Abstract: A portable diagnostic apparatus is brought proximal to a device to be diagnosed. While the device to be diagnosed is operating, a radiofrequency interference signal emitted by the device is received at the portable apparatus and a data representation of it is stored. The data representation is submitted for matching to a diagnostic signal pattern stored in a database of predetermined signal patterns. From the matching is identified and output an indication of a fault in the device to be diagnosed. In one embodiment the submitting is to the database stored in the portable apparatus itself, and in another embodiment the submitting is outputting (data cable, wireless link) the stored data representation to a diagnostic computer such as a personal computer, a peer device, or a remote diagnostic center. The RF interference signal may be one stream and the portable apparatus receives at least a second stream and both streams are used for the matching.