A method carried out in a system, the system comprising at least a diagnosis plug-in device (1) connected to the diagnosis plug (8) of a vehicle, a host server (2), one or more on-board ECUs (3), the method comprising the following steps: /a/ detect a Bus Active condition, either via an electrical pin on the diagnosis plug or via an analysis of activity level on at least one Diagnosis bus accessible at the diagnosis plug, /b/ cause the diagnosis plug-in device to listen only on the at least one Diagnosis bus after the Bus Active condition is detected, /c/ determine a write enable condition, according to evolution of bus load level on the at least one diagnosis bus and/or after a certain time elapsed since Bus Active condition was true, /e/ whenever the write enable condition becomes true, carry out the request/answer services to write requests on the diagnosis bus and retrieve, in the corresponding answers, various data from the board ECUs (3), to be decoded and forwarded to a client fleet management system, /e1/ determining, a cyclic pattern of messages circulating on the bus, with at least busy instants (IB) and quiet instants (IQ), /e2/ place write requests on the diagnosis bus at quiet instants (IQ).