I have a Lennox Pulse 21 furnace and the fan won't work when I have the heat set on auto. The furnace will start, but after about a minute it shuts off without the fan ever starting. If I leave the fan setting "on" it runs constantly at high speed and the furnace also works. I went to a few local dealers who of course didn't want to give me free advice but I finally got one who told me it was probably the "limit control" I think is what he called it. I bought it for $150 and went home and put it on. It has a dial kind of like a thermostat on it. Anyway, after installing it the furnace worked fine for about a day but now is doing the same thing it was before where the fan doesn't kick in when its set to auto. I don't have a manual but could it be some kind of switch or resistor? I don't really know what I am looking for. Any help would be greatly appreciated.