Single-phase pressure drops inside the complex headers and parallel circuits of a microchannel heat exchanger were measured systematically, for the purpose of identifying and quantifying internal manufacturing defects. Results confirmed the Moody chart to be applicable for the submillimeter channel flows. Based on nonintrusive testing followed by destructive evaluation, two types of manufacturing defects were quantified: variation of microchannel port diameters and port blockage by brazing flux. A pressure drop model for the whole heat exchanger was developed, and predicted the pressure and mass flow rate distribution inside the heat exchanger, providing valuable insights for selecting microchannel tube dimensions and heat exchanger circuiting. Most important, simple nitrogen flow tests can now be used to help control the quality of the brazing process, by detecting the presence of blocked or deformed ports in the finished product.