If its not the thermostat, then it's probably a control board component in the furnace. You'll need to get someone out to check it. If your unit is only 3 years old it should still be covered by the warranty.

warranty doesn't mean what you think it does sktn77a.

Parts warranty is usually 5-10 years. You'll probably have to pay a labor charge.

I think I know what the warranty means.

Then you have to say it like it is. Many home owners have no clue about these things. When you just say it's under warranty, they get the impression there is no cost... which is FALSE.

I had exact same problem. Except we had some gas leak outside the home (our furnace is inside) and gas and electric company had tightened the gas valve a little more than need to, so the gas did not flow properly to the furnace. We called our friend who is the HVAC specialist and he increased the gas flow at the furnace and now it seems to respond to thermostat much better.