Originally Posted by Mansderm161
Mercedes told me not to let it run low.
Of course they did. They want to sell you DEF.
Fact is, Schedule B maintenance on a Sprinter Van includes flushing the DEF system, meaning that it will be emptied and refilled. The system will not be harmed by low DEF, only by running out of DEF while the engine is running. DEF is drawn off the bottom
of the tank, so as long as you have any usable DEF still in the tank, the system will work just fine.
However, you're still going to get warning messages if the DEF gets low, the same way that the low-fuel light comes on when you still have about 1/4 of a tank left, because they don't want you to run out.
I always wait until I get a "Check DEF" message— but before I get "XX Starts Remaining"— before I add DEF. My Interstate has almost 50,000 miles on it, and in just a few days I'll tick over 7 years of ownership. I've never had any problem whatsoever with the DEF system, and never once had the van go into "limp home" mode.