My son put a tennis ball in the vent tube and it rolled all the way to the furnace blocking the air from entering the burners. I have a armstrong ultra 90sx furnace and the combustion fan and blower run continuously. I have taken the tennis ball out and it still runs

Reset the vent limit switch on the furnace. Inside the furnace around the vent pipe two wire going to a small disk with a button in the center. Or just try killing power growing to the system and turn it back on.

Swap the tennis ball for a ping pong ball. They will launch out of the vent much more easily when your son wants to practice returning serves.:D