Damaged or corroded high-voltage interlock connector, particularly at the battery pack, inverter, or motor; Broken or chafed interlock wire within the harness; Failed high-voltage component that has an internal interlock switch (e.g., service disconnect, battery pack); Water intrusion or contamination in a high-voltage connector causing increased resistance; Faulty high-voltage control module or interlock detection circuit; Aftermarket accessories or improper previous repairs that disturbed the interlock loop

DTC codes / U35xx · Network communication / U3518

U3518High Voltage System Interlock Circuit C High

Q: What does code U3518 mean?

High Voltage System Interlock Circuit C High. U3518 indicates that the high-voltage interlock loop 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 safety loop that runs through high-voltage connectors and components; when its voltage goes high, the module sets this code to prevent operation of the high-voltage system.

U3518 indicates that the high-voltage interlock loop 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 safety loop that runs through high-voltage connectors and components; when its voltage goes high, the module sets this code to prevent operation of the high-voltage system.

SystemNetwork

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaNetwork communication

Symptoms you may notice

High-voltage system warning light illuminated on the dashboard
Vehicle may refuse to start or enter 'Ready' mode
Reduced power or limp mode, often limiting speed
High-voltage components such as the air conditioning compressor or cabin heater may stop working
Often no symptoms besides the warning light, especially if the fault is intermittent

Common causes, most likely first

Damaged or corroded high-voltage interlock connector, particularly at the battery pack, inverter, or motor
Broken or chafed interlock wire within the harness
Failed high-voltage component that has an internal interlock switch (e.g., service disconnect, battery pack)
Water intrusion or contamination in a high-voltage connector causing increased resistance
Faulty high-voltage control module or interlock detection circuit
Aftermarket accessories or improper previous repairs that disturbed the interlock loop

How to diagnose and fix it

Perform a visual inspection of all high-voltage connectors and wiring for damage, corrosion, or loose connections
Use a scan tool to read live data for the interlock circuit voltage and compare to specifications
Check the service disconnect plug for proper seating and continuity
Measure resistance across the interlock loop at each connector to isolate an open or high-resistance section
Inspect for water intrusion or debris in connectors and clean or dry as needed
Replace any damaged connector, terminal, or interlock wire
If the loop is intact, test the high-voltage control module and replace if faulty

Interlock Circuit High Voltage Isn't Always a Short

A high voltage reading on the interlock circuit often results from increased resistance in a connector rather than a direct short to power, so don't assume a wiring short without testing.
The interlock loop is a safety circuit, not the high-voltage power circuit itself; a fault here can disable the vehicle even if the high-voltage battery is fully charged.
Some vehicles require the high-voltage system to be powered down and the service disconnect removed before inspecting interlock connectors, so follow the manufacturer's procedures exactly.
Intermittent U3518 codes are frequently caused by a partially unseated connector that passes a static test but fails under vibration or temperature changes.
After replacing a high-voltage component, the interlock loop may need to be reset or the code cleared with a scan tool before the vehicle will start.

Can I keep driving with U3518?

Do not drive the vehicle with an active U3518 code, as the high-voltage system may be disabled or operate unpredictably, posing a safety risk. Have the vehicle inspected by a qualified high-voltage technician before attempting to move it under its own power.

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.