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U3518 — High Voltage System Interlock Circuit C High

Manufacturer-specificNetwork / communication

This code indicates a fault in the high voltage interlock circuit, which is a safety system designed to detect if any high voltage connectors are loose or disconnected. A 'High' status typically means the circuit has lost continuity or shorted to voltage, causing the vehicle to disable the high voltage battery to prevent electrical hazards.

Symptoms

What causes it — most likely first

1. Disconnected or loose high voltage connector

Often caused by a connector that was not fully seated during previous service or maintenance, breaking the interlock loop.

2. Faulty high voltage service disconnect

The manual service disconnect plug may not be fully installed, or its internal interlock contacts may be damaged or corroded.

3. Wiring harness damage

Chafed, broken, or rodent-damaged wiring in the low-voltage interlock loop causing an open circuit or short to voltage.

4. Failed high voltage component internal interlock

A degraded connection inside a high voltage component, such as the battery, inverter, or drive motor, causing an open circuit.

5. Faulty HV control module (PCM/ECM)

An internal failure in the module monitoring the interlock circuit, which is very rare and should only be considered after all physical connections and wiring are verified.

How it's diagnosed

  1. Scan the vehicle for additional high voltage or communication codes that may point to a specific component or area.
  2. Safely depower and isolate the high voltage system following the manufacturer's strict procedures before any physical inspection.
  3. Visually inspect all high voltage connectors and the manual service disconnect to ensure they are fully seated, locked, and free of corrosion.
  4. Perform a continuity test on the interlock loop wiring between the control module and the connectors to locate any open circuits.
  5. Check for shorts to power or ground in the interlock circuit wiring.
  6. If all wiring and connectors test normal, monitor the interlock circuit parameters with a scan tool while manipulating the harness to check for intermittent faults.

Code facts & related codes

SystemNetwork / communication
StandardizationManufacturer-specific (P1/B1/C1/U1)

Frequently asked

Can I drive my vehicle with code U3518?

It is not recommended. The vehicle will likely disable the high voltage system, which may prevent the car from starting or cause it to run poorly in a reduced-power limp mode.

What does the high voltage interlock circuit do?

It is a continuous safety loop that verifies all high voltage connectors are securely attached. If a connector comes loose, the circuit breaks, and the system immediately shuts off high voltage to prevent electrocution or arcing.

Will replacing the PCM or ECM fix this code?

No, replacing the computer is rarely the correct fix. This code is almost always caused by a physically loose connector, an unplugged service plug, or damaged wiring in the interlock loop. Replacing the control module without fixing the physical circuit will not resolve the issue.

Is it safe to inspect the high voltage connectors myself?

Only if you follow the manufacturer's procedure to depower and isolate the high voltage system first, and wear appropriate personal protective equipment. If you are not trained in high voltage safety, leave this diagnosis to a qualified technician.