Abstract: A method and apparatus for isolating malfunctions occurring in a communication switching system having a line switching network for establishing communications selectively between calling and called lines under the control of common equipment including a plurality of registers for storing temporarily call processing information received from the calling lines via a plurality of register junctors, a junctor switching network connecting selectively a pool of sending units and receiving units to the junctors, the common equipment including memory, includes establishing a connection between a register junctor and a sender-receiver unit for test purposes, connecting testing circuits to the register junctor via the line switching network, testing the register junctor circuits to determine whether or not they can extend a path from the line switching network to the junctor switching network, testing the sender-receiver unit by means of the testing circuits via the path between the junctor and the sender-receiver uni