ISS Columbus Heat Exchanger Close Call Investigation Case Study

During International Space Station (ISS) Expedition 38, just days before Christmas 2013, the
External Active Thermal Control System (EATCS) on the ISS Columbus Moderate Temperature Loop (MTL) Interface Heat Exchanger (IFHX) shut down due to low temperatures. Engineers on the ground scrambled for several days to troubleshoot a pump module Flow Control Valve (FCV) problem, however; no one recognized that the core temperature of the Columbus MTL IFHX was continuing to drop, to the point that water in the system could freeze. Freezing could rupture the core, resulting in high pressure ammonia entering the cabin of the ISS. With no way to clean up the ammonia, ISS crew would have to evacuate the U.S. segment of the station. Columbus MTL IFHX rupture is considered a catastrophic failure and could potentially result in a loss of crew/loss of vehicle. How did engineers miss this potentially catastrophic issue?