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?

More info needed. What brand/model is the furnace?