The subject of the present invention is a device for diagnosing the loss of a connection to a ground (M) of an electronic control module (BCM) of a motor vehicle. This device (1) comprises a first branch (B1) connecting the electronic control module (BCM) to the ground(M), a second branch (B2) connecting the electronic control module (BCM) to the ground (M) adapted to receive power currents (I(power)) originating from power equipments (EQP) and comprising unidirectional current-conducting means (D), a diagnostic circuit (CD) connected to the second connecting branch (B2) at the level of a measurement point (PM) situated between the unidirectional current-conducting means (D) and the ground (M) and defining a voltage (Vdiag) for diagnosing a loss of connection to the ground (M) of the electronic control module (BCM) on the first branch connecting (B1). According to the invention, the second connecting branch (B2) comprises, between the measurement point (Pm) of the diagnostic circuit (CD)and the ground(M), voltage-generating means(Rs, Idiag) adapted to increase the diagnostic voltage (Vdiag) in case of disconnection of the first branch (B1).