Testing the limit switch in your fan is vital to maintaining an economically efficient heating system in your home. The limit switch regulates both the furnace fan and in most models the burner. The switch is designed to allow owners to easily control the function of their furnace without help from an HVAC professional. If you experience a problem with your switch or it appears to be malfunctioning, contact a licensed professional in your area.
Open up the panel of the furnace to access the inside. Slide the panel off or unscrew it in some models.
Locate the limit switch at or near the upper portion of the open panel. Look for a rectangular box with a switch.
Flip the cover of the box off and out of the way. Inside you should see the rest of the switch and the temperature dial.
Push in or pull out the switch based on what position its already in. Pulling it out will put the fan on manual and force it to come on and off based on the dial settings; pushing it in will cause the fan to run constantly regardless of whether the burner is on. In either scenario, the burner won't be affected since the limit switch acts only as a safety control to shut it off.
Wait 20 to 30 minutes and see if the fan responds to the change you made on the switch. If you pulled it out to run on automatic, the fan should continue running once the rest of the furnace has shut down. If you pushed it in, the fan will shut off when the burner and the rest of the furnace stops.