Loose, partly seated or damaged high voltage connector on the interlock loop; Interlock wiring shorted to a voltage supply or chafed against other wiring; Corroded or bent interlock terminals, or water ingress in a connector; Service disconnect, cover or component lid not fitted correctly after recent work or an accident; Faulty component containing the interlock switch or bridge, such as an inverter, charger or battery pack junction; Fault inside the controlling module or outdated software.

U3520

High Voltage System Interlock Circuit E High.

Generic
Network
Generic code
High severity

How this code is built

U: SystemNetwork
3: Defined bySAE, the same on every car
5: AreaNetwork area 5
20: FaultThe specific fault in that area

What it means

U3520 means a control unit has seen a high signal on high voltage interlock circuit E. The interlock is a low-voltage safety loop that runs through the high voltage connectors, covers and service disconnect, and it tells the system that the high voltage circuit is closed and safe. A high reading means the voltage on this loop is above what the control unit expects. Which parts sit on loop E, and whether 'high' means a short to voltage or an open loop, depends on the vehicle, so check the manufacturer's wiring information.

Symptoms

High voltage system warning light or message on the dash
Vehicle will not go into READY mode or will not start
Loss of drive power or limp mode
High voltage contactors open and the system shuts down
Charging not available or stops part-way
Other communication or high voltage system fault codes stored alongside

Possible causes

Loose, partly seated or damaged high voltage connector on the interlock looplikely
Interlock wiring shorted to a voltage supply or chafed against other wiringlikely
Corroded or bent interlock terminals, or water ingress in a connectorpossible
Service disconnect, cover or component lid not fitted correctly after recent work or an accidentpossible
Faulty component containing the interlock switch or bridge, such as an inverter, charger or battery pack junctionpossible
Fault inside the controlling module or outdated softwarerare

How to diagnose

Read all stored codes and freeze frame data, noting whether the fault is current or historic and what other high voltage or communication codes are present.
Ask about recent repairs, accident damage, battery work or water exposure, since these commonly disturb interlock connections.
Make sure the high voltage system is properly isolated by a suitably trained person before touching any high voltage component or connector.
Visually inspect the service disconnect, covers and connectors on loop E for correct seating, locking tabs, damage and signs of water or corrosion.
Inspect the low-voltage interlock wiring for chafing, pin damage, pushed-back terminals and contamination.
Using the manufacturer's wiring diagram, test loop E for continuity and for a short to voltage with the system isolated, then check the expected signal voltage.
Check the controlling module's power supply, earths and connector condition.
Check for software updates or technical bulletins relating to this code on the specific model.

How to fix

Refit and securely lock any loose connector, cover or service disconnect on the loop.
Repair or replace damaged interlock wiring, especially where there is a short to voltage.
Clean or replace corroded or damaged connector terminals and dry out any water ingress, finding the source of the water.
Replace the component or harness section that holds a faulty interlock switch or bridge, once testing confirms it.
Update the software, or replace the control module, only if the manufacturer's procedure shows it to be at fault.

Can I keep driving?

High severity

Do not keep driving with this code if the car shows a high voltage warning or loses drive power, as the system may shut down without warning. Have it inspected by a technician trained in high voltage systems before further use.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.