Disclosed herein are representative embodiments of methods, apparatus, and systems for performing diagnostic from signatures created during circuit testing. For example, in one exemplary method disclosed herein, a signature produced by a signature generator is received. In this embodiment, the signature corresponds to the circuit's response to no more than one test pattern. The signature is compared to entries of a fault dictionary, an entry of the fault dictionary is matched to the signature if the entry identifies a fault that explains the signature, and the fault is stored in a list of fault candidates.