Prostar/ISX, DEF & CEL light on

Hi everyone, recently I've been blessed with the opportunity to purchase lovely 2015 Prostar with Isx15 and 330K miles on. Truck runs great however I have low def light along with check engine and triangle lights on however dashboard Guage indicates def fluid being 3/4 full. I'd like to fix this issue but I'm wanting to hear opinion on this subject as I haven't dealt with this particular problem before. I've done some research and asked around, was pointed towards def sending unit which makes sense. As of current I do not own INSITE software, all I have available is NED & DLB. This part of the question is likely for Heavyd, am I going to be able to diagnose anything on this truck with DLB software?
Is there any other area of concern I should be looking into besides the def sending unit?
I appreciate the help guys, Jacob

 

I have 3 active codes
1.spn 5862 fmi3
2. Spn 5394 fmi7
3. Spn1569 fmi31

 

Thank you Heavyd, I will pull the def dozer valve and clean it. I'm going to update afterwards

 

Here is the update,
Pulled out the doser injector and yes, the tip had a urea fluid caked on, not bad as I've worse one before but enough to limit the spray out of 3 tiny holes. So I scraped gently all of the baked "Rocky" deposits, cleaned up with brake cleaner, sand down mating surfaces, unfortunately had to use old gasket (for now), put it back together, started engine, codes persisted.
Shut down the engine, ran DLB, codes appeared as active, marked codes for deletion, all cleared except for the "engine derate, torque reduced"
Low def light and check engine light went off except for the triangle. Still, decided to take the truck for a spin and as soon as I moved the truck 10 feet the triangle went off, checked for codes all were inactive. Drove the truck 5 miles everything seemed good, no warning lights. I'm not sure how long it will last but I'm hopeful that the problem is fixed.
Any thoughts on above procedure?
Big Thanks to Heavyd for his smarts, again much appreciated brother
-Jacob