A study of fault collapsing for synchronous sequential circuits is presented. Two phenomena, self-hiding and delay-reconvergence, which invalidate the combinational fault dominance relationship in sequential circuits are identified. These phenomena are caused by the existence of feedback paths and storage elements in sequential circuits. From this analysis, a single-fault fault-collapsing procedure for synchronous irredundant sequential circuits is proposed to reduce the faults for which test has to be generated. This procedure can be applied not only to a nonscan mode circuit, but also to a full-scan and a partial-scan mode circuit by cutting the inputs and outputs of scannable D flip-flops as the primary outputs and inputs of the circuit, respectively. This procedure has been applied to collapse faults for the 31 benchmark sequential circuits, and a 57% reduction in the number of faults as compared with the total number of original faults has been obtained.