In some frozen mode shipments the moisture load can be so high that frost accumulates in the reefer unit. This happens on components above the evaporator coil, the highest component being the return air grid. In reaction to a specification provided by Maersk Line, Carrier has delivered a software solution to that problem (scrl9521.ml3). The aim of this test is to experimentally assess the efficacy of Carrier’s software solution to the ice/frost formation problem in frozen mode shipments with a high moisture load. To that end, a reefer container with Carrier PrimeLine refrigeration unit was placed in a climate chamber. In a series of tests the unit was set at -20 °C and it was subjected to an extreme moisture load for multiple days. The results show that Carrier’s software scrl9521.ml3 is an effective solution to the ice/frost formation problem in frozen mode shipments with high moisture load. However, two concerns remain: 1. It is inherent to the implemented software logic that there is no guarantee of a frost-free return air grid at defrost termination. 2. The alarms generated during the test deserve attention.