A "leads-off indicator" for an ECG apparatus for indicating that one or more of a plurality of ECG electrodes is not properly affixed to a patient and that that obviates the need for a conventional high frequency drive signal, but instead, employs common mode input noise as a drive signal to a reference electrode such that if one of the electrodes defining an ECG vector is not properly affixed, an increase in the ambient noise on an ECG vector associated with the detached electrode occurs as a detectable event. A first algorithm is used to identify whether or not the reference electrode itself is properly affixed to the patient's right leg and, if so, the common mode signal presented to the remaining limb electrodes becomes unbalanced should one of the limb electrodes not be properly connected to the patient. An impedance balancing circuit is provided for developing signals allowing identification of a lose electrode when the ECG system does not utilize a right leg electrode as a reference.