kilo2018

I have a 2005 F150. When I shift it into 4x4 high the yellow indicator light cones on for less than a second and then turns off. The truck is still in 4x4 and works well. Only the dash light does not illuminate.
What is my problem please?
I have noticed that the light itself works well because in startup it illuminates for appx. 3 seconds along with all the other indicators dash lights.