I ordered the sensor on the engine, While the atv was overheating neither the "HOT" light or the fan came on. And when I did the test by unplugging the sensor and turning the key on they both did cycle. I also didnt see a sensor on the rad.

 

OK you I see you have a 05, I think they stop putting a thermal fan switch on the SP500 in 2004.

 

I had the exact problem with my 05 sp with 200 miles on it and it was recalled for the main computer burning up. This faulty computer will cause the atv to overheat at about ever 5 to 10 miles so call local dealer and give them a vin and then post what happens, Polaris man with issues

 

OK i got up there and had a chance to install my new Thermister. SAME PROBLEM.... the fan does not come on when the atv gets hot. But now when the engine is cold and I turn the key to the on position everything is normal, after i start it and get it good and hot (still no fan comming on) then shut it down and turn the key back to the on position the fan will cycle and the "HOT" will flash on the dash as if I had the sensor unplugged. Im lost now...

 

Sorry for not replying on this topic but i fixed mine. We checked fan, thermister. All good.The thing that was bad was the main computer. Mine was recalled for that so i took it to the dealer andthey put it in in oct of 2008 and still never overheated yet.

 

most likely your refering to the ecm that was a recall not the ecu. the ecm was bad in these year models and did cause a wide are of probelms incluiding as described w/ overheating issues.