My 4-5 year old rheem furnace has a problem. Everything seems fine until the burners start up. They fire up, but only stay on for about 3 to 8 seconds, then the blower turns on but only puts out cold air. I don't think it's an ignitor problem because the burner always ignites. It just started doing this a few days ago. Any ideas?

Sounds like flame sensor needs cleaning.or could be bad pressure switch,id say probably flame sensor problem... gas fired unit?