U352A: High Voltage System Interlock Circuit H Performance

What it means

HVIL loop H 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 (HVIL) H lets the vehicle monitor continuity through part of the high-voltage system so it can detect an unsafe or interrupted connection. This performance code sets when the loop's signal timing or pattern is inconsistent with what the control module expects, even if it is not simply too low or too high.

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 communication and HVIL faults to see whether the problem is isolated to loop H or part of a wider network issue.
2. Use live data to watch the HVIL H status while the fault is present and note whether it changes intermittently.
3. With the high-voltage system made safe according to the vehicle maker's procedure, inspect accessible loop H connectors and wiring for looseness, damage, corrosion, or signs of intermittent contact.
4. Have a qualified technician test loop H continuity and signal behavior using the manufacturer's procedure, checking for changes when the harness or connector is gently moved.
5. If loop H tests correctly, check gateway message routing and CAN network termination against the manufacturer's specifications.

Common fixes

• Reseat a loose HV connector in loop H using the manufacturer's safety procedure.
• Repair damaged or intermittently contacting loop H wiring or connector terminals.
• Correct gateway configuration or routing faults if confirmed by network diagnosis.
• Repair or replace a CAN termination fault if testing shows it is outside the manufacturer's specification.

Can I keep driving?

Because an HVIL fault can cause high-voltage functions to shut down or the vehicle to enter limp mode, avoid relying on the vehicle for normal or long-distance driving. Stop safely if power is reduced or warnings appear, and have the high-voltage system checked by a qualified technician.