I referred to page 25 of my trusty 2015 sprinter owner's manual which, states the following:
"To function properly, exhaust gas after treatment must be operated with Diesel Exhaust Fluid (DEF) reducing agent. The DEF range is dependent on operating and driving conditions. Therefore, add to the DEF supply tank "regularly" [Quotations added for emphasis] during vehicle operation or, at the "latest"[quotations at it for emphasis], after receiving the first warning message via the on board computer.
To conform to emission regulations, you must operate the vehicle with DEF and refill the supply "regularly"[Quotations added for emphasis]. The engine management recognizes attempts to operate the vehicle without DEF, with thinned DEF or with another reducing agent. After previous warning messages, the engine management then prevents a further engine start.
If the DEF level drops below 1.5 US gallons (5.5 l) The first warning message it is issued and a warning tone sounds. After the message appears for the first time, and under normal driving conditions, the remaining DEF supply will last approximately 1000 miles (1600km).
If a reserve mark of 0.8 US gallons (3.0 l) is reached, a second warning message is issued and a warning tone sequence sounds. After the first message, the DEF supply will last for approximately 1200 miles (1900km). Subsequently, you will only be able to start the engine a further 16 times.
You should immediately add 2.0 US gallons (7.61) Of DEF(>page 147) or have the DEF supply reservoir filled at qualified specialist workshop.
When the number of remaining engine starts is zero and the engine is restarted the engine management prevents the vehicle from being driven at a speed exceeding 5 mph (8km/h).
If the check engine indicator lamp lights up, the exhaust gas after treatment is faulty or an emission related malfunction occurred (>page 211). You will then only be able to drive a maximum 50 miles (80km) before the engine management restricts the number of starts remaining."
There appears to be an inconsistency in the owner's manual. For example, the manual states the following:
"If the DEF level drops below 1.5 US gallons (5.5 l) The "first" [quotations at it for emphasis] warning message it is issued and a warning tone sounds. After the message appears for the "first" [quotations added for emphasis] time, and under normal driving conditions, the remaining DEF supply will last approximately "1000" [quotations added for emphasis]miles (1600km).
Then, it states, "If a reserve mark of 0.8 US gallons (3.0 l) is reached, a second warning message is issued and a warning tone sequence sounds. After the "first" [Quotations added for emphasis] message, the DEF supply will last for approximately "1200" [quotations add for emphasis] miles (1900km)..."
Maybe I am misinterpreting? After the "first" warning, how many miles can you travel a thousand or 1200?
DEF issues appear to be a common occurrence, especially, if the first message is triggered. As a result, I concur with RonR about the possibility of there being a bug in the system. The owner's manual states the "latest "you should add DEF it is right after the first warning message. In other words, you just can't put your ass and gas in this vehicle! Pay attention! The owners manual also emphasizes the importance of "regularly" topping off the DEF reservoir.
When I first learned about the DEF after-treatment system, my first intuitive thought was to always keep the reservoir full and never let the system trigger because only the Sprinter gods will know what cascading, wallet flushing events could follow? As a result, about every 500 or thousand miles, I top-off the DEF reservoir and so should you!
Mercedes-Benz sells DEF in nifty half-gallon containers with a spring-loaded spout that fastens directly to the vehicles DEF reservoir. I bought two. No doubt they are pricey (about 12 bucks), however, I refill them from less expensive bulk containers from the Zone. The Spring loaded spout minimizes spillage and automatically stops when the reservoir is full. If spillage does occur, I immediately rinse it off. I also make certain the containers do not become contaminated with a crystallized DEF which may or may not dissolve. I don't know I'm not a chemist.
Finally, I have yet to determine what constitutes "normal driving conditions?" I may have read it in the manual but forgot.
May all your travels be free of CEL's, MIL's DTC's!
Bob