U353A: High Voltage System Interlock Circuit M Performance

What it means

HVIL loop M 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

HVIL loop M is a low-voltage safety circuit that confirms high-voltage connections and covers are secure. U353A means the loop M signal's timing or pattern is outside the expected behavior, often because contact at a connector is intermittent. Gateway and CAN faults may affect related reports, so check whether other network codes are present.

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 U353A, other HVIL codes, and CAN communication faults.
2. Monitor loop M status in live data, if available, and look for brief changes that coincide with the fault.
3. With the vehicle powered down according to the manufacturer's high-voltage safety procedure, inspect loop M connectors and wiring for loose fit, corrosion, chafing, or movement-related damage.
4. Have a qualified technician test loop M continuity and signal stability using the manufacturer's procedure for intermittent faults.
5. If loop M remains stable during circuit checks, investigate gateway routing and CAN termination for corroborating network faults.

Common fixes

• Reseat a loose loop M interlock connector.
• Repair loop M wiring or terminals with intermittent contact.
• Replace a loop M connector or interlock component confirmed to have unstable contact.
• Repair verified gateway or CAN network faults if loop M tests correctly.

Can I keep driving?

Intermittent loop M faults can still trigger limp mode or a high-voltage shutdown. Avoid driving if warnings or reduced power occur, and leave high-voltage connector work to a qualified technician.