An external unit for an implantable neuro stimulator system and a method of operating the same, comprising an external coil inductively couplable to an implant coil of an implantable neural stimulator unit when in close proximity to each other. An RF-signal generating module for generating and outputting an RF-signal to the coil for inductive power and/or data transfer to the implantable neural stimulator unit, a measurement unit connected to the coil for measurement of an electrical parameter at the coil and a controller connected to the measurement unit and comprising a comparator and a storage for storing the electrical parameter. The controller is configured to operate at least in normal operating and corrective action mode, and when in the normal operating mode, to repeatedly measure the electrical parameter with the measurement unit, determine with the comparator a change of the measured electrical parameter in relation to the stored electrical parameter switch the controller into a corrective action mode when the change exceeds a threshold； and otherwise modify the stored electrical parameter by the measured electrical parameter.
