What does the D in a circle with a bar through it mean on the warning lights of a 2003 Lincoln navigator?

Answer 1

That indicates that the Overdrive ( for the transmission ) has been turned OFF

using the transmission control switch on your shift lever ( either push the

button again to turn the overdrive on or the next time the vehicle is shut off

and then restarted , the normally allowed overdrive position will be enabled

and the light will go off )