A method carried out in a system, the system comprising at least a diagnosis plug-in device a host server, one or more diagnostics database suppliers, one or more on-board ECUs, the method comprising the following step: /a/ couple the diagnosis plug-in device to a diagnosis port of a vehicle of interest, said vehicle of interest having a certain vehicle type, said vehicle type being defined at least by the Make/Model/Year/Powertrain of the vehicle of interest wherein the method is characterized in further comprising the following steps: /b/ cause the diagnosis plug-in device to listen on at least one diagnosis bus accessible on the diagnosis port, receive frames broadcasted by onboard ECUs and detect therefrom a Bus Active condition, the frames received having each a received frame-ID, /c/ after said Bus Active condition is verified, at the diagnosis plug-in device, determine an instant frame profile according to characteristics of the received frame-IDs, / d1 / determine vehicle type from the instant frame profile, with the help of a frame profile lookup table (22,22a) held in the diagnosis plug-in device and/or in the host server, or /d2/, whenever no vehicle type corresponds to the instant frame profile in the frame profile lookup table (22,22a), then perform a VIN request sequence to alternately retrieve vehicle type, /e/ provide a diagnosis stacks dataset database at the diagnosis plug-in device and/or at the host server, comprising, for each recorded vehicle type, a diagnosis stacks dataset or a link thereto, said diagnosis stacks dataset comprising at least available request/answer services for the vehicle type of interest, /f/ if the diagnosis stacks dataset corresponding to this vehicle type is already available in the diagnosis stacks dataset database, proceed to step /h/, otherwise proceed to step /g/ and then step /h/, /g/ send the vehicle type to one of the diagnostics database suppliers, and receive in return a diagnosis stacks dataset, /h/ carry out the request/answer services defined by the diagnosis stacks dataset to write requests on the diagnosis bus and retrieve, in the corresponding answers, various data from the board ECUs, and report such data in a decoded form to a client fleet management system. /a/ couple the diagnosis plug-in device to a diagnosis port of a vehicle of interest, said vehicle of interest having a certain vehicle type, said vehicle type being defined at least by the Make/Model/Year/Powertrain of the vehicle of interest wherein the method is characterized in further comprising the following steps: /b/ cause the diagnosis plug-in device to listen on at least one diagnosis bus accessible on the diagnosis port, receive frames broadcasted by onboard ECUs and detect therefrom a Bus Active condition, the frames received having each a received frame-ID, /c/ after said Bus Active condition is verified, at the diagnosis plug-in device, determine an instant frame profile according to characteristics of the received frame-IDs, / d1 / determine vehicle type from the instant frame profile, with the help of a frame profile lookup table (22,22a) held in the diagnosis plug-in device and/or in the host server, or /d2/, whenever no vehicle type corresponds to the instant frame profile in the frame profile lookup table (22,22a), then perform a VIN request sequence to alternately retrieve vehicle type, /e/ provide a diagnosis stacks dataset database at the diagnosis plug-in device and/or at the host server, comprising, for each recorded vehicle type, a diagnosis stacks dataset or a link thereto, said diagnosis stacks dataset comprising at least available request/answer services for the vehicle type of interest, /f/ if the diagnosis stacks dataset corresponding to this vehicle type is already available in the diagnosis stacks dataset database, proceed to step /h/, otherwise proceed to step /g/ and then step /h/, /g/ send the vehicle type to one of the diagnostics database suppliers, and receive in return a diagnosis stacks dataset, /h/ carry out the request/answer services defined by the diagnosis stacks dataset to write requests on the diagnosis bus and retrieve, in the corresponding answers, various data from the board ECUs, and report such data in a decoded form to a client fleet management system.