Furnace blowing out cold air even though the ignitor is working

What happens AFTER the ignitor heats up? If you have a flat surface ignitor, it should heat up very brightly, the heat sensor then opens the gas valve and the ignitor lights the gas. If the ignitor is working, the inducer fan must be working too as these are interconnected. If the heat sensor is bad or dirty, you will get the symptoms you indicate. Locate the heat sensor, clean it with fine sandpaper or coarse steel wool. Then, turn off the power to the heater, make sure the thermostat is set 10 degrees below the ambient house temp. Wait 30 seconds, turn on the power to the heater and then raise the temp on the tstat. What happens? If in a fault mode, the control board should have a blinking LED. Count the blinks and match the count to the code on the plenum door.