My two year old furnace does not come on when the thermostat indicates that it should: i.e. 2 degrees below the ambient room temperature. I have set it for 72 degrees but it gets as low as 64 degrees before it will finally come on. I have replaced the thermostat with a brand new honeywell programmable, but it has made no difference. Sometimes it will run for about 30 seconds, then it shuts off, still leaving the house very cold. Other times, it will come on and stay on until the temperature reaches the required shut off temperature on the thermostat. There is no apparent pattern to this behavior. It creates a great deal of discomfort when the heat is off and the furnace refuses to come on. I have tried all of the troubleshooting suggestions in the manual, and occasionally I get results, but sometimes nothing happens.

In order to isolate the problem tie the heating wires on the thermostat together. This should cause the unit to run all the time. If the unit stays running all the time then you have a thermostat problem. If the unit does not stay running all the time with the heat wires tied together then you know you have a furnace problem.

If it turns out to be a furnace problem I am going to suggest you call for service since that furnace has a circuit board and that will need to be checked.

I am sure you have already check your air filter and it is clean or new? I am sure you have already cheched to make sure none of your registers or return air openings are closed or blocked? If you have not done this check them now.

Check and see what the diagnostic light is doing - lower door on furnace small glass window. Count the blinks of the light. It will tell you what's wrong. The code to read the light code will be on the outside or inside of the furnace door. You should have warranty on this unit.