An ECG system and a method for operating a handheld device that provides input to an ECG system are disclosed. The handheld device has a plurality of receiving channels. Each receiving channel includes an electrode that is adapted for receiving electrical signals from a patient's body when the electrode is pressed against the patient's body at a predetermined location on the patient's body. The method includes monitoring an output signal from each of the channels for any of a plurality of invalid signal conditions during a period of time in which the output signal is used to generate a standard lead or precordial lead trace, signaling a user that the handheld device is improperly positioned on the patient's body, and instructing the user on how to correct a placement of the handheld device based on the detected invalid signal condition.