This started the last time I had it out, the fan runs non-stop whenever the key is in the 'on' position. My buddy has the identical ATV and his only runs after the atv idles for several minutes and then his shuts off normally.

 

The heat sensor that tells the fan to run is probably bad. Might not be a bad idea to clean your're rad just to be sure it is notclogged up and making the fan run. The sensor is not expensive so I would change it or mabey the switch is bad and is closed no matter what position it is in. I'm just a novice at this but I work on my stuff and I usually figure it out without too much cost. I hope you don't have the fan hot wired. It will run no matter what.

 

IF YOUR NOT OVERHEATING TRY DISCONNECTING THE COOLANT SENSOR WIRE ON THE HEAD,IF IT STOPS WILL GO FROM THERE, IF NOT IT MIGHT BE THE ECM, I HEAR THEY ARE PROBLEMATIC.