Furnace Fan Limit Control is Bad?

All, My Furnace has been kicking on when the heat turns on but the Furnace Fan is never kicking off. The Fan Control on the Thermostate is set to Auto but still no luck. Sometimes it will turn off but only after a long period of time. The Fan Limit Control is a Disc Fan Limit Control so it can not be set and must be replaced. My question is how easy is it to replace? Is there any tests I can perform to verify that this is the bad part?
Thanks
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You need to give a littel bit more information: Brand, model number, gas?, oil?, etc.
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It is the Original Electric Rudd Furnace that was installed in the house when it was built in 1988. I have a picture of the part I need to replace but I can not find any information on it.
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