Furnace comes on an blows cold air and I was told it could be the heat sensor that needs replacement

There are different things that can cause only cold air to come from the vents. I've even heard of the gas line being off to the furnace -- the crazy thing? The HVAC guy replaced the furnace saying it was bad, only to then realize the gas was off. NICE.

You need a tech to come out, and also make sure you have a carbon monoxide sensor. Some types of furnace problems can be lethal.

Make sure the tech is a manufacturer certified company, and see if they have any credential like NATE certification. You just can't trust everyone. Google "HVAC dealer locator". There is a directory that comes up on that page that shows techs in an area along with their credentials.