A method for diagnosing a mechanism of untimely cut-offs of the power supply to a motor vehicle computer (1) which is programmed to execute a startup routine when woken up and a shutdown routine before being put into sleep mode, includes, at the time of each shutdown routine, generating and storing in storage elements a marker representing a completed execution of the shutdown routine, and at the time of each startup routine, checking for the presence of a marker, and if the marker is present, reinitializing the storage elements of the marker, and if the marker is absent, generating a data element representing a power supply fault.