The aire flo heater starts normally, when the igniter lights it stays lit for a few seconds and then the furnace shuts down. I have observed this several times. Once, the gas did ignite but only weakly and briefly (1 second) then it shut down. I'm not sure what part needs replacing. The furnace has worked well for 3 years.

Make sure the flame sensor is clean, all burner tubes are clean and the gas ports are open. Start there. Without any idea of your heater it is hard to go further.