In modern technologies, process variations can be quite substantial, often causing design timing failures. It is essential that those errors be correctly and quickly diagnosed. Unfortunately, the resolution of the existing delay-fault diagnostic methodologies is still unsatisfactory. In this paper, we investigate the feasibility of using the circuit timing-information to guide the delay-fault diagnosis. We propose a novel and efficient diagnostic approach based on the timing window propagation (TWP) to achieve significantly better diagnostic results than those of an existing delay-fault diagnostic commercial tool. Besides locating the source of the timing errors, for each identified candidate our method determines the most probable delay defect size. The experimental results indicate that the new method diagnoses timing faults with very good resolution.