I have a similar problem re: a gas furnace that stops before reaching the desired temperature on the thermostat. I just replaced my old analog thermostat with a new digital programmable one but the problem persists. I replaced the furnace filter and cleaned the dust from the furnace but this did not solve the problem. When I re-start the furnace using the control switch near the furnace, the fan turns on and the starter light glows but the gas does not ignite. What's wrong with my furnace?