Are you saying that if your set point is say 68 degrees and the room temperature drops below 68; that the t-stat will not put the unit in a call for heat? If the Stat is bimetal with mercury switch, the stat may not be level. If its a digital thermostat there may be an off set temperature or anticipator setting that you dont know about that allows the temperature to drop a few degrees below the set point before calling for heat.

Befroe you replace the t-stat, make sure its not the furnace thats the problem. You can take the stat off the wall and expose the wiring behind it on the sub plate. There will be a R terminal and a W terminal

these are the two wires that get connected when the stat calls for heat. simply complete the circuit from the w-r terminal and the furnace should start imediately. If not you have a bigger problem. There may be a small spark when you connect the two wires, this is 24-28 volts, so do not use two hands to complete the circiut, use something like needle nose plyers with one hand. And do not touch any other wire while doing this.

Also when you do complete the circuit you will have to hold this until the furnace has gone through its entire cycle, after you remove the connection the furnace blower should continue to run for about 90 seconds after you release the contact points. Good luck..

I have carrier furnace that works fine when the temp is set above 71 degress below that it will not light again. Is there a certain gap between the igniter and and pilot light?. The reason I ask is I replaced the spark box and I noticed the part that the plug wire connects to was lose so I tighen it up. Thanks, Niteman