A high-voltage system for a vehicle includes a module having a resonant circuit coupled across powerlines, and a controller. The controller is programmed to, during a power up of the high-voltage system, propagate on the powerlines a first signal at a fundamental frequency for a first period. The controller is further programmed to, in response to a first impedance measured across the powerlines at the fundamental frequency during the period being less than a predetermined value, close main contactors to energize the module.