tried to replace the high limit switch that did not work. well it just did not fix the problem, the unit keeps shuting off on high limit. fan runs cont. even switched the fan to med-high speed. filter is brand new,I don't know what else to look for? any suggestions?

The air moving through the duct system cools the heat exchanger. If insufficient air passes through the exchanger, the high limit shuts the burners off. If the gas pressure changes to the burners, the air flow isn't enough to cool the exchanger and the air gets hot, so the high limit shuts off the burner. If the heat exchanger gets a hole in it so the fire hits the high limit the high limit shuts off.

The furnace is telling you there is a serious problem and you have no time to take a year's course in furnace repair -
Call for service.

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Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.