U3537 indicates that the high‑voltage interlock circuit L is reading a low (ground) voltage condition. The interlock is a safety loop that verifies high‑voltage connectors are properly seated; when it detects a low the control module disables high‑voltage operation. This code is most commonly caused by a wiring or connector problem in that safety circuit rather than a failure of the computer itself.
A short to ground, open circuit, or corroded connector can pull the interlock voltage low, triggering the code.
If the interlock switch or service plug is worn, broken, or not fully engaged, the circuit will read low.
The battery pack contains its own interlock circuitry; a failure inside the pack can cause the same low‑voltage reading.
An inverter, on‑board charger, or DC‑DC converter with an internal short can affect the interlock voltage.
Rarely the PCM itself may misread the interlock signal; this is usually confirmed after all wiring and components have been verified.
It is not recommended. The high‑voltage system is disabled, so the car may run in a limited (limp) mode or not at all, and operating with a compromised HV safety loop can be unsafe.
No. The code will return as long as the low‑voltage condition in the interlock circuit remains.
No. In the vast majority of cases the problem is in the wiring, connectors, or interlock switch; PCM replacement is rarely needed.
It is a safety loop that monitors that all high‑voltage connectors are properly seated. If the loop is opened or shorted, the vehicle shuts down high‑voltage power to protect occupants and the vehicle.
Costs vary widely. Repairing a damaged wire or replacing a service plug is relatively inexpensive, whereas an internal HV battery fault can be costly; a professional diagnosis is required to determine the exact cause.