2005 Chevy Silverado Fan only works on 1,2,3,4, not 5

I need to know what i need to check and change before i take it to the dealer to save a little bit of money. The problem just started for no reason to my view but i just would like to get it fixed and im not sure if its the transistor or a fuse or the blower motor its self im not real sure but when i turn the knob up they all work except 5 .. Thanks