1995 Lennox Pulse 21 furnace. Ironically I did everything from welding the heat exchanger to assembling it to installing it in my house. I am not an HVAC tech or an engineer. The other day I noticed my second problem. A couple of years ago the blower motor failed. Had to replace it with an upgraded motor as the OEM was unavailable at the factory. Very successful operation. I also replaced the air flapper as the old one was caked with dirt. Last week I noticed it was short cycling. The purge blower would run and the for about 5 seconds it would try to fire. Then it would go silent, then the purge blower would run. Then the cycle would repeat. Anyway, I found that if I have the thermostat set at 75 deg instead of the 68 deg I usually run it at, it fires and cycles just fine. Weird. Before I call a real tech, does anyone think the problem could lie in the electronic thermostat?