A diagnostic monitoring system and method is employed for one or more vapor compression systems such as air conditioners and heat pumps having a compressor, an indoor air handler fan coil and an outdoor condensor. Temperature, voltage and current sensors are provided at the outdoor condensor to determine that at least one vapor compression system is operating properly. Data obtained from the sensors is wirelessly transmitted to a receiving-device for use by the systems' custodian or repair service provider and includes information concerning an occurrence of periods when one or more of the vapor compression systems are operating at an abnormal state.