Automobile and truck heat exchangers, including radiators, heater cores, oil coolers, charge air coolers, etc., are predominantly manufactured with aluminum alloys using a process known as controlled atmosphere brazing (CAB). In the CAB process, a fluxing agent (such as potassium fluoroaluminate), is applied to the surfaces to be joined. The fluxing agent melts, dissolves, and displaces the aluminum oxide layer that naturally formed on the aluminum surfaces and frees up and allows the metal filler to flow and form brazed joints. The presence of residual brazing flux in the heat exchangers, along with the high ratio of aluminum surface area to coolant volume within the internal tube surfaces, may affect the physical and chemical properties and corrosion protection of certain engine coolant formulations. The effects may include color fading, ammonia formation/odor, pH increase, rapid depletion of inorganic corrosion inhibitors (particularly nitrite, nitrate, molybdate, and silicate), formation of deposits, coolant passage blockage, and ultimately heat exchanger failure. Some of these effects have been exacerbated by newer, more efficient heat exchangers with smaller internal passages, heat exchangers with internal braze joints along the length of the tubes, and shipment of vehicles worldwide under stagnant coolant conditions. This paper summarizes some of these failure modes in aluminum CAB heat exchangers. It also covers the development of an aluminum heat exchanger screening test to assess the effects of residual brazing flux on engine coolant physical and chemical properties and corrosion protection after aging the engine coolant in a CAB heat exchanger at elevated temperature under stagnant conditions. Test results are provided on conventional, hybrid organic additive technology (HOAT), and extended life coolants (ELC) along with correlations to field performance of coolants. In addition, this paper introduces a test to assess the amount of residual brazing flux in heat exchangers.