A heat exchanger assembly includes a core with a header, an end tank and a joint between the header and the end tank. A seal between the header and the end tank is in an area at the joint between the header and the end tank. The seal normally produces a permanent seal except when a misalignment of the seal at the joint between the header and the end tank produces a temporary seal. At least one groove or channel is defined in the header or the end tank at the joint between the header and the end tank, for detecting the temporary seal. The at least one groove or channel extends beyond a highest point at which the temporary seal is formed. The groove or channel functions as a positive leak path between the temporary seal and the header and/or the end tank in the area at the joint between the header and the end tank.