[JRTJH]

The Ford DEF level sensor is a "high tech" gray tank sensor assembly. It works much better than the RV tank sensors, but it is not without its problems. You may find that when you try to add DEF, the tank won't accept any. If the sensor connector is loose, the computer will read the tank as empty and if the tank "was empty" and recently filled by the dealer, the computer will take several start/stop cycles before it "averages the level" and begins to trust (or believe) what the sensor is sending to the computer.

If you have a fault reader, hook it up to the OBD port and see what it says. If you do decide to "try and fill the DEF tank", pour carefully so you don't overfill it, especially in a cold climate where the expanding DEF "could" expand and crack the tank... That's not covered by Ford's warranty. At least that's what they told me when I was discussing "topping off the DEF" before a trip. My dealer warned me not to "top it off" but to only add a 2.5 gallon container when it was below half full. He suggested, better yet, let it get down to the "500 miles to empty" notice, then add DEF.