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.　©　2019　EPE　Association.