My E has 296K miles but today decided to give me too much heat. The heat is constant and does not respond to changes in the dial thermostat. The heat comes out of the side vents but not in the center or down low. I am using the Economy button. Sometimes the fan keeps blowing after I hit the off button. Sometimes the fan speed buttons work, sometimes not. I can drive with a window open or leave the heat off. I checked the diaphram between the coolant near the battery and it did not seem to be torn. What is the best way to evaluate this?