A device (D) is dedicated to the diagnosis of operating situations) in at least one communication network (RC) having a bus (BU) with N wire(s). This device (D) comprises i) P connection means (MCI-MC3), with P≧N, bypass-connected at least to the N wires of the bus (BU) so as to access the P voltages présent, ii) conversion means (MV1-MV3) devised so as to convert the P voltages into P first digital signais (SI (p,t)), iii) processing means (MT) devised so as to construct a measurement vector (V(t)) associated with a voltages measurement instant (t) and comprising K éléments chosen from among at least the P first digital signais (SI (p, t)), then to compare at least this measurement vector (V(t)) with at least I pairs, with I≧1, of first (Vt(i,t)) and second (Vr2(i,t) référence vectors comprising K éléments representative of K minimum and maximum values that can be taken by the K éléments of the measurement vector (V(t)), so as to deliver I second signais (S2(i)) each représentative of the result of a comparison, each result being représentative of an operating situation on the bus (BU), and iv) I display means (MAI-MAI) each devised so as to signal an operating situation represented by a second signal (S2(i)) delivered.