Method for assisting with diagnosis of a complex system comprising a plurality of mechanical, electric, electronic and/or software components connected to each other, each of the components among a portion of the components being under automatic monitoring and associated with one or more fault sensors, a portion of the components not being under automatic monitoring, the method, implemented by a processor, comprising: - receiving a file containing a modeling of some or all of the complex system by a fault tree defining some or all of the combinations of events causing a top event associated with a failure of the complex system, the fault tree comprising a plurality of intermediate events and a plurality of basic events, - assigning an a priori probability of occurrence value to each of the basic events of the fault tree, in order to generate an augmented fault tree, - assigning failure information to each of the basic events and each of the intermediate events of the augmented fault tree which are associated with a component under automatic monitoring, the failure information being received from fault sensors and describing an observation of the operating state of the component associated with the event of the augmented fault tree, - calculating an ex-post probability of occurrence value for each of the events of the augmented fault tree, based on the assigned a priori probability of occurrence values and the assigned failure information, - assigning a troubleshooting cost to each of the events of the augmented fault tree which are associated with a component not under automatic monitoring and serviceable, the troubleshooting cost defining the time required for troubleshooting said component, - modeling, by a Markov decision process, MDP, based on the augmented fault tree, one or more possible evolutions of the complex system in response to at least one troubleshooting action by the maintenance operator on at least one component, the troubleshooting action corresponding to a procedure for identifying the failure or confirming the proper operation of the component, - determining an optimal sequence of troubleshooting actions for the complex system, according to a decision policy which minimizes the expected value of the sum of the troubleshooting costs and which is determined by applying an MDP-solving algorithm. - receiving a file containing a modeling of some or all of the complex system by a fault tree defining some or all of the combinations of events causing a top event associated with a failure of the complex system, the fault tree comprising a plurality of intermediate events and a plurality of basic events, - assigning an a priori probability of occurrence value to each of the basic events of the fault tree, in order to generate an augmented fault tree, - assigning failure information to each of the basic events and each of the intermediate events of the augmented fault tree which are associated with a component under automatic monitoring, the failure information being received from fault sensors and describing an observation of the operating state of the component associated with the event of the augmented fault tree, - calculating an ex-post probability of occurrence value for each of the events of the augmented fault tree, based on the assigned a priori probability of occurrence values and the assigned failure information, - assigning a troubleshooting cost to each of the events of the augmented fault tree which are associated with a component not under automatic monitoring and serviceable, the troubleshooting cost defining the time required for troubleshooting said component, - modeling, by a Markov decision process, MDP, based on the augmented fault tree, one or more possible evolutions of the complex system in response to at least one troubleshooting action by the maintenance operator on at least one component, the troubleshooting action corresponding to a procedure for identifying the failure or confirming the proper operation of the component, - determining an optimal sequence of troubleshooting actions for the complex system, according to a decision policy which minimizes the expected value of the sum of the troubleshooting costs and which is determined by applying an MDP-solving algorithm.