An apparatus and method of automatically measuring the lead impedance of a high energy shock lead before delivery of high energy therapy used to treat heart arrhythmia. In one example, an impedance measurement circuit measures the impedance between electrodes in a plurality of pairs of electrodes. The measured lead electrode impedance is compared to a predetermined value to detect if the lead is shorted to another lead. If a high-energy shock electrode is shorted to another lead, a shorted lead indicator is set to a fault state. Based on the state of the shorted lead indicator, a processor prevents or allows the delivery of high energy therapy. By checking for a lead short before delivery of the therapy, all of the energy of the therapy is delivered to the patient rather than being bypassed by a shorted lead connection.