Is the valve bad?? Also this Buick has climate control and is not a manual control heat or cooling system. Over the last three to four years getting less and less heat until NONE these last few days as its colder outside. Engine temp is fine as temp guage fluctuates up and down slightly which indicates thermostat is ok. Engine does not over heat and radiator warms up all over equally which indicates radiator is not plugged up. Any suggestions??