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.

The fan and limit is still your problem. Revisit your install of that control. If the fan still works when set to ON on the thermostat the limit and fan control has to be the problem.

The install was simple, there were only 2 wires to connect. The dealer did tell me to clip a tab off it, which I did. It said something about "low voltage."

Also, I replaced the thermostat after I started having the problem and that didn't solve it either.

Another thing is that the new control has a manual button to shut off the fan and the old one didn't, or maybe the old one had been taken off. Either way, the new one doesn't seem to do anything. If I push it in, or pull it out, the fan doesn't shut off when it is running.

I rechecked the wiring and found a loose connection at the other end of a wire (not connected to the limit control) and it works fine now, thanks for the help.

Glad you got it going. Sometimes you have to revisit your work.