I have an Armstrong-Air Gas-Fired furnace G1D91AU and a programmable thermostat which when I set to heat and says system on but the furnace is not turning on. If I set the thermostat to ON the furnace runs but is blowing cold air. It this a problem with my thremostat or my furnace itself?

I would appreciate if you can give me an idea where the problem is.

Thanks!

You need to find out if you are getting 24 volts at the white wire in the furnace when the termostat is calling for heat.