Abstract: A heat exchanger has a core provided with passages for a liquid to keep cool and passages for a cooling medium, a first collection tank for supplying the liquid, a second collection tank for withdrawing the liquid, at least one third collection tank, the core being divided into at least two sections through which the liquid successively flows and which are interconnected by the at least one third collection tank, one of the sections being connected with the first collection tank, while the other of the sections being connected with the second collection tank, a bypass line connected with the third collection tank and in parallel with the other sections, and structure for activating and deactivating the bypass lines.