I've had several DEF issues with my '11 2500 van. The first was the countdown clock coming on and activating after I had just filled the DEF tank with Mercedes' Ad Blue. That was corrected under warranty by an ECU re-flash.
Next was the countdown clock coming on at 9 and staying there, even after several more starts. I filled the tank as soon as I found an auto parts store with the right DEF (which took less than a gallon) and the light eventually went out in a day or 2. It came back on a few days later and I went back to Mercedes. This time, it was a bad sensor, which was replaced.....also under warranty.
Then the DEF light came on and as I was near the dealer, pulled in for a quick diagnostic. Seems that the DEF heater was malfunctioning and had to be replaced......out of warranty by now.
The most recent (and potentially most expensive) episode was the DEF pump. After the DEF light came on the day after an oil and filter change (I don't wait for the service computer, but have the service done at 5000 mile intervals, per the rec. of the Sprinter tech at Mercedes due to my high mileage use) it went back again, and the active code pointed to the pump. After removal and testing, the pump appeared normal but the tech was going to replace it as a prophylactic measure, as I was headed from FL to OR in a few days.......that was until he found out the price of the DEF pump......Almost $700!
At this point, he ran the pump thru a prescribed battery of tests with special equipment and cleaned any part that was accessible and put the pump back in. The tech told me that due to the strict Federal emission standards, it doesn't take more than a blip of a sensor to throw a code that illuminates an idiot light on the dash.
The DEF system has indeed been the only major PIA with this van, but there is no way around it.....yet!!!! I'm presently at 117,000 miles