While driving on the freeway in a construction zone, I smelled a burning oil smell. I considered the possibility it was in relation to road repairs being done (roads being repaved, ie tar resurfacing). I was cut off by another vehicle and depressed my brakes and I saw a poof of smoke over the front passenger area near tire. I noticed all of my gauges were in the normal ranges (temperature, battery voltage, etc.) In less than a minute my truck went into limp mode and advised it had a DEF system issue and it would be limited to a set MPH and a set number of miles. Check engine light came on and another driver alerted me that the underneath of my vehicle was on fire. I pulled over to the side of the highway and my truck became engulfed in flames in. Truck is under factory warranty, unmodified, never been in an accident and has had all recommended services performed at local dealership. There was a significant amount of traffic and luckily I did not collide with anyone or set neighboring dry grass field on fire. Fire department responded and extinguished the fire, but it is a total loss. Insurance company has been notified, and vehicle will be available for inspection by appropriate agencies. While local law enforcement did respond, they did not take a report because I didn't hit anything, and the only property damage was my own. (There is documentation of the event, but just dispatch notes.) Prior to the situation above the truck had been at operating temperature for