The winter of 2013-2014 provided an opportunity to operate off-road vehicles in cold weather for extended time as part of a vehicle/tier 4 diesel engine validation program. An unexpected area of study was the performance of high efficiency, on engine, fuel filters during continuous vehicle operation in cold weather. During the program we observed unexpected premature fuel filter plugging as indicated by an increase in pressure drop across the filter while in service. Field and laboratory testing was completed at John Deere and Donaldson to understand the cause of filter plugging.Although conditions were found where winter fuel additives could cause plugging of high efficiency filters, premature filter plugging occurred even when testing with #1 diesel fuel. This fuel contained no additives and was used at temperatures well above its cloud point. In some cases, laboratory testing of returned plugged filters did not reveal a high pressure drop across the filter and visual examination did not provide additional insight.This paper discusses the vehicle/tier 4 engine and filter testing activity that lead us to conclude that ice crystals, formed when initially dissolved water in diesel fuel is exposed to sub-freezing temperatures, can cause temporary plugging of high efficiency fuel filters. Included is information on filter testing, laboratory testing methods, and results along with information about fuel, temperatures, and dissolved water content.