Path delay fault models are introduced to diagnose failures that cause sequential circuits malfunction and violate timing specifications. For delay faults tests, a pair of test vectors is used for each path to be tested. Diagnosis of failure in asynchronous circuits is more important since the delay faults can change the function of the circuit. .However, asynchronous circuits and their faults are yet to be understood. The purpose of this study is to first identify whether the existing paths will accurately measure delays. Furthermore, a new definition for paths is introduced, which leads to a new category of path delay faults. This new category is different from previously proposed categories by suggesting simultaneous evaluation of pair vectors. Finally, the experimental results for SIS benchmark circuits are shown and the fault coverage is calculated for each circuit.