SSCFI is a rulebased expert system that diagnoses problems in a wide variety of "special circuits," that is, telephone circuits other than regular switched business and residential lines. Special circuits are significantly more complex than regular circuits, and hence more difficult to diagnose. SSCFI diagnoses problems by recursively partitioning the circuit until the responsible fault is isolated. SSCFI selects which circuit to work on, reads its design, selects and initiates analog and digital tests via remotely-activated test equipment, interprets the results of each test in the context of the circuit design, and when done, writes out a detailed description of the problem found and routes it to the party responsible for its repair. SSCFI is entirely autonomous in operation. SSCFI has become an essential element of GTE’s special circuit maintenance operations. It has been in service since 1991 and has been in operation at all GTE’s US sites since 1994. SSCFI testing saves millions of dollars annually and significantly improves the uniformity of testing and quality of the resulting diagnoses. This paper discusses the domain, architecture, and development of the SSCFI system, and the key factors and techniques that made it successful. Lastly, two current projects building on SSCFI’s expertise are discussed -- interactive test assistance and automatic design database cleanup.