The Exhaust Gas Filter and the DEF systems are independent of one another.
When the
Exhaust Gas Particulate Filter fills up, the "Regeneration" cleaning process kicks in. This consists of dumping fuel into the exhaust to heat the filter and burn off the particulates. This process needs to run to completion (around 20 mins at 35 MPH or better).
When towing, the process has a good chance of not even occurring as the Filter is hot enough to burn the particulates without additional fuel.
But as a daily driver, it is very common to take short trips which doesn't allow the system to get to operating temperature and stay there long enough to finish the cleaning. Then the next time you use the truck, it still needs to be cleaning and the same inhibitors occur. Do this enough times and the software throws up its hands and gives you the "Service" message.
The solution is to drive the truck for 20-30 minutes at freeway speeds once you see the "Cleaning" message.
There is a software hack on your 2015/2016 Ford that adds a display of how full the Exhaust Filter is. It is turned on by the mechanic or you can do it with some special software available on the Ford forums. The 2017's and up have this display already AFAIK.
The
DEF system is much simpler in design. It trickles Urea into the system proportional to the amount of fuel being consumed. It really doesn't matter when you fill the tank. As long as there is sufficient Urea, the system is happy. My personal choice is to add when the level gets below 1/2 full.
Here is a You Tube video on "
" which explain a lot.