- Self-diagnostic method of open-circuit faults in power switching blocks of a modular inverter, wherein the modular inverter comprises a plurality (k=1.. n) of n power switching blocks (31a, 32a, 33a, 34a, 31b, 32b, 33b, 34b, 31c, 32c, 33c, 34c) paralleled in a phase leg (5, 5a, 5b, 5c), and being provided with temperature sensors, characterized in that said method comprises a first loop comprising: - sampling and storing temperature data T k p of said n blocks (k=1.. n) in a phase leg measured by the said temperature sensors at relevant sampling times t SD with a relevant sampling interval Δt SD; - at least one step of comparing current temperature data T k p of a block k with current average temperature data ( 1 n − 1 ∑ i = 1 n T i p) of all other blocks of said phase leg at sampling time t SD P, considering said block k as a faulty block kf and raising a fault flag OF on said faulty block kf if the following condition is not fulfilled: T k p ∈ 1 n − 1 ∑ i = 1 n T i p ± Th AV, i ≠ k where Th AV is a predefined average temperature deviation threshold, wherein, said blocks comprising a top switch and a bottom switch (1a, 1b) and shoot-through sensing means for each of said switches, said self-diagnostic method comprises further identifying and localizing the open-circuit fault through triggering a shoot-through test by driving with a drive circuit (7a, 7b) of the said block both the top switch and the bottom switch (1a, 1b) of the block, on which a faulty flag OF is raised, to the on-state for a duration longer than the commutation time of the switches (1a, 1b), but within the safe operating area of said switches, and, testing presence of shoot-through current and raising a confirmed open circuit fault flag OFC on said one of said blocks in case no shoot-through current is detected and wherein in case shoot through current is detected when testing presence of shoot-through current, other diagnostic procedures are initiated. - sampling and storing temperature data T k p of said n blocks (k=1.. n) in a phase leg measured by the said temperature sensors at relevant sampling times t SD with a relevant sampling interval Δt SD; - at least one step of comparing current temperature data T k p of a block k with current average temperature data ( 1 n − 1 ∑ i = 1 n T i p) of all other blocks of said phase leg at sampling time t SD P, considering said block k as a faulty block kf and raising a fault flag OF on said faulty block kf if the following condition is not fulfilled: T k p ∈ 1 n − 1 ∑ i = 1 n T i p ± Th AV, i ≠ k where Th AV is a predefined average temperature deviation threshold, wherein, said blocks comprising a top switch and a bottom switch (1a, 1b) and shoot-through sensing means for each of said switches, said self-diagnostic method comprises further identifying and localizing the open-circuit fault through triggering a shoot-through test by driving with a drive circuit (7a, 7b) of the said block both the top switch and the bottom switch (1a, 1b) of the block, on which a faulty flag OF is raised, to the on-state for a duration longer than the commutation time of the switches (1a, 1b), but within the safe operating area of said switches, and, testing presence of shoot-through current and raising a confirmed open circuit fault flag OFC on said one of said blocks in case no shoot-through current is detected and wherein in case shoot through current is detected when testing presence of shoot-through current, other diagnostic procedures are initiated.