The DEF gauge in my 2015 with 5500 Km / 3400 miles was working fine, gauge was indicating at about 60%. Then today it jumped back up to 100% and stayed there all day. Normal day with some light towing and few stops and starts. Outside temperature was about -5 C / 23 F. Everything else looked good, no engine light and normal range on all stock gauges. I don't have any engine additional engine monitoring equipment or ability to read codes. Any ideas what I should be checking or thinking about.