A method of testing a plurality of repeaters arrayed along a transmission line of a pulse code modulation system, and of locating any faulty repeaters, both testing and fault location being performed automatically. Pulse train groups, each containing a frequency component corresponding to a supervisory signal frequency of one of the repeaters, are successively applied from one terminal of the transmission line. Each pulse train group comprises a plurality of subgroup pulse trains, each of which contains the supervisory frequency component of the pulse train group to which it belongs, with the density of a pulse train in successive subgroups being successively increased. Any faulty repeater is located by determining the pulse train density at which a supervisory signal produced by the repeater fails to increase by a predetermined amount for a known change in pulse train density.