Hard to Understand.

I had a countdown in my 15' GLK 250. Brought to Mercedes and they told me the wires to def needed replacing. It cost me $1,500. 3 years later now(today), I just put 3 bottles of Def in the vehicle last week Monday. This Monday, the countdown appeared! I am confused. Does this mean the wiring needs to be replaced again?? How can I prevent this from not happening again. Great vehicle but this is ridiculous.