Cold temps result in less decomposition of DEF over time. See attached figure from thermochemistry analysis for DEF. This curve shows the number of months it takes for DEF to reach .2% ammonia (the shelf life limit) vs. the temperature it is stored at. As you can see, cold temperatures significantly increase the amount of time it takes for DEF to become contaminated with ammonia.
Freezing of DEF and then thawing multiple times has no effect on the chemical makeup of the solution. DEF becomes slushy as it approaches about 12F, and will freeze solid at around 10F. DEF melts into a solution with the correct 32.5% urea even if the entire tank is not thawed. When DEF becomes liquid again in the vehicle tank, either from ambient temperature or from the DEF tank heater, the urea content of the DEF is maintained, and the vehicle's SCR system works nominally.
The use of heaters in DEF tanks during very cold weather is apparent when observing commercial truck DEF tanks on the freeway or at rest stops. Look for the Blue colored filler cap - that indicates a DEF tank. You'll see no frost or ice buildup on the tank because the DEF is heated by an engine coolant loop or electrical heaters. This system is thermostatically controlled to assure the DEF is not overheated.
One note on storing DEF vehicles in very cold weather. DEF expands when it freezes, so the tank needs to have room in it for expansion. Follow your vehicle manufacturer recommendations to assure that DEF is not overfilled. Never top up a DEF tank if cold weather is anticipated.
This is also why most jugs advise against storing in temperatures lower than 12 F. The DEF will freeze, expand, and burst the jug. This makes for quite a mess.
MN_Traveler: we have family in Waterloo IA, Duluth, and Twin Cities so we get lots of updates on the weather during the winter. It sounds like y'all have had a VERY long cold winter. I'll bet you are looking forward to summer!
Erich Weinberg and Kathryn Whiting
Five Star DEF