Low DEF warning even though tank is full

I have a 2014 with the 2.1 (18,000 miles), we purchased about 2 weeks ago, and twice now we have taken it in for the dreaded Low DEF fluid warning, then the 10 starts remaining countdown. Both times I have taken it to the dealership to have it reset. They told me if it does it again they will dig into it more. That day has come (1 week after the last time), I have it going in for a scheduled service B tomorrow. What is going on here? The tank is completely full, and yet the sensor believes it's on empty or near to it. The funny thing is, the last two times this happened, the dealer reset the counter and it was fine for 5-7 days before the warning came up again. If it was a bad level sensor, I would think it would be bad all the time? I should know more tomorrow when it goes in for routine service, but if anyone out there has any ideas or things I should ask the service guy, please let me know. I don't think the sprinter dealer here is trying to skirt the issue, they seem like a good group of guys and have been nothing but awesome so far. This is under warranty, so no issues with costs to me, just want to get this working 100% so we don't get stranded out of town with a countdown timer.

Why are you getting the first "B" service @ 18,000 miles.
With that mileage, you are 3,000 miles past due for the FIRST "A" service.
The first "B" service on a 2014 Sprinter/OM-651 2.1 liter 4 cyl engine/7G-Tronic transmission is @
30,000 miles.
Roger

A 'B' Service they say...

I suggest that you ask them exactly what your 'B' Service will cover and what it will cost you.

A 'B' Service usually covers everything they can check... including brake pads, cab air filters, tire tread etc. As was previously mentioned...it's a comprehensive 30,000 mile check-up.

Any Warranty Issues found would no doubt be covered but I doubt that the 'B' Service labor is covered by Warranty. Best to check it out.

Remember that your Service Advisor is just that...an advisor.

Talk with a Mercedes Mechanic/Tech.

The 'B' Service on my 2012 cost me $1100. Rob

Are they checking Knox sensors? Are they checking DEF pump and DEF heaters? This has been the cause on our 3.0 Sprinters. Have not had it to happen on the new 2.1 Sprinters yet. Hoping that they had fixed this problem with the newer 2.1. Fingers Crossed?

The previous owner had the schedule A done at 8,900 miles, the service B due light has been on since I bought it, they told me the system knows it's mileage and use and has determined it needs to be done. When I took it in today for the DEF issue they decided to fix that and go ahead with the Service B as well, total cost was $890, which is less then he had originally quoted me. I will pick it up later and get the details on what all was done to it. He mentioned knox sensor and O2 sensor on the phone needing to be replaced to fix the DEF issue. Honestly, the lot I bought the van from didn't know their head from their a$$, and told me they did an oil change on it before I picked it up. Knowing what I know now, they probably didn't use the correct oil/filter, so I wanted to get this to the dealer as quickly as I could to have it serviced correctly.