U3520 indicates a fault in the high voltage interlock circuit that monitors the vehicle's safety isolation. The control module detects an abnormal voltage condition in this safety loop and stores the code.
The interlock switch monitors the HV system’s safety circuit; a failed switch or sensor can trigger U3520.
Wiring harness damage, corrosion, or loose pins in the interlock loop can cause a high voltage fault.
A malfunction in the module that manages the HV interlock can generate the fault, though this is less common.
If service was performed without properly resetting the interlock, the circuit may remain faulted.
A PCM issue can cause the code, but it is rare; the module is considered only after other components are verified.
No, in most cases the PCM is not the cause; the issue is usually a wiring fault or a defective interlock switch.
It indicates a fault in the high voltage safety circuit; the vehicle may enter limp mode and should be inspected promptly.
Yes, after the fault is resolved and the system is reset, the code will clear.
Yes, safety precautions require disconnecting the high voltage system before working on interlock components.