What usually happens when your thermostat goes bad?

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My 450 isn't pumping coolant back into the radiator like it should and the thermostat doesn't seem to be working. What usually happens when they stop working?

[Sjorge450R]

the impeller? Water pump assemble as a whole? Have you checked that stuff out?

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The impeller is on the right side and the thermostat is on the left(where the coolant return hose runs back to the radiator) the impeller and everything is fine, the thermostat def isnt working right but i just was wondering what happens if they stop working.

[Sjorge450R]

well from my understanding, a thermostat typically stays closed when it fails. You can pop the thermostat out and put it in a pot of boiling water and see if it opens. According to service honda, a new thermostat is $27

part number 19300-HP1-671

[S.A.J]

i think its that thermostat/sensor that makes the fan go on.when i bought my 450 with the crank bearing out when the guy fixed it and got it runin it the fan wouldnt come on and itd start spewing coolant. he thought that mayb that was the reason the crank went out was cause it got hot while riding and the guy didnt notice and then the rest is history....not sure though,very good possibilty though

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Thanks for the help guys