99 yamaha kodiak fan problemb?

the fan on my 99 kodiak will not come on when the bike gets hot....if i use a paperclip and jump the two wires that come of what looks like the thermostat (on top of the engine above an oil line) i can make the fan come on. do i need to change the thermo switch. if that is what it is.. or could i have a problemb with some other computer stuff.