Just wondering how often you all add def fluid? I can see where the level reads OK but that's it. How do you know how low you are until you're empty?

 

Not sure on the trucks without the productivity screen but if you have the screen in the dash, it lets you know when your level is low. You can also scroll through the screens and there is one that tells you what you level is. It isn't specific to the point of telling in gallons but it will tell you if it's okay, below half, etc.

 

For the basic screen you have to run a system check.
I think it's SETUP + RESET.
It will report OK or under half.

Otherwise, it will alert you with the EXHAUST FLUID RANGE 800 MTE message although I fill mine prior to that so I've never seen it.

 

The tank has 3 sensors, So any reading only means you are above that sensor in the tank.
Basically
OK means you are above the half tank sensor
Below half means you are between the 1/2 full and above the 800 miles to empty
and the 800 mile count down means you are below the lowest sensor and counting down miles to empty.

Like Kper, I check mine weakly and if I'm below half a tank, I dump in a 2.5 gallon jug.