If you can, this is very simple and is what I would do if called to your home.
Turn the furnace off. Remove the door where the burners are. If you look closely at the rack of burners toward where the heat exchanger
tubes are (where the fire goes) on one end you will see the ignitor. On the other end you will see a flame sensor. This sensor is merely a metal rod that hangs out in the path of the flame. It is connected to a ceramic insulator with a single wire.
Try to find a piece of steel wool, a dry scotchbrite pad or even a crispy dollar bill. And clean the metal rod. This rod, over time has accumulated and attracted dust and film from the products of combustion. Clean the rod as if you were sanding some light surface rust away. Dont use any soap or water or anything. just do it dry.
Sand paper can be used but I dont recommend it. It tends to leave scratches in the metal that only collect dirt faster the next time. It also can contain silica which will impede the flame signal.
This would be normal annual "clean and check" procedure by a good tech. Rarely does a flame rod go bad, they just need cleaned. After all it's just a metal rod with nothing to break (sans the ceramic insulator).
Once you clean this, restart the furnace.
Many people are sceptical. A friend of mine by the name of Aldofo Wurts wrote a great explanatory article on how this works... Allow me to quote the layman part of his article that you may want to read...."Here’s the way it works. The controller applies alternating voltage between the flame sensing rod and the base of the flame (ground). The ions in the flame provide a high resistance current path between the two. Because the surface of the base flame is larger than the sensing flame rod, more electrons flow in one direction than the other. This results in a very small DC offset current. If there is a flame present, the DC offset is detected by the controller, which tells the gas valve to remain open. If there is no current flow, the controller will close the gas valve and the system will purge itself of any remnant gas before trying to re-ignite or lock-out. The DC offset is small, only in micro amps.
It’s very important that the flame sensing rod works properly. Dirt, corrosion, or bad connections in the flame sensing circuit can cause the controller to think the flame didn’t ignite. The gas valve will be shut down prematurely."
Clean the rod, and let me know how it goes... I suspect you will be pleased.