted to ground.
• Clean or replace corroded loop M terminals causing a low signal.
• Replace a confirmed faulty loop M interlock component.
• Correct a verified gateway or CAN network fault if the loop M circuit tests normally.

Can I keep driving?

Stop driving when safe if a high-voltage warning, limp mode, or shutdown occurs; the fault can disable propulsion. Do not touch high-voltage components, and arrange prompt diagnosis.