It is now generally accepted that the stuck-at fault model is no longer sufficient for many manufacturing test activities. Consequently, diagnostic test pattern generation based solely on distinguishing stuck-at faults is unlikely to achieve the resolution required for emerging fault types. In this work we describe a new diagnostic ATPG implementation that uses a generalized fault model. It can be easily used in any diagnosis framework to refine diagnostic resolution for complex defects. For various types of faults that include, for example, bridge, transition, and transistor stuck-open, we show that diagnostic resolution can be significantly enhanced over a traditional diagnostic test set aimed only at stuck-at faults. Finally, we illustrate the use of our diagnostic ATPG to distinguish faults derived from a state-of-the-art diagnosis flow based on layout.