I have a two years old Goodman furance Model # GMP100-5, the blower does not shut off. It only blows air without heat. Can't not turn it off from the thermorstat control. Have to turn it off from the main switch. When the main switch is turn on again and the thermorstat to high(90), only the blows cold air. The burner does not fire up. The unit does not have a pilot.