May need a cleaning. Could be that the crossover tubes are plugged and not igniting the other two. This brings up the question why is it still running on one burner because usually the flame sense rod is on the furthest burner from the ignitor( or burner first to light). Has this furnace been worked on by joe plumber hacker and the saftys been bypassed?this is a serious condition that could cause a delayed ignition or small explosion. Have a qualified technician service it.
