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DTC codes / U35xx · Network communication / U3516

U3516High Voltage System Interlock Circuit C Performance

U3516 is a manufacturer-specific body code indicating that the high-voltage system interlock circuit C has reported a performance fault, typically an unexpected voltage level or continuity issue. The interlock is a safety loop that confirms high-voltage connectors and covers are properly seated before the vehicle allows the traction battery to energize, and the control module sets this code when the circuit's voltage or resistance falls outside the expected range.

SystemNetwork
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaNetwork communication

Symptoms you may notice

  • Hybrid or electric vehicle may refuse to start or enter Ready mode
  • Instrument cluster displays a high-voltage system warning or service vehicle soon message
  • Traction battery contactors may not close, leaving the vehicle without electric propulsion
  • Often no symptoms besides the warning light if the fault is intermittent
  • Charging may be disabled or interrupted on plug-in models
  • Reduced power or limp-home mode may be triggered as a protective strategy

Common causes, most likely first

  1. A high-voltage connector or service disconnect plug not fully latched, allowing the interlock pins to lose contact
  2. Corrosion, water intrusion, or terminal fretting inside an interlock connector
  3. Damaged or pinched interlock wiring harness between the high-voltage components
  4. Failed interlock switch or sensor integrated into a high-voltage component cover
  5. Faulty high-voltage control module or battery energy control module internal driver
  6. Aftermarket or improperly installed high-voltage components disturbing the interlock loop
  7. Loose or backed-out terminal in the interlock circuit connector

How to diagnose and fix it

  1. Inspect all high-voltage connectors and service disconnect plugs for full engagement and secure latching
  2. Check the interlock circuit wiring for chafing, pinching, or corrosion, especially near component covers
  3. Measure interlock circuit voltage and continuity at the control module connector and compare to manufacturer specifications
  4. Perform a wiggle test on connectors and harness while monitoring interlock status on a scan tool
  5. Verify the interlock switch inside the suspect high-voltage component with a multimeter
  6. Repair or replace damaged terminals, connectors, or harness sections as needed
  7. Clear the code and perform a road test or key cycle to confirm the interlock circuit performs correctly

Interlock faults that mimic component failures

  • A partially latched high-voltage connector is the most common cause of this code, and it often happens after service work that did not fully reseat the plug.
  • The interlock circuit is a low-voltage safety loop, so a failure does not necessarily mean the high-voltage battery or inverter is defective.
  • Water intrusion into a connector can create intermittent interlock faults that only appear in damp conditions or after a car wash.
  • Some vehicles set U3516 when the 12-volt auxiliary battery is weak because the control module misreads interlock voltage during low-voltage conditions.
  • Technicians sometimes replace an expensive high-voltage component when the real fault is a chafed interlock wire behind a cover or in a harness bend.

Can I keep driving with U3516?

Do not continue driving with an active high-voltage interlock fault, because the vehicle may disable the traction battery or shut down unexpectedly. Have the vehicle inspected by a qualified high-voltage technician before attempting further operation.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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