This paper presents a user-defined fault model (UDFM) that accounts for silicon behaviors that cannot be explained using traditional stuck-at and transition delay fault models. The new model targets cell-internal faults but does not require time-intensive SPICE simulations because it operates at the logic level. As added benefit, error logs collected using UDFM patterns (instead of traditional models) can be used to generate diagnostic callouts with improved resolution. A workflow that effectively achieves this is presented in the paper along with three case studies that demonstrate the usefulness of the proposed method.