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DTC codes / U35xx · Network communication / U3515

U3515High Voltage System Interlock Circuit C

U3515 indicates that the high-voltage interlock circuit C has reported a voltage above the expected range, which the vehicle's control module interprets as a potential safety hazard. The interlock is a low-voltage loop that runs through high-voltage connectors and covers; when it is broken or shorted, the system disables the high-voltage contactors to prevent electric shock.

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

Symptoms you may notice

  • High-voltage system warning light or message displayed on the dashboard
  • Vehicle may refuse to start or enter 'Ready' mode, leaving you stranded
  • Loss of electric propulsion or reduced power in hybrid/electric vehicles
  • Charging system may be disabled, preventing the high-voltage battery from recharging
  • Often no symptoms besides the warning light if the fault is intermittent
  • Possible audible click or no sound from high-voltage contactors when attempting to start

Common causes, most likely first

  1. Damaged or corroded high-voltage interlock connector pins or terminals
  2. Broken or chafed interlock wire within the high-voltage cable harness
  3. Faulty high-voltage component cover or service disconnect plug not fully seated
  4. Internal failure of a high-voltage component (e.g., battery pack, inverter, motor) affecting the interlock loop
  5. Water intrusion or contamination in a high-voltage connector causing a short to voltage
  6. Aftermarket accessories or improper previous repairs disturbing the interlock circuit
  7. Failed high-voltage control module or interlock detection circuit inside the module

How to diagnose and fix it

  1. Perform a visual inspection of all high-voltage connectors, covers, and service disconnects for damage, corrosion, or loose fitment
  2. Use a scan tool to read live data and check for other high-voltage system codes that may pinpoint the affected circuit
  3. Check the interlock circuit continuity and resistance with a multimeter, following the manufacturer's service information for pinouts and expected values
  4. Inspect the wiring harness for chafing, pinches, or breaks, especially near moving components or sharp edges
  5. Test the interlock loop at each high-voltage component by disconnecting and checking for the presence of the loop signal
  6. If a component is suspected, substitute with a known-good unit only if safe and per procedures, or perform pinpoint tests as directed
  7. Repair or replace the faulty connector, wire, or component, then clear codes and verify the interlock circuit operates correctly

Interlock circuit faults often misdiagnosed

  • A common mistake is replacing the high-voltage battery pack when the actual fault is a simple unseated service disconnect plug or a corroded interlock pin in a connector.
  • The interlock circuit is a low-voltage loop, so a high-voltage insulation tester is not needed; a standard multimeter is sufficient for most continuity checks.
  • Aftermarket alarms or remote starters that tap into wiring near high-voltage components can inadvertently disturb the interlock circuit and set this code.
  • Some vehicles set U3515 when the 12-volt battery is weak or failing, as the interlock detection circuit may not receive stable voltage, so always check the 12-volt system first.
  • The code can be stored intermittently due to vibration or temperature changes, so a wiggle test of connectors and harnesses while monitoring live data is a valuable diagnostic step.

Can I keep driving with U3515?

Do not drive the vehicle if the high-voltage system warning is active, as the vehicle may lose propulsion unexpectedly or disable safety systems. If the light is on but the vehicle drives normally, limit driving to a safe location for diagnosis and avoid high-voltage component access.

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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