The heat exchanger has a plurality of stacked individual boxes for defining locally an inlet chamber and an outlet chamber for fluid. The heat exchanger includes an insert that comprises an inlet and outlet opening for fluid extended into the chambers by a body arranged for fluid communication of the opening with the first or second chamber. The insert that comprises a common part the body of which is provided with at least two openings capable of communicating with the first and second chambers, and it comprises a separating member for isolating the fluid currents between the opening and the chamber with which the opening must not communicate, the separating member being selected between two different shapes each corresponding to the fluid communication of the opening with one of the chambers.