AWD on and off

A friend of mine is trying to figure out why he is getting the AWD is disengaged notice. Is there a way to turn it on and off, or is this a sign of a problem? THANK YOU!

Basic reason for AWD disabled message is bad input from wheel speed sensor. Cause could be tire that is very low on pressure, or sensor (part of wheel bearing/hub assembly) is bad. In many many case though there is wiring problem to one of the sensors (short/corrosion or whatever).