The modular multilevel series-parallel converter (MMSPC) was shown to provide advantages in several industrial applications, but the many individual switches may challenge its reliability. This paper proposes an effective fault diagnosis method (FDM) based on temporary hot-swapping of the look-up table of the scheduler and the use of additional interconnection states to detect and locate transistor open-circuit faults. First, the feature table of all possible switch open-circuit faults is collected via temporarily reconfiguring the scheduler. The converter can intermittently switch to the fault-diagnosis mode for only a few cycles. Features as simple as the square of the error between the output and reference in the fault-diagnosis mode indicate a switch open-circuit fault. Finally, the output voltage characteristics in the fault diagnosis mode are checked and compared with those in the aforesaid characteristic tables to locate the open switch. Validation results reveal that this proposed fault diagnosis method is efficient, accurate, and fast.