Service 4 Wheel Steering Light on 2002 GMC Sierra Denali

Service 4 Wheel Steering msg. came on. Not sure what to do. I bought the pick-up used from a dealer and it is still under the 3 month 3000 mile waranty. I am planning on calling them, but I just wanted to go into it with some knowledge so I don't get screwed.

Asked by for the 2002 GMC Sierra Denali
A scan tool is needed to retrieve trouble codes stored in the computer of your truck. Once the trouble codes are retrieved it will be obvious where to start trouble shooting the problem.
My scanner says no codes found