Diesel Exhaust Fluid (DEF)

Another lesson learned. After the truck's first 5000 miles or so, a message appeared on my dash that DEF needed to be added in about 1000 miles; subsequent messages occurred at 500 miles, etc. Yesterday, with about 4000 miles since I last added DEF, a message - Fill DEF Fluid NOW; otherwise, truck speed will be limited to 65mph in 400 miles. Unlike my previous experience, this time I did not receive the warnings at 1000 and 500 miles. Unfortunately, at the time of these messages, I was in the middle of remote UT where DEF is unavailable; by the time I got to Grand Junction, the truck was very close to being speed limited. When I purchased DEF from the local GMC dealer, I learned that trucks use DEF faster when towing; which I've been doing a lot of lately:). This time, I purchased an extra 2.5 gallons of DEF to keep in the truck for future use - lesson learned.