CaptnD - The high limit switch is a temperature activated safety switch that prevents the furnace from overheating. If it gets too hot inside the furnace the switch will open, breaking the circuit allowing the fan to run until the furnace cools down. Once the switch cools it should close again closing the circuit and allowing the furnace to relight. If you switch is open it is fooling the system into thinking the furnace is still too hot. you need to find out what temp rating that switch is in order to replace it.